2.3 Conventional methods for extraction of zinc from EAFD
2.3.2 Pyrometallurgical methods
2.3.2.1 High temperature processes
Carbo-thermic reduction
Carbothermic reaction is identified as usage of carbon as reducing agent, usually for
metal oxides. These chemical reactions are usually conducted at several hundreds of degree
Celsius. Such processes are applied for production of the elemental forms of many elements
Carbo-thermal reactions produce carbon monoxide and sometimes carbon dioxide.
EAF dust can be treated in Walez Kiln which is a pyrometallurgical process
characterized by the volatilization of non-ferrous metals like zinc, lead and cadmium out of an
oxidized solid mixture by means of reduction by coal in a rotary kiln without generating a
liquid slag this process is considered the most frequently used technology of recovery the zinc
oxide which is presented in EAFD (morcali et al.,2012). In Waelz process, the hot air enters
the kiln; the solid charge is first dried, and then heated up until the reaction starts. The
maximum temperature of the solid reaches about 1200 °C(morcali et al.,2012). The possible
reactions during the reduction process are mainly as the following:
C(s)+ CO2(g) CO(g) Equation 3
ZnO(s)+ CO(g) )+ Zn(g) CO2(g) Equation 4
26
PbO(s)+ CO(g) Pb(l)+ CO2(g) Equation 6
Marcali (morcali et al.,2012) reported that zinc removal in walez kiln process
increases with the increase in time and temperature. The activation energy was found 261.99
kJ/mol for zinc recovery in the pellet form (morcali et al.,2012). Carbothermic reduction is
affected by several factors such as:
1. The flow rate of the inert gas; such as Nitrogen is inversely proportion with
reaction rate when the flow rate of inert gas increase the more (CO(g), CO2(g)) are
carried off. Thus it decrease the CO (g) and CO2 (g) concentration in solid matrixes,
consequently the reaction will decelerate.
2. The Grain size; as the grain size of both Carbon and zinc oxide was small the
reaction was fast.
3. Effect of molar ratio of C/ZnO; high metal content of Carbon produce more
CO andCO2 gases consequently more zinc oxide is converted producing more zinc
metal.
4. Temperature of reaction; as the reaction temperature increased the zinc oxide
will increase.
Industrial Scale processes for EAFD treatment.
Many processes are used worldwide to manage some techniques for metals recovery
from EAFD including pyrometallurgical processes and hydrometallurgical processes. There
are many methods for industrially treating EAFD from carbon steel scrap melting which are
27 Table 9: Commercial processes for EAFD treatment.(morcali et al.,2012)
PROCESS TYPE ZN BEARING
PRODUCT
FE-BEARING
PRODUCT
OTHER
PRODUCT
Waelz kiln (stage 1) pyro ZnO Fe°/FexOy -
Waelz kiln (stage 2) Pyro ZnO - PbCl2/CdCl2
Rotary Heart Pyro ZnO Fe°/FexOy -
Shaft Furnace Pyro ZnO Fe°/FexOy -
Plasma Furnace Pyro ZnO Fe°/FexOy -
Electro Thermal Pyro Zn° Slag/residue -
Ezinex (with electrolysis) hydro Zn° Slag/residue -
Leaching NH4Cl Hydro ZnO Slag/residue -
Leaching in H2SO4 (with
electrolysis)
Hydro Zn° Slag/residue -
Leaching in NaOH (with
electrolysis)
Hydro Zn° Slag/residue -
Leaching in (NH4)2CO3 (with
Calcination)
hydro ZnO Slag/residue -
Nyirenda (Nyirenda,1990) reported the most common industrial high temperature
technologies as shown in Table 10 on his review paper and discussed its economics,
28 Table 10: Most common industrial high temperature technologies. (Nyirenda,1990)
PROCESS AND DESCRIPTION
STATE OF DEVELOPMENT ECONOMICS ADVANTAGES AND
DISADVANTAGES 1. Waelz Kiln
Process:
Feed (pelletized): Fine flue-dust + coke/coal powder + silica/limestone flux. Heat generation: carbon in feed + auxiliary gas/oil burner at discharge end, Max. Charge T, 1100-1250 ºC. Products: Mixed oxide and dry slag.
Most widely used process. Adapted for this purpose since mid '70s. Used in Eurnpe/America/Japan in at least 13 plants. Plant capacities are of the order of 50,000 tpy* and higher.
1987, capital cost for a 1987, capital cost for a 60,000 tpy plant about $20m. Operating costs estimated as $85/t (1988) for a 50,000 tpy plant. Feed preparation relatively Feed preparation relatively simple. Problem of accretion formation/refractory maintenance. Excess usage of carbon, through part is recycled. Favoured by cheap coal. Mixed oxide needs further processing.
2. Tetronics Plasma Process:
Feed (blend of fine dry materials): Dust + fluxes + coal/coke powder.
