The Extractive Metallurgy of Lead (R. J. Sinclair)

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The Extractive

Metallurgy of Lead

By

Roderick J Sinclair

Consultant in extractive metallurgy and formerly with The Electrolytic Zinc Company of Australasia and Pasminco Limited.

The Australasian Institute of Mining and Metallurgy

Spectrum Series Volume Number 15

2009

Published by:

THE AUSTRALASIAN INSTITUTE OF MINING AND METALLURGY Level 3, 15 - 31 Pelham Street, Carlton Victoria 3053 Australia

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No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without permission in writing from the publishers.

The AusIMM is not responsible as a body for the facts and opinions advanced in any of its publications.

ISBN 978 1 921522 02 4

Desktop published by: Angie Spry and Kristy Pocock for

The Australasian Institute of Mining and Metallurgy

Compiled on CD ROM by: Visual Image Processing Pty Ltd

PO Box 3180

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Preface

The history of lead is as old as the recorded history of mankind. Its use as a valuable material in society has been equally long and varied. In more recent times awareness of lead’s toxicity has restricted its widespread use and many older applications have been replaced by newer materials or have been phased out. Today the use of lead is dominated by the automotive lead-acid battery, and a key feature of this application is the ability to achieve a high level of recovery and recycle of scrap batteries. This attribute now makes lead the most recycled metal in use and approaching 60 per cent of the world’s supply of lead is provided by recycled metal. Secondary processing and smelting is consequently as important a part of the extractive metallurgical industry as primary extraction from ores and concentrates.

The growing market share of batteries and the corresponding growing availability of secondary lead has meant that the demand growth has largely been met by secondary lead and the production of primary lead has been static or in decline for many decades. Coupled with increased regulation and controls of the environmental and occupational health aspects of the industry, there has been little incentive to change other than to meet higher regulatory standards. There has been a steady decline in the number of operating primary smelters and technology change has been slow. Nevertheless the industry plays a vital role in the supply of materials to society and there is a need for awareness of processing options so that the most efficient and cost-effective methods of lead extraction and refining can be applied.

There is a singular deficiency in the technical literature of a comprehensive text covering the extractive metallurgy of lead. The purpose of this text is to hopefully fill that gap and to summarise the main processes in use for lead extraction and refining, the reasons why they are used, and the key features of their design and operation. It is primarily written for those in the industry, as an introduction to the issues involved, and to provide a means of developing a broader perspective of the extractive lead industry, and the ramifications of actions within any one sector of the lead production chain. It is by no means an exhaustive exposé of all aspects of individual processing steps in the extractive metallurgy of lead, but it is hoped that it has covered most key aspects and can serve as a reference and guide to stimulate further enquiry as required.

This work follows the completion of a similar text on the extractive metallurgy of zinc, written with the same purpose in mind. The two metals are so closely associated in terms of mineral occurrence and extraction, that it seemed necessary to develop a complementary text on lead and have companion reference volumes covering each metal. Some of the details in this text repeat to some extent sections in the earlier zinc text, such as the coverage of slag fuming, but this has been done to allow each to stand alone, with sufficient information for those only interested in lead.

As with the zinc text, the material is drawn from both the technical literature and from a long term association with the industry and many experienced and competent technical experts over many years.

Much appreciation is expressed to my colleagues for comments, in particular Jim Happ and Denby Ward, and for the support and encouragement from Dr Rod Grant.

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Chapter 1 – Industry Perspective and Introduction . . . .

3

Introduction, Properties and Uses . . . .3

World Supply and Demand. . . .6

The Lead Smelting Industry . . . .7

Primary Smelting. . . .9

Secondary Lead Production . . . .13

References and Further Reading . . . .15

Chapter 2 – Historical Background. . . .

17

Lead Production in Early Times . . . .17

The Lead Blast Furnace . . . .21

Preparation of Blast Furnace Feed. . . 23

Blast Furnace Products . . . .25

Lead Refining . . . .26

Silver Recovery. . . .26

Direct Smelting . . . .27

Secondary Lead . . . .28

Historical Summary. . . .29

Chapter 3 – Raw Materials. . . .

31

Lead Mineralogy . . . .31

Separation and Concentration Methods . . . 32

Commercial Lead Concentrates . . . .36

Commercial Terms for the Purchase of Standard Lead Concentrates . . . 38

Commercial Terms for the Purchase of Bulk Concentrates . . . 40

Commercial Terms for the Sale of Lead Bullion . . . 40

Secondary Materials . . . .41

Part B – Primary Smelting . . . 43

Chapter 4 – Sintering . . . .

45

Process Chemistry and Thermodynamics . . . 45

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The Structure of Sinter . . . .49

Process Operating Parameters . . . .50

Updraught Sintering . . . .55

Sinter Machine Capacity and Performance . . . 56

Gas Handling and Cleaning . . . .60

Sulfuric Acid Production . . . .62

Chapter 5 – The Blast Furnace . . . .

65

Introduction . . . .65

Chemical Principles and Thermodynamics . . . 65

Furnace Performance . . . .67

Slag Characteristics and Composition . . . 72

Furnace Construction . . . .75

Furnace Operation . . . .80

Environmental Issues . . . .85

References . . . .86

Chapter 6 – The Imperial Smelting Furnace ( ISF) . . . .

89

General Introduction . . . .89

Process Description . . . .89

Slag Composition . . . .92

Evolution of Furnace Design and Operation. . . 92

Coke Use and Furnace Capacity . . . .96

Chapter 7 – Direct Smelting Processes . . . .

99

Principles . . . .99

The Boliden Lead Process . . . .105

The Kaldo Process (Top Blown Rotary Converter – TBRC) . . . 106

The Kivcet Process . . . .109

The Queneau-Schuhmann-Lurgi (QSL) Process . . . 116

Top Submerged Lance (TSL) – Slag Bath Processes . . . 119

The Isasmelt Process . . . .120

The Ausmelt Lead Process. . . .123

The Outokumpu Lead Process . . . .125

Chapter 8 – Smelter By-Products and Treatment Processes. . . .

129

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The Conventional Slag Fuming Furnace . . . 135

Top Submerged Lance Slag Fuming . . . 140

High Intensity Fuming Processes . . . 144

Fume Treatment . . . .144

Electric Arc Fuming Furnace . . . .146

Treatment of Lead Smelter Mattes . . . 147

Sinter Plant and Smelter Dusts. . . .147

Chapter 9 – Electrochemical Reduction Processes . . . .

151

Background. . . .151

Processes Based on Molten Salt Electrolysis . . . 153

Processes Based on Aqueous Electrolysis . . . 158

Part C – Secondary Smelting. . . 165

Chapter 10 – Secondary Materials and Pretreatment . . . .

167

Lead-Acid Battery Composition . . . .168

Battery Breaking and Separation . . . 170

Paste Desulfurisation . . . .172

Processing of Secondary Residues . . . 174

Chapter 11 – Secondary Smelting Methods . . . .

175

General . . . .175

Reverberatory Furnace. . . .175

The Blast Furnace . . . .178

The Electric Arc Furnace . . . .180

Rotary Furnace Smelting . . . .181

Top Blown Rotary Converter (TBRC) . . . 188

Top Lance Slag Bath Reactors. . . .188

Electrowinning Processes. . . .188

Refining of Secondary Lead . . . .193

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Part D – Refining of Lead Bullion . . . 195

Chapter 12 – Thermal Refining of Primary Lead Bullion. . . .

