Basics of Extractive Metallurgy Processes
Introduction
Extractive metallurgy is typically concerned with the production of pure metals, alloys or compounds from raw materials. Although metal extraction is often regarded as distinct from the chemical industry per se, it is at heart applied chemistry. Many of the same essential principles and operations are employed. On the one hand, mining and metals production account for a fairly small direct percentage of Western countries’ GNP’s (<5% in the U.S.). However, virtually all other sectors of national economies rely heavily on metals.
There are two main branches of extractive metallurgy. The first, historically, is pyrometallurgy. This involves the conversion of a compound of the metal (often an oxide) into elemental metal (or an alloy) by means of an added reagent and high temperatures. Metal sulfides may be converted into oxides by roasting (combustion of the sulfides). Hydrometallurgy is the branch of metal extraction that employs aqueous solution chemistry for metals production. It is a relatively new player (a bit over a century old in its modern manifestations). Some of the major aspects of these two approaches to obtaining metals from the earth are outlined in Table 9.
For pyromet to be economical the reactions involved must be energetically self-sustaining. Typically a metal sulfide is oxidized by oxygen from air. The metal sulfide is the fuel. The reactions are highly exothermic and the liberated heat sustains the reactions. Hence high grade feeds are required. Often these are obtained by concentration of a low grade ore using physical and physical chemistry processes. When a concentrate cannot be produced, hydrometallurgical methods are the only option. However, given the generally lower temperatures involved, hydrometallurgical processes often take longer (10 to 1000 times longer). In pyromet metal-bearing streams are highly concentrated, whereas in hydrometallurgical metal ions are dissolved in solution and may range from ~1 mg/L (e.g. gold leaching) to at most ~3 M (e.g. zinc electrowinning). All the associated water and other species must also be moved, heated and cooled as the process requires. Pure water has a concentration of ~55.5 mol/L (~1000 g/L ¸ 18.015 g/mole), hence it is inevitable that the vast majority of the solution is water. On the other hand, moving solutions and slurries is generally quite straightforward, compared to moving molten metals and slags.
Hydrometallurgy is often touted as being less environmentally damaging. This may or may not be true, depending on many factors. Pyromet was once notorious for large emissions of gaseous oxides and dusts. Sulfur dioxide (implicated in acid rain) was the main culprit, but dusts containing arsenic, selenium and tellurium, for instance, are also emitted. Currently, tightened emission legislation has required the implementation of scrubber systems that greatly reduce SO2 and dust emissions. This technology is expensive. The captured SO2 may be converted to sulfuric acid (H2SO4), but then this must be either utilized or sold. The acid produced is often impure and must be blended with purer product to meet specifications. Disposal is an expensive proposition. Smelters are also increasingly reticent to accept concentrates high in arsenic, which is difficult to capture. Hydrometallurgical processes dealing with arsenic-bearing streams usually dispose of the arsenic as ferric arsenate, FeAsO4, (scorodite) a stable solid.
Hydrometallurgy may sometimes afford greater flexibility in what to do with the sulfide portion of the minerals being processed. Elemental sulfur can be produced in some processes or sulfate. Sulfur can be sold; sulfate must be disposed of as a suitable precipitate. “Metallurgical sulfur” may be contaminated with arsenic, selenium, tellurium and other species, rendering it of little economic value. Increasingly strict limitations on disposal are making it more difficult to get rid of hydrometallurgical wastes. Hydrometallurgy also faces other pollution/disposal problems. Tailings solutions may contain metal ions and process reagents and decomposition products that must be treated before release. Suspended solids in tailings may also contain leachable metal ions.
On balance, hydrometallurgy can afford greater flexibility for treatment of a wider variety of ores. However, optimizing processes is a challenging undertaking. It is also often the case that hydrometallurgy and pyrometallurgy are both employed in specific situations for certain ores. For example zinc is commonly produced by a pyrometallurgical treatment of ZnS to form ZnO. The latter is then hydrometallurgically leached with sulfuric acid and zinc metal is recovered by electrolysis from the solution. And, it is frequently the case that the best way to refine an impure pyrometallurgical metal product is through dissolution into an aqueous solution and electrolysis (electrorefining). It is often the case that hydrometallurgy is employed when pyrometallurgy is impossible or impractical. For instance, if the metal to be recovered is more reactive than the impurities in a pyromet process (e.g. aluminum; formation of the metal from the oxide by common reductants such as coal is not thermodynamically favourable) or if the ore grade is very low and cannot be upgraded by common mineral processing methods. Numerous other factors may also come into play such as environmental legislation and rather complex political considerations.
| Table 9 - Comparison of pyrometallurgy and hydrometallurgy | |
|---|---|
| Pyrometallurgy | Hydrometallurgy |
| Rapid reaction rates (~minutes) | Slower rates (~hours-years) |
| More economical for high grade ores | Suitable for low grade ores |
| Requires high grade feeds (e.g. concentrates), but can handle inhomogeneous feeds | Can utilize low to high grade feeds, but requires more uniform feeds and careful attention to mineralogy |
| Unsuitable for complex ores due to difficulty forming concentrates | Has flexibility to treat complex ores and produce various by-products |
| Unsuitable for processing secondary sources or separating chemically similar metals | Can process secondary sources; it is possible to separate chemically similar metals |
| Strongly exothermic reactions (sulfide minerals act as fuels) | Metals in solution may be very dilute (as low as 1 mg/L. Some leaching reactions may be energetically self-sustaining; many are not. Water and other species must also then be heated. |
| Convenient separation of metals from by-products, e.g. Ag, Au, Se, Te, PGM’s | Separation of by-products may be complicated. |
| High concentration of metal in process stream (molten metal or matte) | Metals concentration at most ~3 M, usually much less |
| High temperatures involved: 300-2000°C | Ambient to moderate temperatures (5-270°C). (Water at 1 atm boils at 100°C.) |
| Sulfide converts to SO2 gas. Must be scrubbed; usually oxidized to H2SO4. Other elements, e.g. Se also convert to volatile oxides; must be scrubbed/precipitated (expensive). Contaminates acid product. The impure H2SO4 must be sold or disposed of. | Generally no toxic gas products. Sulfide may be converted to elemental sulfur (often contaminated with Se, Te, As etc.), or to sulfuric acid/sulfates. Sulfates may be precipitated. |
| Pollution/disposal problems: SO2 gas, dust (containing toxic materials), slag (may or may not be toxic; can be visually unappealing; large tonnages) | Metal-bearing tailings solutions and suspended solids. Tailings may contain toxic metal ions/process reagents. Solids may release metal ions into environment. |
| Economics best suited to large scale, high throughput; has high capital cost | Small to large scale; capital cost less sensitive to plant size in lower production range |
| Must handle molten metals, slags, mattes | Solutions/slurries can be readily handled. Corrosion can be an issue. |
| Engineering not considered complex | Often requires sophisticated control systems; more complex engineering; more like a chemical plant. |
Specific factors relating to production of a particular resource are as follows:
- Mineralogy, grade and the like.
- Availability of fuels, water and chemical reagents.
- Electricity availability/generation.
- Labour requirements and availability.
- Transportation, storage and handling of raw materials, reagents and products.
- Environmental impact and waste disposal.
- Instrumentation for process control and optimization.
Hydrometallurgy
Hydrometallurgy figures prominently in the production of many metals, including, Al, Cu, Zn, Pb, Ni, Co, Al, U, Au, Ag and PGM’s, as well as many other minor metals and by-product elements.
