Raw Material Preparation and Pellet Production

4 . 3 . 1 R a w M a t e r i a l P r e p a r a t i o n

Analogously to the c o m m e n t s given u n d e r item 3.2, an a d e q u a t e l y e q u i p p e d pelletizing laboratory should also include the most i m p o r t a n t facilities and e q u i p m e n t for the beneficiation of ores.

(a) Washing of Ores. Finely disseminated ores should be g r o u n d to m i n u s 0.5 m m . In a p r e l i m i n a r y test, a small quantity of ore is slurried in an agitating tank and its suspension b e h a v i o u r is examined. A f t e r a certain settling time, the suspension is separated f r o m the sedimented solids. A f t e r weighing and chemical analysis of the separated solids portion, a metal balance is to b e established. In semi-industrial tests for the p r o d u c t i o n of m a j o r concentrate a m o u n t s washing a p p a r a t u s of types which are gen-erally used in industrial plants, such as washing d r u m s , spiral classifiers, wet screens a n d hydrocyclones are utilised. A flowsheet based on a test with an earthy, limonite-containing h e m a t i t e f r o m G o a is described in Fig. 33. In this case the iron concentration effect was of secondary interest while the decrease of the a l u m i n a content was m o r e i m p o r t a n t .

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71 3 Raw Materials and Their Preparation for Pellet Production

Fig. 33. Flow sheet and material balance in a pilot-scale washing plant for earthy hematite

(b) Gravity Methods. T h e characteristics of the ores are firstly ascertained with only m i n o r quantities. By h e a v y - m e d i a s e p a r a t i o n in organic liquids of a higher density t h a n water, the g a n g u e is separated f r o m the iron-bearing material. T h e density of the heavy m e d i a — generally a r o u n d 2.8 g/cm3 for iron ores - is a d j u s t e d to o p t i m u m selectivity. T h e s e laboratory tests are to be confirmed by experiments on a semi-industrial scale. Suitable e q u i p m e n t for such tests is available on the m a r k e t .

(c) Flotation. D u r i n g flotation the ore surface is, in d i f f e r e n t ways, altered by using specific chemicals. T h e effect of individual chemicals is studied.

Since, in contrast to m a n y physical beneficiation processes, the time factor plays a certain role d u r i n g flotation, the next testing p h a s e is carried o u t in a continuously operating pilot plant. O n this occasion, not only the t i m e factor but also t h e influence of middlings in internal circuits on the process are investigated. Such tests on a larger scale are the prerequisites to the eleboration of a suitable flowsheet. W i t h a view to a good filtrability of the concentrates a n d , if necessary, an efficient wettability of the grain surfaces during pellet p r o d u c t i o n , the choice of reagents is of i m p o r t a n c e .

(d) Magnetic Separation. According to their b e h a v i o u r in a magnetic field, the minerals can be divided into three classes, n a m e l y highly-, weakly-, and n o n - m a g n e t i c minerals. S o m e minerals i m p o r t a n t for pelletizing are

specified b e l o w3 8) , with the b e h a v i o u r of iron in a m a g n e t i c field serving

In conformity with these properties, low-intensity m a g n e t i c separators are used for highly m a g n e t i c ores a n d high-intensity m a g n e t i c separators for weakly m a g n e t i c ores. F o r the d e t e r m i n a t i o n of the liberation size, the ores are g r o u n d to different fineness.

T h e Davis tube, which already with a few grams of material gives representative results a n d guide values for f u r t h e r tests o n a larger scale, is employed for wet separation. Both types can b e arranged in a mass flow circuit so that the data thus obtained is a l r e a d y valid for the design of in-dustrial plants. Subsequently, representative pelletizing tests are run on the m a j o r concentrate quantities thus p r o d u c e d .

Fig. 34 shows, for example, the beneficiation flowsheet f o r a magnetite ore.

However, if weakly m a g n e t i c ores (hematites, limonites) are to be upgraded, substantially stronger m a g n e t i c fields of u p to 2.5 Tesla are necessary. T h e s e separators are, therefore, called high-intensity magnetic separators. In this case, a test with small a m o u n t s of material can yield first data on the field intensity required. F o r the p r e p a r a t i o n of flowsheets, a p p a r a t u s o n a semi-industrial scale with a t h r o u g h p u t of several h u n d r e d kilograms per h o u r are used. In the event of disseminated ores with an a d e q u a t e g r i n d i n g degree, wet separators are m o s t suitable. D r y separa-tors are e m p l o y e d for m o r e coarsely intergrown ores.

(c) Magnetizing Roasting. L e a n hematite bearing ores are r e d u c e d to magnetite u n d e r a weakly reducing a t m o s p h e r e . This process calls for

Fig. 34. Alternative routes for wet magnetic separation of magnetite ore in a pilot plant

relatively h i g h processing costs so that it is only used rarely on an in-dustrial scale. Intermittently operating small rotary kilns or f l u i d - b e d reactors are e m p l o y e d for the p e r f o r m a n c e of orienting tests in which the principal characteristics are to be d e t e r m i n e d . Before the construction of an industrial plant, pilot plants are generally erected and o p e r a t e d over a period of at least several months.

( f ) Electrostatics. If this process is t a k e n into consideration, the b e h a v i o u r of the minerals in an electric field is first tested with small quantities.