197

Methods and Equipment. . . .199

Copper Removal or Copper Drossing . . . 199

Softening for Arsenic, Antimony and Tin Removal . . . 205

Removal of Silver and Other Precious Metals . . . 210

Separation of Thallium . . . .217

Separation of Zinc from Lead . . . .217

Separation of Bismuth . . . .219

Final Caustic Refining . . . .220

Refining of Secondary Lead . . . .221

Summary of Common Impurities, Their Control and Recovery . . . 221

Chapter 13 – Electrolytic Refining of Lead . . . .

227

Process Principles . . . .227

Practical Operations . . . .230

Current Modulation . . . .236

Periodic Current Reversal. . . .237

Bipolar Electrode Cells . . . .238

Final Refining of Cathode Lead . . . .238

Anode Slimes Treatment . . . .238

Other Electrolytic Refining Systems . . . 239

Chapter 14 – Alloying and Casting . . . .

243

Handling Molten Lead and Alloying . . . 243

Specifications . . . .243

Casting . . . .243

Part E – Environmental and Economic Issues . . . 247

Chapter 15 – Health and Environment Issues . . . .

249

Lead in the Environment. . . .249

The Toxicology of Lead . . . .249

Exposure Pathways . . . .251

Occupational Standards and Controls . . . 251

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Chapter 16 – Energy Consumption . . . .

259

Purpose and Scope . . . .259

Energy Consumption for the Sinter Plant–Blast Furnace. . . 259

Thermal Refining of Lead Bullion . . . 261

Electrolytic Lead Refining . . . .261

Direct Smelting Processes . . . .262

Electrochemical Lead Extraction Processes . . . 264

Comparison of Extraction Processes . . . 266

Energy Consumption in Supply of Lead Concentrates. . . 266

Energy Consumption for Secondary Lead Production . . . 267

Chapter 17 – Costs and Economics of Lead Production . . . .

269

Purpose and Basis . . . .269

Smelting by the Sinter Plant–Blast Furnace . . . 269

Smelting by the Kivcet Process . . . .274

Smelting by the Isasmelt Process . . . 277

Comparison of Smelting Technologies. . . 279

Lead Refining . . . .280

Metal Pricing . . . .284

By-Products . . . .285

Overall Economics for Refined Lead Production . . . 286

Economics of Secondary Lead Production. . . 289

Appendix 1 – Properties of Lead and Associated Compounds. . . .

293

Lead Metal Properties. . . .293

Binary Lead Rich Eutectics. . . .294

Properties of Lead Oxides . . . .294

Vapour Pressures . . . .295

Silver Metal Properties . . . .295

Thermodynamic Properties of Compounds Involved in Lead Extraction . . . 296

Heat Capacities at Constant Pressure . . . 297

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PART A

GENERAL CONTEXT

This part of the text covers the general structure of the lead smelting industry, including its scope, its history and details of raw material supplies used for the recovery of lead metal.

Chapter 1 Industry Perspective and Introduction Chapter 2 Historical Background

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Industry Perspective and Introduction

INTRODUCTION, PROPERTIES AND USES

Lead is a metal of wide historical significance. It is now a very mature commodity and as such exhibits declining intensity of use, with broad replacement in many of its traditional uses. Much of the replacement results from acute awareness of the effects of lead on human health and the environment.

Lead was widely used in ancient times, dating back over 7000 years. It was often mined and produced as a co-product of silver, which was highly prized for ornamentation and jewellery and later for coinage. Lead served as a collector for silver and gold and often smelting was conducted primarily for this purpose. Lead was separated from the precious metals by oxidation in the ‘cupellation process’.

The Phoenicians and later the Romans mined silver and lead in Spain. Lead was mined at Laurium in ancient Greece and on the islands of Rhodes and Cyprus. The Romans also produced lead in Britain and in ancient Gaul. In the Middle Ages, silver and lead mining and production flourished at Rammelsburg, in the Hartz region of Germany, and in the Erzgebirge, and in Upper Silesia. Large deposits were later found and developed in the New World – in the USA, Mexico and Canada, as well as in Australia and these deposits represent major supplies of lead today.

In Roman times, lead was used for making water piping, for lining water tanks and baths, as a roofing material and as a seal for weatherproofing buildings. It was used in soldered lead sheets by the Assyrians in the Hanging Gardens of Babylon. The Latin word ‘plumbum’ for lead has been synonymous with the working of lead metal for handling water, hence the trade of ‘plumbing’. Lead’s low melting point and softness enabled it to be used to seal bronze and iron connectors into building stone, and this can still be seen in many ancient buildings and ruins today.

It was used for the construction of large windows from smaller fragments of glass at a time before large-sheet glass production was possible. Stained glass windows still remain as a prominent example of this art.

Because of its high density and ease of moulding, lead was used as a projectile in warfare, initially for slingshots and catapults, but following the invention of gunpowder and firearms, was primarily used for the manufacture of ammunition. The production of lead shot using a high tower to form small spherical shapes was a significant industry up until the 19th century.

Also due to the ease of moulding, as well as the hardness of its alloy with antimony, lead was used by Gutenberg in the first printing process for the fabrication of moveable type, and is still the basis of large-scale printing type setting where this is still used. However, the new technologies of offset and electronic printing are rapidly replacing this use.

Lead’s oxides as red and white lead were used as paint pigments dating from ancient Egyptian times until the mid 20th century. They provide good pigment coverage and relatively stable colour, but have been phased out of use in recent times for health and environmental reasons.

The unique electrochemical properties of lead in combination with its oxides and sulfates provided a means of constructing high capacity and high powered electrical storage batteries. This low cost application has developed into the major use for lead today, principally for starting, lighting and ignition (SLI) power supply in the automotive industry. Traction batteries for fork-lift trucks, buses and other heavy vehicles are commonly lead-acid. Large installations of lead-acid batteries are used for standby and uninterruptible power supplies, and for electrical energy storage from renewable

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energy sources. Because lead in this application remains concentrated and is not dispersed, it can be readily recovered, making lead the most recycled metal, at around 60 per cent of total world supply. This has given rise to a major part of the lead smelting industry being structured around secondary sources of feed.

The use of lead-acid batteries for portable power sources such as power tools has not been high due to lead’s high density, and alternatives such as nickel-cadmium, nickel-metal-hydride and lithium-based batteries have predominated, with much higher energy-to-weight ratios. In this regard, the future trends in electric vehicles are not yet clear, but lead-acid batteries have the advantage of relatively low cost, with high power delivery.

As a pure metal, lead is soft and malleable with low mechanical strength. This is an advantage in some applications such as weatherproofing, but one consequence is that under stress the metal will easily deform to relieve that stress, or ‘creep’, and this can take place over long periods of time. Indeed, lead can creep under its own weight, and to avoid this effect the safe tensile stress is 1.7 MN/m2and in compression, 2.75 MN/m2. Lead can be alloyed to improve its strength properties, and antimony was commonly used as a hardener. Pure lead is in fact rarely used.

The corrosion resistance of lead is due to the formation of dense coherent surface films such as oxide, carbonate or sulfate. This, coupled with its ability to be alloyed and rolled into sheet, has enabled lead to be used as a construction material in the chemical industry, particularly in sulfuric, phosphoric or chromic acid environments. For these applications it was often used as a protective coating on steel, applied by melting and wiping, or ‘burning’, the lead onto the steel surface.

