7 Agitated Autoclaves

Agitated Autoclaves

When temperatures in excess of 100°C are required to obtain suitable reaction rates, pressurized autoclaves are needed. Temperatures of up to 300°C are required in some instances. Steam pressure at 300°C is 85 atm. Autoclaves are suitable for concentrated feeds. They are expensive and costly to run, so the metal values in the feed must be high to justify their use. Leaching times are typically short (10 min – 2 h).

Autoclaves are long and tube-shaped (like submarines). They may be horizontally or vertically oriented. Vertical orientation allows for more autoclaves in a given area. Often they are divided into a number of compartments. In a horizontal autoclave (Figure 17) slurry flows in from one end and over weirs from one compartment to the next. Each has its own stirrer. Good agitation is essential

Figure 17. Schematic illustration of a horizontal, brick-lined autoclave vessel for leaching of a metal sulfide concentrate. Source: R. Berezowsky et al, Journal of Metals, 1991, pp. 9-15.

to suspend the solids, which promotes good leaching kinetics. (Minimizing residence time in these expensive vessels is imperative.) Leached slurry is discharged from the end of a horizontal autoclave via a pipe dipping below the surface. Modern autoclaves are operated by continuous charging and discharging, and not in batch mode. Vertical autoclaves are commonly used in the alumina industry (Bayer process) and sometimes in processing of nickel laterite ores.

Horizontal autoclaves are filled to 65-70% of their volume. Reagents may be injected into each compartment. Oxygen gas is injected for oxidation of sulfides. The extent of oxidation of the sulfide depends on the relative amounts of O₂ and mineral sulfide. Due to reagent consumption costs, it is preferable to leach sulfides to elemental sulfur (1/2 mole O₂ per mole of S^2-) rather than sulfate (2 mole O₂ per mole S^2-). Oxygen from air is readily concentrated, e.g. by a cryogenic process to 99%. A horizontal autoclave is needed if oxygen is to be used. Inert gas impurities (like N₂ in the O₂ or CO₂ generated from reaction of acid with carbonate minerals) is continually vented. This maintains a high partial pressure of O2. Where sulfide minerals are involved, oxidation by O₂ liberates heat. Some of this heat may be recovered to preheat the incoming feed slurry. If the sulfide content of the solids is high enough, the process will be energetically self-sustaining (autogenous). This is certainly desirable from an economic standpoint.

Autoclaves must be designed to withstand high pressures and temperatures. They also contain harsh chemical environments, which promote corrosion of steel. In caustic systems (Bayer process) mild steel may be used, largely because of a protective scale that forms. Some Ni/Co sulfide concentrates are leached using oxygen in an ammoniacal medium, i.e. NH₃/(NH₄)₂SO₄. In this case the autoclave may be made of stainless steel, or have stainless steel cladding. For acid systems (H₂SO₄ medium in particular) the autoclave will can be carbon steel, but lined with brick and lead. Titanium has been used because of its high strength and good corrosion resistance in acid solutions, so long as an oxidizing condition is maintained. Titanium is also more resistant to corrosion by chlorides than is steel. The TiO₂ scale that forms on titanium is very strong and adherent, protecting the underlying metal from corrosion. However, titanium itself is strongly reducing and fresh titanium metal surface exposed to high pressure oxygen will combust, causing an extremely hot fire. Erosion is another possible means by which fresh titanium metal may be exposed. The use of titanium autoclaves requires great care.

Some important examples of autoclave leaching are given below:

• The Bayer process mentioned previously uses autoclaves for leaching.
• Nickel laterite ore leaching, particularly in Cuba. The ore is comprised of oxides and hydroxides, especially of iron, as well as aluminosilicates. Strong sulfuric acid at high pressures and temperatures is used to leach nickel and cobalt into solution. Oxygen is not needed for these reactions. The nickel and cobalt are then precipitated with H₂S to form a synthetic concentrate of NiS and CoS.
• The NiS/CoS concentrate from Cuba mentioned above is shipped to Fort Saskatchewan, Alberta to be leached and then converted into pure nickel and cobalt. The leaching process uses ammonia/ammonium sulfate and oxygen gas as oxidant. After purification [Ni(NH₃)₆]⁺² and then [Co(NH₃)₆]⁺³ in solution are reduced to the metals using hydrogen gas, again in autoclaves.
• Sphalerite concentrate (ZnS) is leached using autoclaves at about 150°C and oxygen gas. The mineral is oxidized in sulfuric acid to form aqueous ZnSO₄ and elemental sulfur. Oxygen is the oxidant.
• Gold is sometimes very finely disseminated in pyrite, i.e. FeS₂. Grinding does not liberate the gold particles and thus leaching of ground pyrite concentrate with cyanide directly may be unsuccessful. In such cases the pyrite may be first oxidatively destroyed in an autoclave using acid and oxygen. The residue can then be leached with cyanide to extract the gold. Since a concentrate of pyrite is made, the gold value in the pyrite may be quite a bit higher than in the ore as a whole. This may justify the added cost of autoclave processing.