With this process, the corresponding p a r a m e t e r s m u s t b e closely observed.

The operating conditions m u s t only vary w i t h i n very n a r r o w limits, they have to be precisely ascertained in previous tests. It is also possible to condition the ore grain surface in order to increase the differences in the conductivity of t h e various minerals.

4 . 3 . 2 G r i n d i n g

The most i m p o r t a n t prerequisite to green ball f o r m a t i o n is a sufficient fineness a n d a suitable size distribution of the raw materials. W i t h the exception of concentrates, which o f t e n are already o b t a i n e d in a sufficient fineness f r o m the beneficiation plant, natural ores, coarser concentrates

and additives have to be ground or re-ground. T h e grinding tests carried out in the laboratory h a v e virtually two purposes:

(a) P r e p a r a t i o n of grinding samples with a representative size range for the p e r f o r m a n c e of pelletizing tests.

(b) D e t e r m i n a t i o n of the m o s t e c o n o m i c m e t h o d taking the capital in-vestment, energy c o n s u m p t i o n and w e a r into consideration with the d e m a n d for high-grade pellets being of greatest importance.

There are f o u r variants for the p e r f o r m a n c e of fine grinding:

— Open-circuit d r y grinding in ball mills, - Closed-circuit dry grinding, grinding test facilities f o r this purpose. T h i s particularly applies to pelletizing p l a n t constructors.

In order to limit the scope of test work accordingly it is advisable to run pellet firing tests in parallel to the grinding tests since o f t e n after a few tests p r e l i m i n a r y decisions can already be taken. Moreover, the fineness this regard, the use of air-blown screens has proved to be suitable. A f t e r dry grinding, s o m e ores tend to f o r m loose agglomerates. In such cases, the wet screening m e t h o d is preferable. A screen analysis is reliable for the particle size range below 0.02 m m . F o r the precise d e t e r m i n a t i o n of these very fine fractions in connection with the grinding degree, it is additionally necessary to ascertain the specific surface area (see item 3.2.2.1).

(b) Surface Determination. F o r this purpose, the m e t h o d according to Blaine, k n o w n f r o m the cement industry, was first taken over and is nowadays a d o p t e d for pelletizing also. In s o m e laboratories, the Svenson m e t h o d is preferred.

N e i t h e r m e t h o d is q u i t e i n d e p e n d e n t of h u m a n errors, i. e. of the p e r s o n previous r e p u l p i n g in alcohol for dissolving the agglomerates has also

proved to b e successful3 9). closed-circuit grinding, care should b e taken t h a t the classifiers b e sufficiently d i m e n s i o n e d since o f t e n they only ensure a representative classification f r o m a certain capacity o n w a r d which m a y involve the use of correspondingly greater mills. This detail should b e particularly considered for wet g r i n d i n g with hydrocyclones as classifiers.

The p e r f o r m a n c e of large-scale tests in mills with e.g. 1.0 m diameter at a throughput of 1—3 t/h is r e c o m m e n d e d for ascertaining the exact energy con-sumption. T h e throughput is dependent on the grindability of the ore and of the required fineness. Consequently a larger ore quantity of u p to 5 0 - 1 0 0 t obtained d u r i n g grinding tests in large-scale pilot mills.

In these tests, the ore was g r o u n d in various g r i n d i n g systems to the same fineness. Consequently, the quality of green a n d i n d u r a t e d pellets is roughly identical within t h e scope of test accuracy.

Wet grinding Dry grinding

As regards t h e energy consumption, closed-circuit wet grinding yields the lowest values. In the case of closed-circuit dry grinding, the energy c o n s u m p t i o n is almost the same 40).

In some cases, the specific p o w e r c o n s u m p t i o n for grinding m a y be determined with a certain precision by a d o p t i n g the Bond m e t h o d4 1) . But often, in the case of surface grinding, this m e t h o d is not precise enough.

4 . 3 . 4 F i l t r a t i o n

Wet grinding is followed by filtration in all cases in o r d e r to achieve a moisture content of the filter cake which is at or better below the o p t i m u m green pellet moisture content. As concerns t h e filter cake moisture content to be attained, small-scale tests with filter plates give indications. H o w e v e r , continuous filtration test's with the corresponding filter type, such as disc or d r u m - t y p e filters are r e c o m m e n d e d for d i m e n s i o n i n g the filter plant.

Table 11. Influence of various grinding methods on grinding energy and properties of hematite pellets

In the event that the filter cake is difficult to dewater, hot steam filtration may be advantageous. A t h e r m a l secondary drying is not only expensive but also often leads, primarily w h e n d r u m - t y p e dryers are used, to un-desirable agglomerates w h i c h m a y bring a b o u t great difficulties d u r i n g green ball f o r m a t i o n . T h e s e agglomerates hinder:

(a) the desired close size range of green pellets f r o m being achieved;

(b) a h o m o g e n e o u s mixture, particularly with additives a f t e r drying.

A further possibility for the s u b s e q u e n t regulation of the filter c a k e moisture content is the use of drying additives. In this connection, the highly w a t e r - b o n d i n g bentonite has, in particular, a considerable influence as has also calcium oxide ( C a O ) w h e n special m e a s u r e s are taken.