The high density of lead, and the fact that its oxides will dissolve in glass without causing colouration, have enabled its use to increase refractive index and form decorative ‘crystal’ glass products. High quality crystal can contain up to 70 per cent lead and was first introduced in the 17th century.

Many lead compounds have unique properties with corresponding useful applications. The organo-metallic compound tetraethyl lead has been important as an additive to automotive fuel to control pre-ignition in the internal combustion engine. It is effective in very small amounts, and the petrol-driven internal combustion engine and, indeed, the automobile itself, owed much of their early development to this use. Tetraethyl lead represented a large use of lead in the mid 20th century, but health and environmental concerns have seen this largely eliminated in the early 21st century.

Lead compounds are also used for a range of plastic stabilisers to overcome the degradation of the plastic by heat and UV radiation. This is particularly applied to polyvinyl chloride (PVC), where it is used for construction applications such as house siding, window frames and rainwater products. Degradation causes decomposition and loss of HCl from the polymer structure, in turn causing discolouration and brittleness. A number of base metal salts, particularly lead, zinc, tin and cadmium, are effective in HCl bonding and preventing free HCl formation. The lead salts are usually tri-basic lead sulfate, phosphate or stearate. There are some legislative requirements that products of this nature must be recycled because of their lead content.

Other significant uses of lead are for the sheathing of electrical and communication cables, and for protection against high energy radiation. Its high coefficient of absorption of X-rays and gamma rays at 0.48 cm-1, combined with the ability to dissolve lead oxides in glass, provided for the construction of cathode ray tubes for television and computer monitor applications. This has been an important use for lead, although there is a significant trend to replacement by more compact alternative display technologies, such as LCD and plasma screens. Apart from addition to glass, radiation shielding in many forms in the nuclear industry relies on the use of lead.

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A generalised list of the major uses of lead may be given as follows:

•

batteries:

•

automotive SLI, and

•

energy storage,

•

sheet for the building industry,

•

sheathing of power and telecommunication cables,

•

plastic stabiliser chemicals,

•

radiation shielding:

•

cathode ray tubes, and

•

general applications,

•

ammunition,

•

corrosion protection:

•

chemical lead applications,

•

glass additive for production of crystal,

•

glazes for ceramics,

•

colouring pigments for plastics,

•

paints:

•

both pigment and preservative uses,

•

weights,

•

sound insulation barriers, and

•

automotive fuel additives.

Many old uses of lead such as for ammunition remain, but many have been phased out with the availability of new materials, and because of the recognition of the health hazards associated with some of those old uses. Health concerns have also seen lead removed from paints and petrol. Lead poisoning and its effect on mental health has been known about for many years, and regulations covering permissible maximum lead levels in the blood of those working with lead have been introduced and progressively tightened. Children are more susceptible to lead poisoning, and the effect on childhood mental development has been a significant issue in the formulation of environmental controls. The ancient practices of using lead in cosmetics and sweetening wine by storing it in lead containers or adding lead acetate have long been eliminated. These practices have even been given as a cause for the downfall of the Roman Empire, for the madness of King George III of England and his subsequent loss of the American colonies. Lead shot in cartridges for hunting waterfowl has largely been replaced with iron shot because of concerns about the poisoning of birds from shot ingestion collected with feed from the bottom of waterways. With these health and environmental pressures, the pattern of lead use has shifted markedly, as illustrated in Table 1.1 which shows lead end uses for 1960 and for 2005.

Due to environmental and health concerns, the clear general trend is to replace lead in the dispersive uses and to concentrate its application to uses where it can be recycled. This trend will necessarily see an increase in the proportion of lead supply derived from secondary processing, and minimal growth or a decline in the future demand for primary lead.

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WORLD SUPPLY AND DEMAND

World consumption of lead totalled close to 7 800 000 t in 2005, of which about 3 400 000 t was derived from mine and primary smelter production. The balance came from secondary production from recycled scrap products – predominantly batteries.

Total world lead consumption and mine production since 1970 are illustrated in Figure 1.1 and 2005 production figures are given in Table 1.2.

CHAPTER 1 – Industry Perspective and Introduction

End use 1960 2005

Batteries 28% 75%

Pigments and chemicals 10% 8%

Rolled extrusions 16% 6%

Alloys and ammunition 15% 5%

Cable sheathing 18% 2%

Miscellaneous (including tetraethyl lead) 13% 4%

TABLE1.1

End uses for lead in 1960 and 2005 (source: Lead Development Association).

0 1000 2000 3000 4000 5000 6000 7000 8000 1970 1975 1980 1985 1990 1995 2000 2005 Year

Consumption Mine Production

Lead

’000

(t)

FIG1.1 - Global lead consumption and mine production.

Region Metal (t) Mine (t)

Europe 1 702 000 256 000 Africa 130 000 130 000 America 2 043 000 1 013 000 Asia 3 486 000 1 322 000 Oceania 276 000 715 000 World total 7 636 000 3 436 000 TABLE1.2

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Figure 1.1 shows a slight decline in mine production, although it has been relatively steady over the past two decades. Mined lead will correspond closely with primary metal production at around 3 300 000 t/a. The gap between mine production and total consumption closely matched the production of lead from secondary sources, which was 4 200 000 t/a in 2005 and growing significantly. This increase in secondary lead production matches the increasing proportion of lead being used for lead-acid batteries.

Primary lead is produced largely from the smelting of lead sulfide (galena). This is often mined in conjunction with zinc sulfides where both metals are sought. However, the growth in demand for zinc has outstripped the growth in demand for primary lead and there has been a relative decline in the mined lead to zinc ratio, to match smelter requirements. The ratio of lead to zinc mined was 0.7 in 1960, declining to 0.5 in 1983 and to 0.32 in 2005.

THE LEAD SMELTING INDUSTRY

The lead smelting industry is divided broadly into primary and secondary smelters, producing a crude lead bullion, and refineries, removing impurities from the crude bullion to achieve the market grade of refined lead as set by the London Metal Exchange (LME) or the customer. Refineries may be directly associated with the smelting operations or may be separate independent operations, taking crude bullion from the smelters. There are, for instance, large independent refining operations in Japan and in the UK. Japan, for example, has a surplus of stand-alone refining capacity and has traditionally purchased bullion. This can be an efficient approach where a primary smelter is located at a mine site and the refinery is located close to final refined lead markets. Refining of primary bullion is more complex than for secondary bullion, and most independent refineries have the capability for handling primary bullion. Secondary refining can be relatively simple with few impurities to remove, and is usually part of the secondary smelter.

A broad schematic of the structure of the industry is shown in Figure 1.2.

Primary lead production is based on the smelting of lead sulfide concentrates. There is a large disparity between regional mining and smelting operations and a significant world trade in lead concentrates. Table 1.3 shows the major lead mining countries and Table 1.4 the major smelting capacities in 2004.

