GB1594851A

GB1594851A - Extraction of zinc

Info

Publication number: GB1594851A
Application number: GB20466/77A
Authority: GB
Original assignee: Interox Chemicals Ltd
Current assignee: Solvay Interox Ltd
Priority date: 1977-05-16
Filing date: 1977-05-16
Publication date: 1981-08-05
Also published as: SE7805439L; FR2391284A1; ES469686A1; ZM4778A1; AU520274B2; BE866937A; JPS53142317A; AU3569978A; FI781345A7; IT7849361A0; IT1103492B; NL7805099A; CA1116872A; ZA782397B; AR215928A1; BR7803060A; DE2821309A1; NO781675L

Description

(54) EXTRACTION OF ZINC (71) We, INTEROX CHEMICALS LIMITED, of Hanover House, 14 Hanover Square, London W1R OBE, England, a British Company do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for extracting metal from its ore, and more particularly to a process for the extraction of zinc from its ore.

Several of the ores from which it is commercially viable to extract zinc are sulphidic in nature, and are currently processed pyrometallurgically. Such processes require considerable heat energy, so that in consequence there is an increasing incentive to locate processes which can be effective at moderate temperatures.

According to the present invention there is provided a process for the extraction of zinc from a zinc sulphide-containing ore, comprising the step of agitating the ore in ground form with an aqueous sulphuric acid leach liquor containing as the primary oxidising agent one or both of hydrogen peroxide and monoperoxosulphuric acid at an elevated temperature up to the boiling point of the leaching liquor.

The term "primary oxidising agent" is used herein to indicate the oxidising agent which provides the major proportion of oxidation. Thus, e.g. oxygen or air may be sparged through the vessel containing the mixture of leach liquor and ore, if desired, but it is the hydrogen peroxide and/or peroxomonosulphuric acid, hereinafter referred to as PMS, which oxidises the major proportion of zinc sulphide, thereby enabling the zinc to go into solution.

The process of the present invention is particularly applicable to those ores in which the zinc sulphide is present as sphalerite (zinc blende). Although, preferably, the zinc sulphide represents the major proportion of the ore, the process according to the present invention can also be employed to extract zinc from ores in which the zinc sulphide represents only a minor proportion of the ore, for example less than 20% w/w. Other components of the ore can include chalcopyrite and arsenopyrite. It is particularly difficult to extract zinc from sphalerite ores using only sulphuric acid leaching liquors containing only oxygen as the oxidant at atmospheric pressure, in that no more than approximately a third of the available zinc can be leached out in any reasonable length of time.By the use of a process according to the present invention, however, under comparable conditions, not only can a higher proportion of zinc be extracted, but the rate of leaching is also faster.

It is highly preferable for the temperature of the leaching liquor to be maintained during the leaching at a temperature of at least 700C, preferably at least 80"C. Although the upper limit of the temperature range is the boiling point of the leaching liquor under the conditions employed, i.e. pressure and concentration of solvents in the sulphuric acid solution, a temperature in excess of 100"C is normally not employed. In practice, the temperature of the leaching liquor is often maintained in the range of 90 to 100"C.

Another important parameter, which is independent of the temperature of the leach liquor, is the particle size of the ore. As the particle size of the ore decreases, so the efficiently of utilisation of the primary oxidizing agent increases. Thus, although the present invention process can be employed in respect of particles retained on a British Standard 41D:1962 mesh number 50, abbreviated herein to BSS 50 mesh, it is preferable to use particles which pass through BSS 50 mesh, and more preferably particles which pass through a BSS 100 mesh. In highly preferred embodiments, the ore is ground to about BSS 200 mesh or smaller before being leached. The lower limit of the particle size is, in practice, determined by the apparatus to be employed for separating the leach liquor from the spent ore.Generally, at least the major proportion of the particles are retained by a standard BSS 250 mesh.

