Sulphide minerals such as copper, nickel, zinc, gold and the like are recovered from their ores by a number of well known processes. One such process uses the relative solubility of the mineral in solution to allow the mineral to be leached from the ore. Conventional leaching processes require expensive equipment and a high level of technical expertise to maintain and use the equipment. Thus, it is not uncommon for an oxidative hydrometallurgy leaching plant to be located some distance away from the ore body and even in another country. This in turn significantly increases transportation costs, and it should be realised that transportation of ore or only partially enriched ore containing perhaps only a few percent of the desired mineral is extremely wasteful and undesirable, but in the absence of being able to recover the metal of value from the minerals on-site, there is little real alternative.
The processing methods of oxidative hydrometallurgy are commonly used in many different applications. Due to the refractory nature of many of the mineral species treated in such processes, these applications generally require leaching conditions of high temperature and pressure and require substantial supplies of oxygen. For example, base metals such as copper, nickel and zinc can be recovered by hydrometallurgical processes which usually embody pretreatment, oxidative pressure leaching, solid/liquid separation, solution purification, metal precipitation or solvent extraction and electrowinning.
According to conventional technology, oxidative leaching processes usually require very aggressive conditions in order to achieve acceptable rates of oxidation and/or final recoveries of metal. Under these conditions, which often mean temperatures in excess of 150.degree. C. or alternatively temperatures in the range 150-200.degree. C. and total pressures in excess of 1500 kPa, the chemical reactions which occur use large quantities of oxygen, both on stoichiometric considerations and in practice where amounts in excess of stoichiometric requirements are used due to process inefficiencies.
An example of oxidative hydrometallurgy is the treatment of refractory gold ores or concentrates. Refractory gold ores are those gold ores from which the gold cannot readily be leached by conventional cyanidation practice. The refractory nature of these gold ores is essentially due to very fine (sub microscopic) gold encapsulated within the sulphide minerals. This gold can often only be liberated by chemical destruction (usually oxidation) of the sulphide structure, prior to recovery of the gold, which is usually done by dissolution in cyanide solution. Of course, other lixiviates such as thiourea and halogen compounds and the like may also be used.
A number of processing options are available for treating refractory gold ores which contain sulphide minerals like pyrite, arsenopyrite and others. Pressure oxidation, typified by the so-called Sherritt process, is one such process which typically consists of the steps of feed preparation, pressure oxidation, solid/liquid separation, liquid neutralisation and gold recovery from oxidised solids usually by cyanidation.
A cryogenic oxygen plant is usually required to supply the substantial levels of oxygen demand during the pressure oxidation step, which is the heart of the Sherritt process. Typically, the conditions for the pressure oxidation step require temperatures in the region of 150.degree. C. to 210.degree. C., a total pressure of 2100 kPa, a pulp density equivalent to 20% to 30% solids by mass, and a retention time of two hours to three hours.
The typical oxidative hydrometallurgical processing methods referred to above generally have oxidation reactions that are carried out in multicompartment autoclaves fitted with agitators. In order to withstand the generally highly aggressive conditions of the reactions, the autoclaves are very costly, both to install and maintain. These vessels must be capable of withstanding high pressure, and linings of heat and acid resistant bricks need to be used. The agitators are made of titanium metal, and the pressure relief systems utilised are also costly and require high maintenance. These high costs and the sophistication of the technology (skilled operators are generally required) detract from the wider acceptance of high pressure/high temperature oxidation, particularly for use in remote areas or by small to medium size operators.
Cooling of the agitators also presents problems, and expensive cooling coils and heat exchange jackets are required to keep the leach temperature at optimum conditions.
The aggressive leaching conditions outlined for recovery of metal values from base metal concentrates are required to achieve acceptable leaching rates from the minerals. Under conditions of atmospheric pressure, the leaching rates of the mineral species are too low to support an economically viable leaching process.
Attempts have been made to reduce the aggressive conditions and to lower the pressures in order to lower the cost in building and operating a leaching plant. For instance, it is known to initially fine grind the ore or the ore concentrate (it being known to use flotation as an initial step to concentrate minerals in the ore), prior to oxidative hydrometallurgy to leach the ore. The fine grinding increases the surface area to volume ratio of the ore particles to improve extraction. A fine grind to an 80% passing size of 15 micron or less is used. The initial fine grind results in acceptable leaching rates being observed with less aggressive conditions, and leaching can be carried out at temperatures of 95-110.degree. C. and at a pressure of about 10 atmospheres or about 1000 kPa.