Primary smelters are often associated with major mining operations, but are usually centrally located in major industrial centres. Because of the environmental issues associated with lead smelting sites in the past, it will be very difficult in the future to obtain licences for the construction of new

Country Mined lead production (tonnes of contained lead)

Australia 654 000 China 618 000 USA 464 000 Peru 308 000 Canada 200 000 Mexico 152 000 Others 804 000 Total 3 200 000 TABLE1.3

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Country Total smelter production (t/a)

Production from mined lead (t/a)

Secondary lead production (t/a) China 1 533 000 1 350 000† 183 000† USA 1 338 000 290 000 1 048 000 Australia 390 000 365 000 25 000 Japan 375 000 175 000 200 000 Canada 370 000 250 000 120 000 Germany 350 000 75 000 275 000 Kazakhstan 330 000 300 000 30 000 Italy 283 000 130 000 153 000 UK 176 000 0 176 000 Others 1 685 000 135 000 1 540 000 Total 6 820 000 3 070 000 3 750 000 † Estimated. TABLE1.4

Major lead smelting production in 2004.

Mining Mineral Processing Primary Lead Smelting Lead Refining Scrap Collection Breaking and Separation Secondary Smelting Slag Fuming Ore Waste Plastics

Zinc and Copper Concentrates Tailings Lead Concentrate Slag Waste Slag Zinc Oxide Sulfuric Acid Lead Bullion Lead Bullion Antimonial Lead Silver and Gold

Copper Matte

Refined Lead Residues

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primary lead smelting sites. As a result there have been very few new greenfield primary smelters constructed in the past 20 to 30 years. This is reinforced by the fact that primary smelting of lead has been static over that period, so there has been no need for additional capacity. This approach is likely to continue, and existing primary smelters will tend to be upgraded or replaced with improved technology at existing sites.

Secondary lead smelting tends to be localised around major population centres and the supply of waste batteries, due to the relatively high cost associated with the transport of used batteries. The secondary smelting technologies used are also suited to relatively small-scale operations in comparison with primary smelters, which can benefit significantly from the economies of scale.

The balance of lead metal flows for the total lead industry is illustrated in Figure 1.3, with the horizontal width of the bars representing the annual tonnage of metal produced and used. This illustrates the relatively high level of production by secondary lead recycling in comparison with new lead from mine production. New lead essentially reports to a growing inventory of lead-acid batteries and other metal uses, and to losses from the system as dispersive uses. The inventory effect reflects both the growing demand for batteries and the life of the battery before it is scrapped and recycled.

PRIMARY SMELTING

Primary lead smelting is largely based on the treatment of lead sulfide (galena) concentrates. A number of processes are used but the traditional sinter plant–blast furnace technology (as illustrated in Figure 1.4) has predominated.

The sinter plant eliminates sulfur and produces an agglomerated material with lead feed and fluxes present as oxides, which is subsequently reduced to lead metal in the blast furnace using metallurgical coke. Crude lead bullion is refined either by the thermal process, which individually separates impurities, or by the electrorefining process, to give a refined lead with less than 0.01 per cent total impurities.

Secondary Smelting Primary Smelting Mine Production

Batteries Metal Other

Uses

Losses Losses

Recycle

Recycle Metal Scrap

Recycle Residues Scrap Batteries

Lead Residues from Zinc / Copper Smelting

Lead Metal Produced

Inventory Growth Recycle Scrap

Batteries

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More recent process developments have been applied to direct smelting in which sulfur elimination and oxide reduction take place in the one unit, enabling the heat of sulfide oxidation to be utilised, and thus improving the overall thermal efficiency of the process. Direct smelting processes avoid the use of metallurgical coke as a relatively high cost fuel and reductant. The incentive to change from the sinter plant–blast furnace technology has also been driven by environmental issues, since these operations are difficult to contain, and can contribute significant emissions of lead particulates to the atmosphere.

The major primary lead smelting processes in use are:

•

the sinter plant–blast furnace combination (see Chapters 4 and 5),

•

the Imperial Smelting Process (also a sinter plant with closed top blast furnace for co-production of zinc) (see Chapter 6), and

•

direct smelting processes (see Chapter 7):

•

the Kivcet process,

•

the QSL process,

•

the ISASMELT and Ausmelt processes,

•

the Boliden process, and

•

the Kaldo TBRC process.

As indicated in Table 1.4, production of lead from primary sources is of the order of 3 100 000 t/a. However, the capacity of primary smelters is significantly in excess of this figure, since most primary smelters also accept varying proportions of secondary materials as part of their feed. These additional feeds are commonly in the form of lead residues, containing oxide lead as well as sulfates. These residues may arise from scrap processing or from other metal extraction such as zinc and copper,

Sinter Plant Gas

Cleaning

Sulfuric Acid Production

Blast Furnace Gas

Cleaning Lead concentrates

Fluxes – silica, lime, ironstone

Sulfuric acid Dusts

Dusts Slag to waste or fumer Lead bullion to refinery

Recycles

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which give rise to fumes and leach residue containing high levels of lead. In the latter case, the lead produced from such materials is still primary lead, but is not accounted for in the statistics for mine lead production. These materials can represent around ten per cent of total lead output from the sinter plant, and lead bullion production from the blast furnace can be significantly greater due to direct feeds to the furnace, particularly if those materials contain metallic lead. It is therefore difficult to arrive at a figure which truly represents primary lead production capacity; however, it will be of the order of 4 000 000 t/a (2005). The number of primary smelters listed in 2004 totals 53, and Table 1.5 shows the distribution of the world’s primary lead smelters by the process used.

The sinter plant–blast furnace technology represented over 90 per cent of total primary lead capacity in 1980, so there has been a significant replacement of that technology. There has been virtually no additional primary capacity in that period, hence there has been a net closure of blast furnaces and the remaining plants are relatively old. It is likely that there will need to be progressive closures of sintering–blast furnace operations and replacement by direct smelting technologies in the future.

The capacity distribution of primary smelting capacity is shown in Figure 1.5. The vertical axis represents cumulative capacity of plants above a given plant size, as given by the horizontal axis.

Figure 1.5 indicates that almost half of the primary production capacity (or 1 750 000 t), is attributed to plants above 100 000 t/a, and 80 per cent of the primary production capacity (or 2 800 000 t) is attributed to plants above 50 000 t/a capacity, of which there are only 25 presently operating, as detailed in Table 1.6. Of those 25, three use the Kivcet process, three use the QSL

Process type Capacity (t/a) Percentage of total Number of smelters

Sinter plant–blast furnace 2 470 000 70% 34

Imperial Smelting Furnace 280 000 8% 8

Kivcet 360 000 10% 3

QSL 270 000 8% 3

Other processes 160 000 4% 5

Total 3 540 000 100% 53

TABLE1.5

Distribution of primary smelter capacity by process type in 2004.

0 500 1000 1500 2000 2500 3000 3500 4000 0 50 100 150 200 250

Smelter Capacity ’000 (t/a)

Cumulative Cap acity Above ’000 (t/a)

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process, one uses Kaldo and two Ausmelt technology. The remaining 16 use sinter plant–blast furnace. The average primary smelter capacity is 67 000 t/a of lead bullion, whereas the median capacity is 50 000 t/a.