An important variable in the process of the present invention is the acidity of the leach liquor. This parameter is independent of both the particle size of the ore and the temperature of the leach liquor. Desirably, the leach liquor is maintained at a pH of below 1.5. During the course of the zinc extraction in solution, there is a tendency for the pH of the liquor to rise. Allowance can be made for this tendency by ensuring that enough acid is present initially, or by adding further amounts of acid during the course of the reaction, or by employing a combination of the two techniques. In preferred embddiments, the acidity of the leach liquor is maintained at a pH of up to or below 1.0, because use of such an acidity enables a higher proportion of the zinc to be extracted in solution at a faster rate than if a less acid solution is used.Such an acidity often can be achieved by employing an initial sulphuric acid concentration of at least 150 gpl, desirably in the range of 170 to 250 gpl, but it will be recognised that the actual pH obtained will vary with the pulp density. It will be recognised, that in the case of the use of PMS as primary oxidising agent, PMS is normally employed in solution in sulphuric acid, and allowance for this sulphuric acid must be made in calculating the amount of sulphuric acid present in the liquor. Moreover, where PMS is added during or throughout the extraction process, its introduction, coupled with that of sulphuric acid, means that less acid need to present initially. However, the presence of excess acid is not in itself inherently disadvantageous to the process, except to the extent that use of unnecessary amounts of any material renders any process less attractive commercially.Instead of treating the zinc sulphide mineral in a single stage, the process can also be effected in two or more stages, in successive stages increasingly depleted zinc sulphide mineral being contacted either with fresh sulphuric acid solution containing or to which is added the primary oxidising agent or with solution that increasingly approaches fresh solution. It will be recognised that whilst the fresh solution desirably has a pH as indicated above, when the solution passes to successive stages in the process it contacts less depleted zinc sulphide mineral and a progressively rising pH may be tolerated.

The hydrogen peroxide or PMS can be introduced into the leach liquor in various methods. In the first method, all the hydrogen peroxide or PMS to be used is present initially in the leach liquor. This method is less preferred because it normally results in greater losses of hydrogen peroxide or PMS through decomposition than do the other methods described herein, and also could introduce an undesirable constraint upon the pulp density that could be employed. In other methods, a part of the hydrogen peroxide or PMS as introduced during the extraction period. Such methods are preferred, in that they tend to result in a more efficient utilisation of the primary oxidant. The introduction of the primary oxidant can be either at prearranged intervals or at a pre-arranged rate throughout the leaching period.In such methods, the total amount of primary oxidising agent required can be pre-determined by, for example, carrying out preliminary tests on a sample of the ore under the other conditions that would be employed, i.e. particle size, temperature and acidity of the leach liquor and duration of the leaching period. In other and more preferred methods, the hydrogen peroxide or PMS is added during or throughout the leaching period in amounts and/or at a rate determined by the ore itself. Although it will be recognised. that some variations can be easily achieved without departing from the spirit of this method, it is highly convenient to employ the output from a standard electrode pair monitoring the electrochemical potential (emf) to regulate the introduction of the hydrogen peroxide or PMS.Such regulation can be achieved by establishing a pair of limits about the desired emf, herein measured with respect to a standard calomel electrode which is normally at least 450 mV, and often from 450 to 650 mV, so arranging that when the lower limit is reached, introduction of the hydrogen peroxide or PMS into the leach liquor is triggered, and that when the emf is at or above the upper limit, the flow of hydrogen peroxide or PMS is cut off. If desired, the flow of the primary oxidising agent into the leaching liquor can be cut in and cut off sharply at the respective limits, or alternatively, by using a proportioning pump which pumps the primary oxidising agent solution at a rate in inverse relationship with the emf, a smoother and a closer control of the emf can be achieved. Use of methods in which a part of the primary oxidising agent is added during or throughout the leaching period. and in particular in those methods in which the introduction is controlled so as to maintain a substantially constant emf, enables more efficient use to be made of the primary oxidising agent. By employing appropriate control of the introduction of the primary oxidising agent, in combination with the grinding of the ore to an appropriate particle size and by using a leach liquor of the appropriate acidity, relatively efficient use of the primary oxidising agent can be made.Thus, in a process in which the emf is maintained at 450 mV or higher, the particle size is below 100 mesh (BSS) and preferably below 200 mesh (BSS) and the acidity is at or below pH 1.2, the utilisation of the primary oxidising agent normally will be within the range of about 100% to about 200% of the theoretical amount required to extract 100% of the zinc from the ore into solution, in the case of hydrogen perioxide and about 80% to 100% in the case of PMS. The further that the process strays away from such a combination of parameters, the greater the tendency to waste primary oxidising agent. Thus, e.g., use of an ore containing a high proportion of particles in the range of above 100 mesh (BSS) can require over 500% by weight of the theoretical amount.Similarly, where the introduction of the primary oxidising agent is not controlled properly, the rate of extraction of zinc into solution can suffer, and utilisation of the primary oxidising agent can also suffer: Often, since zinc sulphide ore can contain as an impurity iron compounds, such as pyrite, the leaching liquor can contain undesirably high levels of iron compounds. The level of such iron compounds, however, can be drastically reduced by reducing the acidity of the leaching liquor to within the range of pH 2.5 to 4.0, preferably about pH 3.0, and maintaining the pH of the liquor at such a value until ferric salts have precipitated out, often a period of up to an hour. The pH adjustment can be effected by any suitable alkaline hydroxide, such as sodium hydroxide or ammonium hydroxide.Alternatively, the pH can be adjusted to a pH of approximately 1.5 by the addition of ammonium hydroxide, and the iron is precipitated out of solution as a jarrosite salt.