Co-Current and Counter-Current Leaching Methods

In continuous leaching there are two basic configurations: co-current leaching and counter-current leaching. Counter-current leaching was already mentioned briefly in the context of vat or percolation leaching. The important thing to bear in mind is that we have two streams – a solid stream of material being leached, and a solution stream doing the leaching. These streams may flow in the same direction, or in opposite directions. When the solids and solution flow in the same direction it is co-current leaching. When they flow in opposite directions it is counter-current leaching. A schematic illustration of the two types is shown in Figure 18. Making the solids and solution flow in the same direction is easy enough. Usually there is a solid-liquid separation after the final leach step. This is indicated in part (a) of Figure 18. Moving the solids and solution in opposite directions is a bit more challenging. The way that this is accomplished is by having a solid-liquid

Figure 18. Schematic illustrations of (a) co-current, and (b) counter-current leaching. Note the net flow of solution from right to left and flow of solids from left to right in part (b).

separation after each leaching stage. Fresh leach solution enters from one end and PLS leaves at the other (right to left in the diagram (b) above); solids enter from the other end and flow against the direction of the solutions (left to right in diagram (b) above). The basic differences noted here are sufficient to distinguish co-current and counter-current leaching processes.

The advantages of co-current leaching are simplicity and lower capital and operating costs. The disadvantages may sometimes be either loss of valuable minerals, or excessive use of reagents and associated costs of coping with high left over reagent concentrations. Note that countercurrent processing need not always present this trade-off, but sometimes it does. The advantages of counter-current leaching may be both higher extents of leaching of valuable and more efficient use of reagents. The disadvantages of counter-current leaching are a more complex and expensive leaching process (both capital and operating costs). And, finally, the minerals being leached or the nature of the leaching process may not always allow for a counter-current option, e.g. high-pressure auotclaves.

Obviously the counter-current system is more complex and has more equipment; thus it is more expensive. There has to be a good reason to go to employ this extra complexity and cost. If recovery of valuable metal can be substantially enhanced and reagent costs substantially lowered, compared with co-current leaching, then counter-current leaching may be worthwhile.

One instance where counter-current leaching may be pursued is when metal values are contained in more than one mineral, at least one of which requires aggressive conditions to leach. This is best illustrated with an example. The main zinc mineral in nature is sphalerite, ZnS. This is separated from ores as a concentrate. However, concentrates are never pure and inevitably some pyrite and other minerals report to the concentrate. Sphalerite concentrates are commonly roasted in air. The product is called zinc calcine. This forms ZnO (the main constituent of roasted sphalerite concentrate). However, in the process ZnS and FeS2 also form zinc ferrite, ZnFe₂O₄. Zinc oxide is readily leached in dilute sulfuric acid:

[latex]ZnO_{s}~+~H_{2}SO_{4~aq}~=~ZnSO_{4~aq}~ +~H_{2}O_{l}[/latex]

On the other hand, ZnFe₂O₄ requires stronger acid and higher temperatures in order to leach. In the co-current leach system it would be necessary to have high concentrations of acid in the latter stages of leaching in order to leach ZnFe₂O₄, after the ZnO had been leached. (The ZnO-ZnSO₄ mixture buffers the pH so that the low pH needed to leach ZnFe₂O₄ cannot be attained until the ZnO is consumed.) Thus the solution exiting leaching would have a high H₂SO₄ concentration. This would have to be neutralized prior to solution purification and metal recovery, which would cost too much. Hence in the earlier versions of the roast-leach-electrowin (RLE) process zinc in ZnFe₂O₄ was not recovered.