Region, country and company name Location Process Annual capacity (t) Europe

Belgium

Umicore Hoboken S-BF 125 000

France

Metaleurop Noyelles Godault S-BF 110 000

Germany

Berzelius Stolberg Metaleurop Weser Blei

Binsfeldhammer Nordenham QSL Ausmelt 100 000 90 000 Italy Eniresorse

Porto Vesme Kivcet 100 000

Kazakhstan

Kazpolymetal Ust Kamenogorsk Chimkent Kivcet S-BF 140 000 160 000 Serbia

Trepca Kosovska Mitrovica S-BF 125 000

Sweden

Boliden Mineral AB Ronnskar Kaldo 55 000

Americas USA Doe Run Doe Run Herculaneum Glover S-BF S-BF 205 000 95 000 Canada Teck-Cominco Brunswick M&S Co Trail, BC Belledune Kivcet S-BF 120 000 108 000 Mexico

Met Mex Penoles Torreon S-BF 180 000

Peru

Centromin La Oroya S-BF 93 000

Asia and Oceania China

Zhouzhou Smelter Baiyin Northwest Smelter Fankou Mine Shenyang Smelter Zhouzhou Baiyin (Gansu) Fankou (Guandong) Shenyang (Liaoning) S-BF QSL S-BF S-BF 100 000 52 000 60 000 70 000 India

Hindustan Zinc Chanderiya (Rajasthan) Ausmelt 50 000 Japan

Toho Zinc Co Chigirishima S-BF 90 000

South Korea

Korea Zinc Co Onsan QSL 120 000

North Korea

Korea Metals and Chemicals Mumpyong S-BF 90 000

Australia Nyrstar Xstrata Zinc Port Pirie Mount Isa S-BF S-BF 220 000 150 000 TABLE1.6

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Refining is a significant and separate part of primary lead smelting, and two different approaches are used involving pyrometallurgical separation processes or electrorefining. Pyrometallurgical methods involve the oxidation of selected impurities from molten lead bullion for collection as a slag or dross, or the precipitation of impurities to form a dross or crust by the addition of reagents and/or by changes in temperature. A number of steps are usually applied for removal of copper, arsenic and antimony, silver and precious metals, zinc, then bismuth, and finally, residual minor impurities and drossing reagents by treatment with caustic soda. Operations are usually conducted in externally heated crucibles or ‘kettles’, holding between 100 and 300 t of molten lead.

A significant part of the refining operation involves the recovery of by-products, particularly the precious metals silver and gold.

Electrorefining involves the transfer of lead from an impure anode sheet, through an electrolyte to a high purity lead cathode. Crude bullion, after copper, arsenic and antimony removal, is cast into anodes, which are placed in tank cells. The electrolyte commonly used in the Betts Process is a solution of lead fluorosilicate and free fluorosilicic acid. Lead is deposited on lead starter sheets, which are removed from the cells and melted to high purity refined lead. Impurities are contained in the anode slimes and are collected and processed by pyrometallurgical methods for recovery of precious metals, bismuth and copper.

SECONDARY LEAD PRODUCTION

Secondary lead is primarily sourced from scrap lead-acid batteries but also processed scrap metallics such as sheet and pipe. Secondary operations are characterised by relatively small plants in comparison with primary smelters, and are sized to handle scrap availability within a local area. This is determined by the economics of scrap battery collection and transport to the secondary operation, and it follows that the largest secondary plants are located in the high vehicle density areas of the USA.

The first step in secondary lead processing is the breaking and separation of scrap batteries. In this step, batteries are shredded or disintegrated, then the battery components are separated by physical methods into metallic components, pastes containing lead oxides and sulfate, plastics from battery cases and plate separators, and waste battery acid, which is usually neutralised with lime to form gypsum. Polypropylene recovered from cases is a valuable material and can be recycled for reuse. The metallic components may be simply melted to recover lead, largely contaminated with antimony, but the battery pastes are treated in a smelting process in which they are reduced, using a carbon-based fuel, to lead bullion and a waste slag. Processes used for secondary smelting include the following:

•

blast furnace,

•

reverberatory furnace,

•

short rotary furnace,

•

rotary kiln,

•

submerged lance slag bath reactor (eg Isasmelt or Ausmelt Processes),

•

electric furnaces, and

•

leaching and electrowinning processes.

Some refining of secondary bullion is required for the removal of antimony, arsenic, copper and tin. This is usually done in kettles using standard pyrometallurgical refining techniques, but is far less extensive than for primary lead.

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Secondary lead is recovered either as ‘soft lead’ or as ‘hard’ or antimonial lead. The metallic components of automobile batteries such as plate grids and posts may be made from antimonial lead alloys containing up to ten per cent antimony, but usually less than three per cent. This provides the source of antimony in secondary lead, but it can be controlled to some extent by separately processing metallics and non-metallic scrap. There is a trend to the use of calcium lead alloys in place of antimony for sealed batteries, which significantly reduces the quantity of antimonial lead produced by secondary smelters.

0 500 1000 1500 2000 2500 3000 3500 4000 0 20 40 60 80 100 120 140 Capacity ’000 (t/a) Cumulative Cap acity Above ’000 (t/a)

FIG1.6 - Distribution of world secondary lead smelter capacity.

0 10 20 30 40 50 60 70 80 90 100 0 to 20 20 to 40 40 to 60 60 to 80 80 to 100 100 to 120 120 to 140 140 to 160 160 to 180 180 to 200 200 to 220 Capacity Range ’000 (t/a)

Primary Secondary Number of Plant s

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There are around 150 secondary smelters worldwide with a median capacity of 15 000 t/a of lead, although there are many small plants and the first quartile size is 6000 t/a. Figure 1.6 shows the distribution of world secondary capacity as the cumulative capacity above a given plant size, and compares with Figure 1.5 covering primary smelters. Clearly secondary smelters are much smaller than primary smelters.

The number of primary and secondary smelters within a given size range is illustrated in Figure 1.7, which shows the significant difference in numbers and in plant capacities. The scale of these plants also has an impact on the technologies used for secondary smelting in comparison with primary smelting, and the most common approach is the use of the short rotary furnace.

REFERENCES AND FURTHER READING

Henstock, M E, 1996. The Recycling of Non-Ferrous Metals, 342 p (International Council on Metals and the Environment: Ottawa).

International Lead Association website, <http://www.ila-lead.org>.

International Lead Association Europe website, <http://www.ila-europe.org>. Lead Development Association International website, <http://www.ldaint.org>.

Siegmund, A H J, 2000. Primary lead production – A survey of existing smelters and refineries, in Proceedings Lead-Zinc 2000, pp 55-116 (The Minerals, Metals and Materials Society: Warrendale).

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C

HAPTER

2 Historical Background

As already discussed, lead has been used by humans for over 7000 years. Lead oxides were used as pigments in ancient Egypt and also as glazes for pottery, and objects made of the metal have been found dating from 3800 BC. The Chinese used lead coinage dating back to 2000 BC. Lead sheet was reportedly used in the construction of the Hanging Gardens of Babylon, and it is known that lead was used at that time for embedding bronze and iron connection brackets into stone blocks used in construction. It was extensively used by the Romans in building construction, for water pipes, for coinage and in warfare. The Romans were also familiar with lead-tin alloys for use as solder.

Mining of lead ores is recorded at Mount Laurion in Greece in the fifth century BC. It was mined by the Phoenicians in Spain and later by the Romans in the Rio Tinto region, as well as in Derbyshire in Great Britain and widely throughout Europe, but particularly in Silesia, Bohemia and the Hartz Mountain area of Germany.

The history of lead is also inextricably linked with the mining and recovery of silver, which was produced for its value as a currency of trade, as well as a precious metal for the manufacture of jewellery and artefacts. Because of lead’s association with silver and its potential use for degrading silver coinage, lead mining and smelting operations were often closely controlled by the application of strict laws.