The concentration of hydrogen peroxide solution which is .introduced into the leaching liquor can .be any commercially available concentration up to 65% w/w. Concentrations often employed are in the region of 6% w/w, 10% w/w, 20% w/w, and 30-35% w/w.

Preferably the solutions contain stabilisers.

In highly desirable embodiments, the principal oxidising agent is PMS, because it tends to.

be more stable under the process conditions than does hydrogen peroxide. In consequence, ore that has been ground to some extent less finely can be employed than if hydrogen peroxide were to be used. The concentration of PMS will normally fall within the range of 5 to 75% by weight, the concentration of from 10 to 65% by weight often being used. In one convenient method for its producton, PMS solution is made by reacting aqueous hydrogen peroxide with oleum or concentrated sulphuric acid. Consequently, solutions containing PMS often contain in addition a small percentage- of hydrogen peroxide.

Suitable conditions for the production of PMS are described in British Patent Specifications Nos 738407 and 844096. Broadly speaking, it is particularly desirable to employ concentrated hydrogen peroxide solution, e.g. in the range of 60 to 85% w/w hydrogen peroxide together with oleum since such a combination enables conversion of the' sulphate species to peroxymonosulphuric acid to occur to a greater extent than when more dilute solutions are employed. Provided that precautions normal in respect of exothermic reactions, such as cooling, are carried out, PMS can be generated safely and efficiently by the method outlined above.Alternatively, if desired, PMS can be obtained by hydrolysis of a peroxydisulphate, especially peroxydisulphuric acid produced, e.g. by electrolysis, or the sodium potassium or ammonium salts thereof, hydrolysis to PMS rather than continuing to hydrogen peroxide, occurring most rapidly at temperatures in the range of 50 to 700C.

Because PMS solutions tend to lose their available oxygen content upon storage, even atambient temperatures, it is preferable to use freshly prepared PMS, for example made and used on the same day. It is convenient, in some embodiments of the present. invention to produce PMS at the rate at which it is consumed, such as by controlling the rate at which hydrogen peroxide and sulphuric acid are fed into a reaction chamber by the rate at which the PMS solution is' introduced into the leaching liquor. Such control can be effected independently from or in conjunction with the method of controlling the rate at which PMS is introduced into the leaching liquor.It will be recognised that apparatus for controlling the rate of flow of fluids, such as valves and proportioning pumps are well known, as are mechanical and electrical-systems for trasmittihg the control signal from a detector to these control means.

The pulp density of the mixture of ore and leach liquor can be any density up to the maximum at which the mixture can be agitated. In practice, the pulp density is often around 40 to 50% because at such pulp densities the equipment is being used relatively efficiently and the final concentration of zinc in the leaching liquor is relatively high, thereby expediting its subsequent removal from the leach liquor and purification. Lower pulp densities, e.g. in the range of 10 to 20% can be used, if desired, and are particularly suitable for demonstrating effectiveness of hydrogen peroxide and PMS on the small scale.

Reference herein has been made to the treatment of ore, but it will be recognised that the present invention is especially useful in respect of dressed ores, such as ores subjected to a differential flotation to separate the zinc concentrate from, e.g., galena.

Zinc which has been extracted into solution in the leach liquor can thereafter be removed and purified by methods conventionally employed when sulphated calcine has been leached with sulphuric acid. Consequently, the process can include neutralisation with zinc oxide to precipitate iron, arsenic, and antimony, clarification of the pulp in thickeners, and displacement of cadmium by addition of zinc dust. One benefit of the present invention is that the proportion of arsenic extracted into solution from arsenopyrite, the arseniccontaining mineral most commonly associated with sphalerite is comparatively low, thereby reducing the problems of disposing of extracted arsenic.One additional feature arising from the use of hydrogen peroxide and PMS in the present invention is that, whilst on the one hand, the amount of poisonous hydrogen sulphide gas involved is exceedingly small, to the point of being frequently undetectable, a very substantial proportion of the sulphur is produced in elemental form rather than being oxidised through to gaseous sulphur dioxide.

