WINNING METALS WITH WATER IS relatively new science J. H. Canterford, Commonwealth Scientific and Industrial Research Organization offers some attractive xtractive metallurgy, the science of recovering metals and non-metals options for recovery of from their ores, may be divided into hydrovietallurgy and pyronietallur- low-grade, complex and gy. Whereas in pyrometallurgy, heat plays a major role, in hydrometallur- small-body ores, and Egy, solution in water (or, in certain cases, solvents other than water) is an essential feature. readily lends itself to Thus, hydrometallurgy generally involves converting the desired metal or outomation. metals in an ore, concentrate or intermediate product into a water-soluble form, followed by recovering from solution a more highly refined product. In certain cases, however, hydrometallurgy is used to remove easily-soluble waste or gangue minerals from an ore, leaving an insoluble concentrate. As Wadsworth [I]has pointed out, hydrometallurgy, with a history dating back some ti00 years, has only recently attained the status of a science. This compares with a 6,OOO-yew history for pyrometallur~T. Because many hydro processes require the use of electrical energy, the widescale commercial use of hydrometal- lurgy dates back only some 120 yews. The significance of hydro processes can be well illustrated by reference to the wide range of papers presented at the Third International Symposium oti Hy(Irometd1urg-y , held in 1YX3 I.! 1. Annual reviews, including those of Warren I .I 1, provide useful data on cun’ent de\c.loi)”irnts, while the personal viewpoints of i3urkin 14 I, Habashi 151, I’icketi~gand Canterforti I(;], and Weir arid Masters I;] may give those associated with research and development of hydro processes a new slant on their problems. I3ecause hydro processes are can-ied out at relatively low temperatures (2(L 250°C) compared with pyro processes (600-2,0OO0C), rates of lust be carried out on a large scale, and addition of extr; reaction are much slower and, in most cases, control the rocessing capacity is very costly. productivity of a given size of plant. Wadsworth and Miller Another advantage of hydrometallurgy is that it is we1 [SI provide an excellent review of the rates of a wide range of uited to low-grade and more-complex ores. This is impor hydro reactions. ant, because the world’s high-grade ores that are readill Conventional hydro processes consist of three basic steps: eneficiated into concentrates suitable for pyrometallurg 1. Dissolution (leaching) of the wanted constituents. re becoming depleted. 2. Purification of the metal-containing solution (leachate). A good example is nickel: Although about 70% of thc 3. Recovery of the dissolved metals. rorld’s production is presently derived from sulfide ores Fig. 1 shows a simplified flowsheet of a typical hydro pro- iainly by pyro techniques, the sulfide ores account for on11 cess. As can be seen, there are numerous methods applicable bout 30% of the known land-based reserves. It is obvioui for each of the basic processes, the actual combination cho- hat nickeliferous (containing nickel) laterites will becomc sen depending on many factors. (Cementation is a process of icreasingly used as the source of nickel. surrounding a solid with a powder and heating the whole so Other advantages of hydrometallurgy are: Processes arc that the solid is changed by chemical combination with the eadily carried out on a continuous basis, leading to readj powder; electrowinning is the recovery of a metal via utomation and control; and sulfides can be eliminated in tht electrolysis.) lemental form, rather than as sulfur dioxide. Of course, hydro processes suffer from several importan! Why choose hydrometallurgy? isadvantages. One d the most important is that they art There are a number of metals, alloys or oxides that can be nergy-intensive. Other disadvantages are: They involvt produced only by a hydro or a pyro process. Uranium oxide andling large volumes of often dilute but corrosive an( (U,O,) and steel, respectively, are good examples. How- ?metimes poisonous solutions; and they produce residue: ever, there are many ores and concentrates that can be iat are difficult to filter, wash and dispose of in an environ treated by a variety of both hydro and pyro techniques. ientally acceptable manner. The choice between hydrometallurgy and pyrometallurgy, During the late 1960s and early 1970s, much of the impetus and indeed between several alternative hydro processes, )r the development of new hydrometallurgical routes cam( involves consideration of many factors, some of which be- -om the need to treat nonferrous metal sulfide concen. come apparent only during pilot-plant operations. Non-met- mates. Pyro treatment of these concentrates leads to tht allurgical factors - including geographical location, avail- roduction of large volumes of sulfur dioxide. If this is no1 ability of manpower, water and power, and market xovered and converted to sulfuric acid, then the potentia requirements - have a marked influence on the choice be- nd real