LEAD PRODUCTION IN EARLY TIMES

Lead can be reduced from its oxide at relatively low temperatures compared with other metals, and the use of a wood fire is sufficient to produce lead metal. Early lead smelting methods used a stack of wood and ore piled in a hollow or ‘bole’ on the side of a hill crest to utilise strong winds to intensify the fire. In Britain these smelting sites were known as ‘bolehill’ or ‘bloomery’ sites, and were common for metal smelting in general. A small retaining wall could be built around the base to retain a bed of coals and provide a reducing zone. Channels allowed molten lead to run out from the furnace.

The next development was the application of hand, or foot, operated bellows to provide an air blast to the hearth, which enabled the smelting site to be more conveniently located near ore supplies. The furnace was constructed as a short, square shaft with a bottom opening for the bellows and to allow metal and slag to run out. The shaft was packed with charcoal and ore. An example of this type of furnace is the Catalan Forge, introduced in Spain about 700 AD.

These furnaces were primarily used for iron production and evolved into the blast furnace in time. The furnaces in use for general smelting applications in the 1500s, including lead and silver, have been described by Agricola, in the first detailed descriptions of smelting practices up to that time. (Agricola, 1950). The furnace was typically a rectangular shaft 370 mm wide by 460 mm deep and 1500 mm high, equipped with a single tuyere through the rear wall close to the hearth, and operated by bellows. It was constructed of stone for the rear and side walls, with brick for the front wall. The hearth was made of rammed clay mixed with powdered charcoal. A number of furnaces were constructed against a large stone wall, behind which were located a series of bellows – one for each furnace – operated by a shaft linked to a water wheel. Figure 2.1 gives an illustration taken from De Re

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The medieval furnaces operated on charcoal, but wood was also used when smelting lead ores, which was the simplest of the various smelting operations for which the furnaces were employed. For the smelting of silver and gold ores, lead was also added to the furnace and to the forehearth, as a solvent for the precious metals. Production of slags and matte were also common with precious metal smelting and there was considerable recycling and reworking of these materials.

The furnace generally operated only for a few days and was then cleared of accretions and the walls were replastered with ‘lute’, a paste of clay and fine charcoal. The skill of the furnace operator was most important in regulating the air blast from the bellows and in the placement of the ore charge towards the front of the shaft so as to avoid the formation of a sintered mass or ‘sow’. Natural fluxing materials such as fluorspar were often also added to the charge depending on the nature of the ore.

In the 16th century, lead smelting tended to develop towards the use of a shallower hearth akin to the blacksmith’s forge, with rear fixed tuyeres blown by bellows that were driven by water power, and a mix of selected lump lead ore or concentrate and charcoal was piled on the hearth and hand rabbled. Crushing and simple gravity or hand-sorting of ores was becoming more common at this time. Hearths of this type were the Scotch hearth, as shown in Figure 2.2, and the Moffat ore hearth. Early hearth dimensions were 0.6 to 0.9 m2, with a central depression of around 100 mm deep to retain lead.

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The hearth was started with a charcoal fire, onto which lead ore was added with more fuel. The charge was worked by hand-stirring with iron tools. Lump material was removed onto the front working stone, was broken to allow oxidation and then pushed back onto the heap. The basin in the hearth filled with molten lead, which then overflowed into a cast iron collection pot located at the front of the hearth. Lime was added at around one to 1.5 per cent of the ore charge to cover the molten lead and enhance the formation of a crumbly slag, which allowed good blast penetration and sulfur removal. The temperature was kept as low as practical to maintain this slag regime and to minimise lead fuming. A lumpy slag was removed periodically and generally reported around 20 per cent lead. The high lead slag was stockpiled and, with improved smelting techniques, many of these old hearth slags were reworked to recover additional lead.

The hearth furnace required lump material of high lead grade to avoid excessive dusting and fuming, and to minimise slag formation and loss of lead in that slag. It was consequently favoured by early lead smelting operations in the Mississippi Valley with clean high-grade galena ore. At these sites a water-jacketed version was developed, constructed of water-cooled cast iron panels in a U-configuration on the long axis, and termed the ‘American Water Backed Hearth’. A later development favoured by the Missouri lead producers was the Newman Hearth, a mechanically rabbled version of this technique. An eight foot (2400 mm) long by 20 inch (203 mm) wide hearth containing an eight inch deep bed could produce three tons of lead in eight hours from high-grade concentrates. The mechanical version relieved the smelterman of the laborious task of constantly hand-rabbling the charge to break up accretions and ‘sows’, with exposure to heat and fumes.

CHAPTER 2 – Historical Background

To Chimney

Charge Door

Lead Pot Lead Pool in

Hearth

Charge and Fuel Bed

Tuyere

Air Blast

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The copious fume emissions from these furnaces and the need to process finer ores eventually favoured the use of the reverberatory style hearth furnace, which did not require the use of bellows and is illustrated in cross-section in Figure 2.3. These furnaces probably evolved from the open oven style hearth furnaces used at Carni in Austria, and the Saxon furnaces, which resembled baking ovens.

For the standard reverberatory furnace, a batch of galena was added to the furnace hearth and was roasted with hand-rabbling for about two hours, in which time part of the lead sulfide was directly oxidised to lead sulfate. The resulting mixture of lead sulfate and unreacted lead sulfide was thoroughly mixed and the temperature of the furnace was increased. This allowed the ‘roast reaction’, as given in Equation 2.1, to take place, with copious emission of sulfur dioxide:

PbSO₄+ 2PbS = 3Pb + 2SO₂ (2.1)

Any silica in the concentrate tended to react with lead oxide (PbO) to form lead silicate, and in the final stage of the process lime was added to the furnace charge and mixed in with the slag and unreacted ore, for the purpose of decomposing the lead silicate in accordance with Equation 2.2:

2PbSiO₃+ 2CaO + C = 2CaSiO₃+ CO₂+ 2Pb (2.2)

Following this step, molten lead bullion was tapped from the base of the furnace hearth as a crude impure or ‘hard’ lead.

In some operations, particularly for those processing lump feed, the roast-reduction cycle was repeated a number of times, with the temperature raised for each cycle. In this situation silver tended to concentrate in the first run lead bullion, and could be four times the silver content of the final lead run. This was a useful approach to handling high silver ores, so as to reduce the effort required in silver recovery by the Pattinson Process or by cupellation.

The earliest reverberatory hearths were introduced around 1720 in Silesia and England. Early types were the Corinthian furnace, the English or Flintshire furnace and the Silesian furnace. Silesian furnaces used in Germany were up to 25 foot (7600 mm) long by 8 foot (2400 mm) wide, with five working doors in each side. Labour requirements were 15 to 20 man hours per tonne of lead produced, with coal consumption close to 0.8 tonnes per tonne of lead produced.

Firebox

Hearth

Tap hole

Flue Filling port

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In later practice during the 1800s two hearth furnaces were commonly used: one for calcining with gases going to a chamber-type sulfuric acid plant, and the second or ‘flowing’ furnace operating at a higher temperature, in which calcined material and coke were melted with the addition of lime and fluorspar to produce a fluid slag. Often pig iron or scrap iron was also added as a reductant. As well as lead bullion and a low lead slag, matte (or ‘regulus’) was also produced and could contain significant amounts of lead. The matte was reworked by calcining and returned to the reduction furnace, thus producing a second matte enriched in copper. Matte could be recycled a number of times.