This is a feature that becomes increasingly advantageous as restrictions upon the admission of sulphur-containing gases into the atmosphere are tightened. Moreover, transportation and sale of solid sulphur is much easier than gaseous sulphur dioxide which would require either additional processing stages or to be liquefied before it could be transported readily.

In view of the fact that PMS has generally greater stability to decomposition than does hydrogen proxide, PMS can be used as a secondary oxidising agent over a wide range of conditions from ambient up to the boiling point of the leaching liquor. When the secondary oxidation agent is added under the conditions described it is highly desirable for it to be present during a substantial proportion of the time, preferably added incrementally or continuously during the leaching period.

Various zinc sulphide ores contain additionally copper sulphide as chalcopyrite. The primary oxidising agents described herein can be employed to leach out the zinc at a faster rate than the copper, thus effecting a partial separation. A suitable method for preferentially extracting zinc from an ore containing sphalerite and chalcopyrite comprises the steps of agitating the sphalerite and chalocopyrite-containing ore in ground form at a temperature of from 65 to 75"C with aqueous sulphuric acid at a pH of below 1.5 and containing sufficient hydrogen peroxide to maintain the oxidation potential at above 450 mV, preferably between 450 mV and 700 mV, and separating the ore from the leach liquor after a pre-determined proportion of the zinc has been extracted in solution.In practice, the pre-determined proportion of zinc would generally fall within the region of 80 to 95%, typically approximately 90% by weight. Such a process would produce a leach liquor containing a comparatively small proportion of copper, and therefore reduce the cost of production of substantially pure zinc metal. If desired, thereafter, the ore which has been separated from the zinc containing leach liquor could be contacted with fresh sulphuric acid leach liquor containing hydrogen peroxide or PMS at a temperature of between 75 and 100"C, and a pH of below 1.5 to extract the remainder of the zinc and copper into solution, thus producing a leach liquor that is comparatively rich in copper.

In an alternative method for preferentially extracting zinc from a sphalerite and chalcopyrite-containing ore, the ground ore can be agitated with a PMS and sulphuric acid containing leach liquor at a temperature of above 70"C until a predetermined proportion of zinc has been extracted into solution, preferably from 75 to 90%, typically 80%, and thereafter the partially spent ore and leach liquor are separated. The partially spent ore can be contacted with fresh leach liquor as described in the immediately preceding paragraph.

Having described the invention generally, specific embodiments will now be described more fully by way of example. Descriptions of alternative processes, shown by the prefix 'C' are present by way of comparison only.

In Examples El to E8 and comparisons C1 and C2, the ore was an Australian concentrate, typical of material currently processed pyrometallurgically, containing as the major constituent sphalerite, and also containing pyrite, with traces of chalcopyrite and metallic iron. Its composition was 52.0% Zn, 32.2% S, 10.0% FeO, 1.0% Pb, 1.4 SiO2, 0.4% Al203, 0.24% CaO, 0.03% MgO, percentages being by weight. The size distribution of the concentrate used in Examples El and E2 was as follows TABLE 1 BSS Mesh % by weight Cumulative Weight % + 52 7.0 7.0 - 52 +100 25.4 32.4 -100 +200 21.0 53.4 -200 46.4 100.0 In the remaining Examples E3 to E8 and comparisons C1 and C2 the concentrates had been ground to pass through BSS mesh 200.

In Examples E9 to E12, the ore was an arsenical mixed sulphide concentrate produced as a by-product of tin operations, and currently processed pyrometallurgically. The major phase in the concentrate was pyrite, and minor phases were sphalerite and arsenopyrite and chalcopyrite, the latter being associated to some extent with the pyrite. Its composition was 11.5% Zn, 2.5% Cu, 4.0% As, 31.0% Fe, 0.8% Sn, 37.0% S, percentages being by weight.

The concentrate had been ground to pass through the BSS mesh 200.

The compositions of hydrogen peroxide employed are shown in Table 2 with other conditions and results, and the composition of the PMS solution was peroxomonosulphuric acid 10%, sulphuric acid 24%, hydrogen peroxide 1.2%, and water 64.8%, percentages being by weight. The PMS solution was obtained by reacting 70% w/v aqueous hydrogen peroxide and 98% w/v sulphuric acid with continuous stirring and cooling behind 9 safety screen, and thereafter diluted to a concentration of 10% w/v PMS by addition of distilled water.