Heat generation: Electrical energy.
DC argon stabilized arc between cathode above charge and
Two commercial plants inUSA of capacities 7,200, and 11,000 tpy. Sited in Jackson and Blytheville,
respectively. Both commissioned 1989. Same
furnace type used for stainless steel dusts, Sheffield, UK.
Cost data for CS dust process unavailable. Total investment for 8,000 tpy SS dust process estimated as £2.2m in 1986. (SS dust process has no condenser). In 1977, similar concept had estimated operating costs of $88/t for a 30,000 tpy plant.
Direct recovery of metals.
Selective reduction mode, forms corrosive FeO rich slags; higher refractory costs, also CO2 higher in off- gas, lower Zn recovery. Operates between total and
29 anode in furnace hearth. Furnace T about 1500 ºC. Products: PW zinc; Pb with 2% Zn; molten slag; molten Fe. If selective reduction used, no Fe forms.
selective reduction. Zn recovery (70%) presently, lower than for other processes. Favoured by hydro- electric power (HEP) use.
3. Flash Furnace Process (flame reactor process)
Feed: Fine dry powder pneumatically
injected. For most dusts fluxing is unnecessary.
Heat generation: Pulverized coal or gas fired, with O2
enrichment.
Furnace T above 1600 ºC.
Products: Mixed oxide & liquid slag.
Prototype unit has been tested (Monaca, USA). 50,000 tpy gas fired plant being engineered (Bartlesville, USA).
1989 estimates, $5.3m. for a 40,000 tpy plant; $1.7m for a 5,000 tpy unit.
Operating cost, $109/t.
Stated as idea for
small scale processing. Low cost carbon or gas fuel. Mixed oxide needs further processing. Requires O2, but cost estimates include this. 4. Electric Furnace Process (Elkem Multi- purpose Furnace):
Feed (briquetted with
Furnace in commercial operation for rock wool production (Iceland).
Demonstration plant has been operated for dusts (Norway).
Cost data not available. Direct metal recovery. Fe metal formation, energy & reductant waste. Elaborate feed
30
H2O-free binder):
Dust + fine coke + silica flux. (All needed dry).
Heat generation: slag resistance to electrical flow. Slag at 1500 ºC. Products: Zinc metal (PW grade); Pb bullion (with 2% Zn); molten Fe (≤ 20% of Fe in feed). Liquid slag.
Uses imperical smelting process Zn condenser.
preparation.
Favoured by HEP availability. Furnace said to need lower refractory
maintenance, frozen slag part of lining.
5. Electrictrothermic Process:
Feed: Sintered dust + pea coke. Before sintering, dust de- leaded by roasting (1300 ºC) in presence of CaCl2.
Heat generation in furnace: Electrical energy mainly. Coke, for reduction and current flow. Reaction zone, 1350 ºC. Production: Pb-rich dust, from roasting &
Plants initially built primary Zn industry. Japan (Ryoho) plant, 45,000 tpy (35,000 tpy dust used since 1974. USA plant (Monaca) 100,000 tpy total (since 1980 an unspecified % of this has been flue-dust).
Economic data not available. Old plants whose feed has now changed to include steelmaking flue-dust.
High purity ZnO product (98%). High energy requirements for roasting, sintering, reduction furnace. Expensive quality coke, 42% sinter weight used. Some unused coke & unsmelted sinter recycled – obtained
as magnetic separation tailings.
31
sintering. ZnO, by oxidation of Zn(g)
from furnace. Dry slag.
6. Half-Shaft Furnace process
Feed (Briquetis): Dust + Zn/Pb residues + fine coal + binder. Heat generation: Coal combustion with pre- heated air.
Products: Liquid slag. Mixed oxide.
Part of old Zn processing plants where furnaces were originally constructed for Retort process residues.
Cost data unavailable.
Plants are part of old primary zinc production works whose feed has now changed to include steelmaking flue-dust.
Mixed oxide produced. Lower Zn recoveries.
Productivity low for single unit. Matte enables Cu and Ag recovery, though matte and slag not easily separated.
7. Sirosmelt Process:
Feed: Slag (or other feed material) + coal. (Optional addition of Pb to collect Cu, Ag, Au into bullion). Heat generation: Fuel oil/coal combustion. Submerged injection, during reduction/fuming stage. Bath temperature, 1350 ºC. Products: Mixed
Development and marketing since 1980.
Initially for treatment of old process slags has been extended to non-ferrous smelting flue-dust. Said to be suitable also for processing neutral leach residues of zinc plants.
Cost data unavailable. However, capital and operating costs are said to be low.
Transportable unit. Ideal for small scale processing.
Relatively simple process. Submerged injection, high smelting & reduction rates but at
cost lance wear. Mixed oxide requires further processing.
32