The above smelting operation still resulted in a significant amount of lead being volatilised, forming a fume in the exit gases from the furnace, probably rich in toxic elements such as arsenic, as well as sulfur dioxide. This caused the destruction of vegetation around the smelting operation, and the poisoning of farm animals and cattle feeding nearby. To reduce this effect, long horizontal flues up to 1.5 km in length were constructed from the furnace to the final vent stack, allowing the bulk of the fume to settle out onto the walls and base of the flue. Collected fume contained of the order of 33 per cent lead and was recycled. Collection methods were improved by the addition of drop-out or condensing chambers immediately following the smelting furnace, which were introduced in England around 1780.

The crude bullion from the smelting furnace was allowed to oxidise in shallow open pans and the dross skimmed from the top removed arsenic and antimony, ‘softening’ the resulting lead metal.

The use of iron metal additions to reduce galena or lead oxide was first noted in India in the 14th century. In this method, iron was combined with lead sulfide to form metallic lead and an iron matte as in Equation 2.3, thus limiting the formation and emission of SO₂(Dube, 2006):

PbS + Fe = FeS + Pb (2.3)

Lead ore, charcoal and iron were placed in crucibles within a furnace and later removed to separate and recover the lead. In the late 18th century in Europe, iron reduction was applied with the addition of high-grade lump ore, charcoal and iron to a small shaft furnace. Iron use was around 12 to 15 per cent of the ore charge. This practice first appeared in Claustal in the Upper Hartz region and later at Tarnowitz in Silesia and Przibram in Bohemia.

The use of iron for lead reduction also occurred in Japan, but using an open pan hearth filled with burning charcoal into which lead ore and pig iron were charged. Iron use was two to three times higher than reported in European practice.

THE LEAD BLAST FURNACE

The hearth furnace processes described above were only efficient for the smelting of lump high-grade ores with low levels of associated ‘earthy materials’ such as silica and iron minerals, as well as zinc. To handle lower grade materials it was preferable to use a higher temperature process and produce a molten slag containing the gangue minerals, and hence the blast furnace was applied.

The blast furnace for the production of iron had evolved from the early charcoal, fuelled hearth and shaft furnaces blown by water-powered bellows. Coke was first used by Abraham Derby at Coalbrook Dale, in England in 1713, and with the invention of the steam engine and its use for operating blowing engines, together with the development of the hot blast by Neilson in Glasgow in 1830, the iron blast furnace was well established by the mid 19th century. Furnaces were originally constructed from massive sandstone blocks bound with iron straps. Later, thin steel shell construction was used lined with refractory brick.

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Early shaft furnaces for lead were used in Freiberg and in the Harz region in Germany. These evolved from the earlier hearths, using a rectangular cross-section with one tuyere at the back of the furnace facing a tapping access at the front. This developed into a horseshoe shape with a number of tuyeres and finally into a circular shaft. The Castilian furnace from Spain was an example of the early circular shaft furnaces for lead. It was constructed of sandstone blocks and is shown in Figure 2.4.

Later furnaces, such as the Lower Harz furnace, the Claustal furnace and the Przibram furnace (small versions of an iron blast furnace), were of brick construction. In 1863 the Pilz furnace in Germany introduced water-cooled cast iron plates at the base of the furnace in the tuyere zone. This was followed in 1891 by the American Water Jacketed furnace, at Great Falls in Montana, and the Globe smelter furnace at Denver, Colorado. Water jackets contributed significant benefits by enabling rapid repairs and much longer operating campaigns, due to the reduction in accretion formation in critical narrow areas of the furnace. With the adoption of water jackets, furnace design tended to move from circular to rectangular cross-section for construction simplicity, also allowing significant increases in productivity from each furnace. The Rochette furnace used at the Atenau smelter near Claustal in the Upper Hartz region introduced the concept of a long rectangular hearth, with rows of tuyeres on each of the long sides and tap holes at each end. The main driver for this change was the realisation that blast penetration from the tuyeres was a limiting factor; the only way to get higher production was therefore to retain optimum width for this purpose and to make the furnace longer. Around the end of the 19th century large furnaces were 36 inches wide (914 mm)× 108 inches long (2740 mm) at the tuyere level (2.5 m2hearth area), and around 16 ft high (4870 mm).

Sandstone Block Shaft

Rammed Hearth Tuyeres (5) Lead Pot Slag Car Flue Charging Ports

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With the use of higher blast pressures at the Port Pirie smelter, blast penetration at the base of the furnace was increased to an optimum width of around 1200 mm. Capacity was further increased by extending the length of the furnace, but this reached limits at around 7.5 m, set by the ability to tap slag from one end. By tapping slag at both ends or from the centre, the length could be extended to around 11 m, giving a hearth area of around 13 m2, which is similar to the typical lead blast furnace today.

Although the width at the base of the furnace was limited to achieve blast penetration, narrow shafts permitted shaft accretions to readily bridge across and block the furnace. This was corrected by expanding the width of the furnace above the tuyeres using a short sloping section or ‘bosh’, with either a tapered or straight upper shaft. In the early 1900s the upper limit to the furnace width was around 2000 mm. In 1935 Port Pirie further extended the width at the tuyeres to 1524 mm and added a ‘chair jacket’ and a second upper row of tuyeres with a width of 3048 mm. Further, in 1940 the formation of accretions was minimised by extending the water jackets to the full height of the furnace. This design evolved to the largest lead blast furnace currently in operation. The Port Pirie blast furnace is shown in Figure 5.6 (Chapter 5).

The use of oxygen enrichment of blast air has also enabled the capacity of the blast furnace to be further increased, and is applied in most operations.

Details of blast furnace performance and operation are covered in Chapter 5.

In 1960 the first standard commercial scale Imperial Smelting Furnace (ISF) was constructed at Swansea in the UK, as an adaptation of the lead blast furnace, to simultaneously produce zinc and lead. The furnace operated with a hot top to retain zinc in the vapour phase. The top was sealed and gases passed through a lead splash condenser to strip zinc from the gas phase into a lead-zinc bullion which could be cooled for separation of crude zinc and lead metals. The ratio of zinc to lead production from these units is generally more than 2:1, and lead production from the standard unit is close to 40 000 t/a. Thirteen plants were constructed around the world but due to unfavourable economics a number of these have now closed. Details are given in Chapter 6.

PREPARATION OF BLAST FURNACE FEED

Originally, lump sulfide ores were fed to the blast furnace, but this tended to produce large quantities of matte, requiring appropriate levels of iron flux to form the matte. Silica fluxing was also important to displace lead from sulfates and to form a slag. Limits on the input of sulfur due to excessive matte formation were generally around 15 per cent in the feed material. Speiss (an iron-arsenic-antimony intermetallic) was often also present as a separate phase.

For lower grade ores with higher sulfur contents, and for ores with high arsenic content, it was necessary to eliminate these elements by roasting prior to feeding to the blast furnace. Early roasting was done in open heaps over fuel beds with a tunnel beneath to supply combustion air. This was applied at Broken Hill, Australia for the sintering of lead slimes, which were formed into bricks and dried, and then stacked in an open heap over a fuel bed and burned for ten to 15 days. The resulting material was sintered and quite suitable as blast furnace feed.