The apparatus used in each Example and comparison comprised a 250 ml reaction vessel fitted with a five necked lid, and heating element, a thermostat accurate to +20C, a thermometer, a propeller stirrer operating at 450+ 100 rpm, a water cooled condenser, a peristaltic pump for pumping the solution, and a standard platinum/calomel electrode to measure the potential. In addition, in the Examples, the apparatus included a fine bore glass tube, which was so shaped and positioned as to introduce the solution of oxidising agent immediately beneath the stirrer.

In each'Example or comparison, a slurry (10% w/v 15g/150ml) of ore in a sulphuric acid solution of a given concentration was introduced into the reaction vessel, stirred continuously and heated to the temperature indicated in Table 2. Throughout the reaction period the aqueous solution of the selected primary oxidising agent was introduced into the slurry intermittently so as to maintain an oxidative environment, normally within the range of 450 to 700 mV, although some variation outside that range is tolerable. After 1, 2, 4 and 6 hours samples of the leaching liquor were taken and analysised for zinc content and expressed as a percentage of the total amount of zinc known to be present in the ore initially. After 6 hours, the electro chemical potential (Eh, expressed in mV with respect to the calomel electrode), the pH, and the iron concentration in the leach liquor were measured.

In one minor variation, the entire amount of oxidising agent in Example E8 was'added during the first three hours. In comparisons C1 and C2 no solution of oxidising agent were pumped into the leach liquor, but instead respectively oxygen and argon were sparged into the reaction vessels continuously during the extraction period.

The reaction conditions and extraction results are summarised in Table 2 herein below.

In Table 2, the heading % TA is the amount of oxidant employed expressed as a percentage of the theoretical amount of primary oxidising agent required to enable all the zinc to be extracted into solution in Examples El to E8 or all the copper and zinc in Examples E9 to E12, assuming in El to E6 and E9 and E10 oxidation solely by hydrogen peroxide, and E7, E8, Ell and E12 oxidation solely by PMS, and basing the %TA upon measured total zinc content of the solution and residue. The heading Vol Added is the volume of either H2O2 or PMS solution added, and the heading Fe Conc. is a measure of iron in solution, whether in a ferrous, or principally, ferric state.

TABLE 2 Examples Vol. H202 H2SO4 Comparison Temp. Added strength % TA Conc.

Number "C ml % w/v gl- El 70 60 30 444 128 E2 90 80 3: 593 128 E3 90 140 6 207 128 E4 90 91 6 134 171 E5 90 112" 6 165 213 E6 90 16 30 118 213 E7 90 lqs 79 128 E8 90 1 82 128 E9 90 104 6 570 32 E10 70 .98 6 539 32 Ell 90 130 353 32 E12 100 100 272 32 C1 90 Sparged Oxygen 213 C2 90 Sparged Argon 213 TABLE 2 (continued) % Zinc Recovery After Final Readings Example Comparison Eh pH Fe Conc Number lh 2h 4h 6h mV gl- El N.D.' N.D.* N.D.* 33.0 675 0.8 2.0 E2 14.7 33.9 N.D.* 84.3 645 1.1 5.2 E3 13.3 31.2 77.5 79.9 585 1.2 6.1 E4 33.7 60.0 88.4 99.0 550 0.8 4.1 E5 41.6 70.1 98.2 99.3 565 0.6 4.0 E6 41.8 66.6 86.9 93.6 480 0.5 5.0 E7 46.5 88.1 98.3 98.8 610 0.1 4.3 E8 67.3 95.1 96.5 98.1 595 0.6 , 4.7 E9 48.3 61.0 91.0 99.0 450 0.4 ' 1.7 E1û 47.4 55.2 92.6 98.2 510 0.4 5.4 Ell 72.5 81.4 92.0 99.6 465 0.1 6.8 E12 48.0 86.0 94.5 99.0 575 0.6 4.9 C1 N.D. * 32.8 N.D.* 33.3 288 0.35 N.D.

C2 N.D.* N.D. * N.D. - 30.5 0 0.3 N.D.

N.D.* not determined From Table 2 it can be seen that the effect of raising the temperature from 70"C to 90"C significantly speeds up the rate of extraction of zinc in solution from less finely ground ore.