Counter-current leaching offers a way to recover this zinc and avoid excessively high acid concentrations in the final leach solution. This is illustrated with the flowsheet in Figure 19, which shows how the leach process fits into the overall process. As the calcine passes through leaching, the more easily leached minerals (ZnO, in particular) dissolve first. The most resistant (or refractory) minerals persist until the final stage of leaching. Further, the strongest acid and highest temperature (the most aggressive conditions) are employed in this stage in order to leach ZnFe₂O₄. This is often called the hot acid leach (or just the acid leach). After solid-liquid separation the solids exit as tailings and the solution is directed back to the preceding stage, still high in acid and somewhat enriched in ZnSO₄. In the first stage the fresh calcine enters the leach circuit. This readily dissolves even in quite dilute acid (typically pH ~4). This is called the neutral leach due to the weakly acidic pH. The excess of strong acid concentration is thus substantially used up and much of the ZnO dissolves. The highly enriched leach solution exits the each circuit after solid-liquid separation to go on to purification and metal recovery (by electrowinning for zinc), while the solids proceed on to further leaching.

This has two beneficial effects that work together. One is that zinc values locked up in ZnFe₂O₄ are leached, and the main reagent (H₂SO₄) is efficiently used. The former generate revenue; the second saves money. In RLE processes for zinc, there are numerous variations on the counter-current leaching theme, and many are economically viable.

Figure 19. A counter-current leaching flowsheet for zinc production by the RLE process. Acid is generated in electrowinning and thus spent electrolyte is recycled back to leaching. Some additional acid may be needed in leaching, as indicated by the dashed line. Modified from source unknown.

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Note: Zinc processing by the RLE process has the further complication that iron must be removed from the leach solution, and this is typically done within the leaching process (not shown in Figure 19). There are three basic ways to do this, depending on the form of ferric ion to be precipitated. These are formation of: Fe(O)OH (goethite), Fe₂O₃ (hematite) and M^IFe₃(SO₄)₂(OH)₆ (jarosites; M^I = M⁺¹; usually NH₄⁺ or Na⁺). Jarosite precipitation has been traditionally a common method for iron removal. The leach stages and jarosite precipitation may be integrated in numerous possible ways, and it is not uncommon to perform jarosite precipitation on the solution after hot acid leaching and before directing it back into neutral leaching. In all this additional complexity, one can often discern an underlying counter-current leaching process, despite intermediate processing of the leach solution between the hot acid leach and the neutral leach.

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An example of co-current leaching is next. Figure 20 is a flowsheet for a bacterial leach process for pyrite (FeS₂). The pyrite contains finely disseminated gold (too fine to expose even by fine grinding). The pyrite from the ore is concentrated, and thus removes most of the gold from the ore. The pyrite must then be destroyed, which is accomplished by using thiobacilli bacteria to oxidize pyrite to ferric ion and sulfate. This then leaves a solid residue that contains unleached gangue and the gold. This is an instance where it is the solid residue that is desired rather than the solution; the solution is simply neutralized as cheaply as possible (CaCO₃) and disposed of. The solid residue is directed to a cyanidation plant for gold recovery. Note the presence of the cooling system. Oxidation of pyrite concentrate generates a lot of heat. If excess heat is not removed the temperature in the leach tanks will rise to >40°C and kill the bacteria. Note also that air is introduced into each tank to facilitate oxidation of pyrite. Sulfate medium is ideal for this process since the bacteria are oxidizing sulfur (formally S₂^2- in FeS₂) to sulfate and dilute sulfuric acid solutions are not too severely corrosive.

Leaching occurs in a series of tanks and the solids and solution both flow in the same direction. There is a single solid-liquid separation process at the end of leaching. These two features are diagnostic of co-current leaching. In this case there is no need for the more involved counter-current mode. There is one main mineral of interest (pyrite). Gold is not locked up in other more resistant minerals, hence no hard-to-leach minerals that might otherwise warrant counter-current operation. In addition, the bacteria required a fairly narrow range of conditions. Hence increasingly extreme conditions would be detrimental. Finally, the only real reagent used in the leaching process is oxygen from air, which comes at a uniform concentration of about 21% O₂. (Sulfuric acid is a product of the reaction.) Overall, then, counter-current leaching is not really even feasible in this case.