Open compartmented pads or stalls were also used, often venting to a central collection flue for combustion gases. However, roasting at low temperatures produced fine calcines, which were not a practical feed for the blast furnace, and higher temperature roasting to melt or sinter the material was necessary, so that lump material could be produced. To effectively achieve this, roasting furnaces were developed in the form of reverberatory hearths, and were introduced around 1790. Typical roasting furnaces were up to 5 m wide and 20 m long, with a fire at one end and exhaust flue at the other end. A series of doors along the side of the furnace allowed access for hand-raking and

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movement of material along the length of the furnace from the cold feed end. Lead concentrate and ironstone, lime and silica fluxes were fed at the cold end. The hot end of the hearth, near the fire, contained a depression or sump called the ‘fuse box’ where the calcine melted and from where it was manually scrapped out into slag pots. The slag was cooled and solidified, and then broken into lump material suitable for blast furnace feed. A typical roaster of the size indicated would process 5 t/d of lead feed using three men per shift and consuming 3 t/d of coal.

At the Pontgibaud smelter in France, a reverberatory hearth was used for batch calcining, followed by elevation in furnace temperature to cause surface melting and sintering of the calcine into an agglomerated mass, rather than complete melting of the charge. This was withdrawn from the furnace, cooled and broken into lump for blast furnace feed.

In order to improve the intensity and efficiency of the roasting process, the Huntington-Heberlein process was introduced in the 1890s. Partly roasted material from the hearth roaster was moistened and placed in a pot fitted with a lower grate (or converter), with a layer of hot material on the grate, and was subjected to an air blast. The construction of the roasting pot or converter is shown in Figure 2.5.

These converters could be considered the forerunners of today’s updraft sintering machines. However, they had serious deficiencies: they were batch operations, they generated much fume, and the work of manually handling and breaking the sintered material from the pots was laborious and unhealthy.

This approach significantly improved the productivity of both the roasting operation and the blast furnace, and lead smelters in the early 1900s had large numbers of converters producing blast furnace feed.

Air Blast

Trunnions

Grate Hood

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Alternative processes at the time were the Bradford-Carmichael process, in which lead ore was mixed with dehydrated gypsum (plaster) as a binder, was formed into lumps and then processed in the converter, the Savelsberg Process, which fired ore and limestone over a fuel bed in a converter to produce a sintered material as blast furnace feed.

A circular rotating furnace using a downward blast through a bed of ore and limestone, covered with a surface layer of wood chips as a starting fuel, was developed by the Cerro de Pasco Corporation in Peru to provide a suitable sintered blast furnace feed.

The major advance to overcome the disadvantages of the pot roasting methods came with Dwight-Lloyd sintering machine. Originally this consisted of a series of boxes with an open grate base, running on rails over a suction box. The boxes or ‘pallets’ were firstly pushed through a small reverberatory furnace to ignite the top surface of the charge in each pallet box, and then continued over the suction box until combustion was completed. The pallets were inverted to empty the contents and were returned to the beginning of the process. The principle was extended to the development of the standard downdraft sinter machine around 1910 and later to the updraft machine in 1955. Details of current sintering processes are covered in Chapter 4.

BLAST FURNACE PRODUCTS

Early blast furnace operations processing lead ores or concentrates from hand-sorting or gravity separation methods had to contend with much higher levels of impurity metals than later operations processing flotation concentrates. In particular, blast furnace feed contained high levels of sulfur, iron, arsenic, copper and zinc. As well as bullion and slag, the blast furnace produced significant quantities of matte and speiss, and the presence of zinc created significant problems with furnace accretions and high slag viscosities.

As well as containing iron at around 40 to 45 per cent, matte contained about 12 per cent lead, most of the copper from blast furnace feed, about half the zinc and a substantial proportion of the silver. Matte was initially roasted in open heaps or stalls to remove sulfur and was then recycled to the blast furnace, but later reverberatory roasting furnaces were used. A shaft kiln was used at the Harz smelter in Germany.

The recycle of roasted matte to the blast furnace resulted in the progressive enrichment of the copper content of matte, until it reached a grade where it could be processed to blister copper in a converter. Table 2.1 shows the composition of matte produced after five recycling stages from a smelting operation around the 1890s (Hoffman, 1899).

Zinc proved to be a particularly troublesome problem and many techniques were developed to remove zinc from blast furnace feed. Mineral separation techniques using gravity had limited effectiveness for some ores, and in particular cases it was necessary to leach zinc from roasted ores

CHAPTER 2 – Historical Background

Matte production cycle 1 2 3 4 5

Lead content (%) 13.5 8.3 10.0 9.0 9.0

Iron content (%) 48 44 31 20 12

Copper content (%) 5.7 12.8 27.8 42.9 50.9

Sulfur content (%) 25 20 21 18 21

TABLE2.1

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either using water after low temperature sulfation roasting, using dilute sulfuric acid to extract zinc sulfate, or water and SO₂to remove zinc as a sulfite. The development of practical froth flotation from around 1913 substantially improved the separation of zinc and lead sulfides and presented cleaner lead concentrates to the lead smelters. This also increased the fineness of concentrates and necessitated the introduction of sintering methods for blast furnace feed preparation.

Speiss was the other significant blast furnace product from earlier smelting operations. It contained significant amounts of entrained particulate lead as well as silver and gold. Speiss was often roasted in heaps or a calcining furnace and recycled to the blast furnace. At the Trail smelter in Canada it was treated in a bottom blown converter with the addition of molten lead. The lead captured most of the silver and gold and no doubt significant amounts of arsenic were volatilised into the gas stream.

LEAD REFINING

Simple cooling of the lead bullion from the smelting furnace initially allowed the separation of a black dross containing most of the dissolved zinc, iron, tin and oxygen in hot furnace bullion. This dross could be skimmed off and worked up if the tin content was high enough. With further cooling, much of the copper and sulfur content came out of solution forming copper-rich crusts. The process was termed ‘copper drossing’.

In early lead-refining practices, further purification firstly involved the oxidation of bullion in shallow open pans. The dross formed contained antimony and arsenic and was skimmed off until the lead was ‘softened’. Oxidation softening in a reverberatory furnace was practised in the mid 1800s and was developed into a continuous operation in the early 1900s. The alternative Harris Process, for removal of arsenic, antimony and tin by the addition of caustic soda and sodium nitrate, was introduced in 1920.

Separation of low levels of silver was not practised until the development of the Pattinson Process in 1829 as detailed in the next section. The Parkes Process for silver and gold removal by zinc addition was introduced in 1872.

Remaining copper and other impurities were originally removed by the addition of zinc, and the removal of zinc was by chlorine to form a zinc chloride dross or by drossing with caustic soda. Vacuum dezincing of lead was developed as a practical technique in 1946.

‘Fine’ copper removal by sulfur drossing was developed in 1923.

Bismuth became a significant issue as the uses of lead became more demanding and was the prime reason for development of the Betts Electrolytic refining process in 1902. The alternative Kroll-Betterton process involving the use of calcium and magnesium metal additions was introduced in 1936.

Details of thermal refining practices are covered in Chapter 12 and electrolytic refining in Chapter 13.

SILVER RECOVERY

From early times silver was an important source of wealth, but particularly so during the Middle Ages in Europe. Many early lead smelting operations were for the prime purpose of recovering silver, and lead could be regarded as a collector for silver and as a by-product. Extensive mining of silver with co-product copper and lead occurred throughout central Europe, notably in Austria, Saxony and the Harz district of northern Germany. Most silver ores are sulfides and contain argentite or silver glance (Ag2S), although there are also many complex mixed sulfides with antimony, arsenic, copper and