Thus, while some progress has been made in reducing the operating parameters and thus the cost of the leach system to date, the leach still must be carried out under pressurised conditions. Pressure leach systems are expensive to build. Because of the high capital and processing costs of pressure leach systems, these systems are economical only for high grade concentrates. High grade concentrates are required because
(1) operating cost per unit of contained metal considerations PA1 (2) less heat generation/exchanger problems with high grade concentrates PA1 (3) capital cost per unit of contained metal is lower balancing up with large initial capacity outlay to metal recovery. PA1 (a) milling said composition to a particle size P80 of 20 microns or less, PA1 (b) leaching said composition with a solution comprising sulphuric acid and ferric ions at ambient pressure whilst sparging with an oxygen containing gas in an open tank reactor at a temperature of up to about the boiling point of the solution, whereby at least some of the acid and at least some of the ferric ions are obtained from dissolution of the iron containing mineral, and ferrous ions generated by the leaching reaction are substantially re-oxidised to ferric ions in the leaching solution; PA1 (c) precipitating iron and separating said iron and solid materials from the leaching solution; PA1 (d) extracting desired metal ions from the leaching solution by solvent extraction with an organic solvent to form an organic phase and raffinate comprising sulphuric acid and ferric ions; PA1 (e) returning the raffinate to the open tank reactor and blending with further milled composition; PA1 (f) separating the metals from the organic phase obtained in step (d) by stripping with electrolyte from an electrowinning cell and electrowinning. PA1 (a) milling said composition to a particle size of P80 of 20 microns or less and PA1 (b) leaching said composition with a solution comprising sulphuric acid and ferric ions at ambient pressure whilst sparging with an oxygen containing gas in an open tank reactor at a temperature of up to about the boiling point of the solution, whereby at least some of the acid and at least some of the ferric ions are obtained from dissolution of the iron containing mineral, and ferrous ions generated by the leaching reaction are substantially re-oxidised to ferric ions in the leaching solution. PA1 (a) milling said ore to P80 of 5 micron and PA1 (b) leaching said ore with a solution comprising sulphuric acid and ferric ions, at ambient pressure whilst sparging with an oxygen containing gas in an open tank reactor at a temperature of up to about the boiling point of the solution.
It is also known to oxidatively leach sulphide mineral species with ferric ions. Ferric ion is a relatively effective oxidising agent which enables oxidation to be carried out at pressures less than that normally required when oxygen is the oxidant. However, there are a number of practical difficulties associated with using ferric ions as the oxidant. First, at ambient pressure the reaction is inherently slow. Also during the leaching reaction, ferric ions are reduced to ferrous ions. A build up of ferrous ions in the leaching solution adversely affects the rate of leaching. Also the ferrous ions must normally be removed from the leach liquor prior to further processing which is difficult.
Leaching solutions are generally recycled. However, before a ferric leaching solution can be recycled the ferrous ions must be re-oxidised to ferric ions. This is because it is important for maximum effectiveness of the leach that most of the iron is in the ferric form. The leach solutions can be regenerated by electrolytic oxidation, use of strong oxidisers such as permanganate, oxidation under high pressure of oxygen, or oxidation by bacteria. Each of these methods suffer from disadvantages which limit their application. For example high pressure oxidation is limited by the costs of the autoclaves involved. Oxidation by oxygen under ambient pressure can occur but only at an inherently slow rate. Catalysts may be used to increase the rate but such catalysts are expensive and are not economical for recovery from low grade ores.
Each of the above processes either require expensive autoclaves or other equipment and/or the addition of expensive reagents used for oxidation or regeneration of ferric ions. This means that it is only economically viable to process high grade ores by these methods. Another disadvantage of these processes is that they generate significant amounts of waste products such as gypsum, sulphuric acid and jarosite. These products must be disposed of in an environmentally acceptable manner which also adds to the cost.
Many valuable copper or zinc bearing ores are found in association with iron containing ores such as pyrite. Pyrite is of little value and is effectively a diluent of the valuable ores. Further, leaching of pyrite produces iron species which interfere with extraction of the desired metals. Pyrite is therefore generally removed from other ores prior to processing. The pyrite may be removed by methods such as flotation. Such separation also adds significantly to the cost and in some cases it is not economically feasible to process some low grade ores at all.