environmental problems associated with the gener. tween two competing processes. The situation is not normally one of hydrometallurgy versus pyrometallurgy, but one of establishing the most favorable process for a given ore in a given location. As we are required to process lower-grade and more-complex feed materials, as energy costs continue to spiral, and environ- mental constraints become more severe, many of the new- generation processes will tend to incorporate both hydro and pyro stages. The roastileach/electrowin (RLE) process for recovering metallic zinc from zinc sulfide concentrates repre- sents one of the best-known examples of a process with a pyro “front end.” At the present time, hydro and combined hydrolpyro processes are used to recover a wide range of metals and nonmetals, including the following: uranium, gold, alumina, Solvent extraction, ion exchange, zinc, nickel, cobalt, copper, the platinum-group metals, tita- Cementation, precipitation, nium, niobium, tantalum, zirconium, molybdenum, tung- crystallization sten, beryllium, the rare earths, boron, halite (NaCl), car- nallite (KMgCl3-6H20),and sulfur. It is generally recognized that one of the advantages of Cementation, gaseous reduction, hydrometallurgy compared with pyrometallurgy is that the electrowinning former can be used to process relatively small ore deposits, using relatively small processing plants. Hence it is possible to design and economically operate a portable, trailer- mounted hydro plant. Good examples are the so-called purificaoon Waste PURE (Portable Uranium Recovery Equipment) plant de- signed by Dravo [9], and the solvent-extractiodelectrowin- ning unit developed by Holmes and Naver Inc. [IO]. igure 1 - The basic concepts of a hydrometallurgical On the other hand, to be economic, most pyro processes rocessing route are shown in this hypothetical flowsheet 42 CHEMICALENGINEERING/OCTOBER a. iw lition of extra ation of acid rain from the emitted SO2 cannot be ignored. treatment to convert the desired metal into a leachable form, Many metallurgists, process engineers and corporate or to reduce gangue mineral dissolution, that is, increase that it is well managers believed that alternative routes based on hydro- selectivity. For example, ammoniacal ammonium carbonate This is impor- ~netallurgywould solve many of the problems, particularly leaching of nickeliferous laterites is carried out with pre- at are readily it1 view of the fact that it was known that some of these reduced feed. wometallurgy routes were capable of converting sulfides to the more Research is continuing on the development of alternative environmentally acceptable elemental form of sulfur. leachants, purification reagents and operating procedures. t 70% of the Some went so far as taking the view that hydrometallurgy For example, there are several groups investigating the use sulfide ores, kvas the panacea of all environmental ills associated with the of thiourea as an alternative to cyanide for the dissolution of mount for only extractive metallurgical industry. In reality, the situation is gold from a range of ores, concentrates, secondary products It is obvious not clear-cut. On one hand, there have been significant and residues. Many would regard the replacement of envi- l will become j improvements in sulfur dioxide recovery procedures; on the ronmentally unacceptable cyanide with thiourea as a very 1 other hand, hydro routes often produce solid and liquid desirable change. 9ocesses are residues that are difficult to dispose of, as discussed Electrowinning is energy-intensive, and considerable ef- ling to ready previously. forts are being directed towards the development of alterna- unated in the tive routes - particularly hydrogen reduction of the dis- Process selection solved metal on a continuous basis. Solvent extraction and .a1 important At a first glance, Fig. 1 might suggest that the hydrometal- ion exchange have been used for many decades in analytical hat they are lurgist is faced with (a>a difficult choice to be made between chemistry, but were only adopted in hydrometallurgy about 'hey involve the numerous alternatives available for each stage, or (b) the twenty years ago. They are now regarded as standard hydro mosive and advantage of having a wide range of alternatives that could unit processes. uce residues be used for a given ore in a giien location. In fact, these various hydro technologies have advanced to I an environ- In reality, the position is somewhat different. For any the stage that some reagents (extractants and resins) are given ore, there are normally only two or three reagents that now being manufactured for a specific hydro feed-solution. 'the impetus are commercially applicable for, say, leaching. The type of This contrasts with the position about 10 years ago, when a routes came ieuchant that can be used is largely determined by the process had to be designed to produce feed solution that was lide concen- mineralogical form in which the desired metal occurs, by the compatible with the reagents then commercially available.