The effect of grinding the ore to below BSS mesh 200 is to significantly improve the efficiency of utilisation of hydrogen peroxide. An increase in the concentration of sulphuric acid tends to result in an increase in the proportion of zinc that can be extracted within a given period, from the ore, but that above a certain concentration, substantially all the zinc is extracted. Moreover, it will be seen that when PMS is used, the initial concentration of sulphuric acid can be substantially lower than if hydrogen peroxide were to be employed.

Finally, it can be seen that there was no significant increase extraction of zinc into solution gained from sparging oxygen in Cl instead of a chemically inert gas, argon, in C2 and that both of these methods were commercially unacceptable even at 900C and.employing ore milled to pass through BSS mesh 200. By comparison, however, employing the same temperature conditions and milled ore, and the same or in some cases even longer sulphuric acid concentration, both hydrogen peroxide and PMS achieved substantially complete extraction of zinc from the ore during the same time period.

In Examples E9 to E12, the rate of extraction of copper into solution was also measured after 1, 2, 4, and 6 hours. The results are summarised in Table.3 herein below.

TABLE 3 Example % Copper Recovery After No. lh 2h 4h 6h 9 12.7 82.1 96.0 98.0 10 6.8 17.3 24.0 92.0 11 21.7 33.2 44.3 46.4 12 14.5 38.5 72.5 73.6 From a comparison of Table 2 and Table 3, it will be observed that whereas a very high proportion of the zinc had been extracted after 4 hours employing hydrogen peroxide and a temperature of 70"C, only a very small proportion of the copper had been extracted.

WHAT WE CLAIM IS: 1. A process for the extraction of zinc from a zinc sulphide-containing ore, comprising the step of agitating the ore in ground form with an aqueous sulphuric acid leach liquor containing as the primary oxidising agent one or both of hydrogen peroxide and monoperoxysulphuric acid at an elevated temperature up to the boiling point of the leaching liquor.

2. A process as claimed in claim 1 wherein the pH of the leach liquor is maintained at below pH 1.5 during the leaching.

3. A process as claimed in any preceding claim wherein the initial sulphuric acid

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

TABLE 2 (continued) % Zinc Recovery After Final Readings Example Comparison Eh pH Fe Conc Number lh 2h 4h 6h mV gl- El N.D.' N.D.* N.D.* 33.0 675 0.8 2.0 E2 14.7 33.9 N.D.* 84.3 645 1.1 5.2 E3 13.3 31.2 77.5 79.9 585 1.2 6.1 E4 33.7 60.0 88.4 99.0 550 0.8 4.1 E5 41.6 70.1 98.2 99.3 565 0.6 4.0 E6 41.8 66.6 86.9 93.6 480 0.5 5.0 E7 46.5 88.1 98.3 98.8 610 0.1 4.3 E8 67.3 95.1 96.5 98.1 595 0.6 , 4.7 E9 48.3 61.0 91.0 99.0 450 0.4 ' 1.7 E1û 47.4 55.2 92.6 98.2 510 0.4 5.4 Ell 72.5 81.4 92.0 99.6 465 0.1 6.8 E12 48.0 86.0 94.5 99.0 575 0.6 4.9 C1 N.D. * 32.8 N.D.* 33.3 288 0.35 N.D.

C2 N.D.* N.D. * N.D. - 30.5 0 0.3 N.D.

N.D.* not determined From Table 2 it can be seen that the effect of raising the temperature from 70"C to 90"C significantly speeds up the rate of extraction of zinc in solution from less finely ground ore.

The effect of grinding the ore to below BSS mesh 200 is to significantly improve the efficiency of utilisation of hydrogen peroxide. An increase in the concentration of sulphuric acid tends to result in an increase in the proportion of zinc that can be extracted within a given period, from the ore, but that above a certain concentration, substantially all the zinc is extracted. Moreover, it will be seen that when PMS is used, the initial concentration of sulphuric acid can be substantially lower than if hydrogen peroxide were to be employed.

Finally, it can be seen that there was no significant increase extraction of zinc into solution gained from sparging oxygen in Cl instead of a chemically inert gas, argon, in C2 and that both of these methods were commercially unacceptable even at 900C and.employing ore milled to pass through BSS mesh 200. By comparison, however, employing the same temperature conditions and milled ore, and the same or in some cases even longer sulphuric acid concentration, both hydrogen peroxide and PMS achieved substantially complete extraction of zinc from the ore during the same time period.

In Examples E9 to E12, the rate of extraction of copper into solution was also measured after 1, 2, 4, and 6 hours. The results are summarised in Table.3 herein below.

TABLE 3 Example % Copper Recovery After No. lh 2h 4h 6h

9 12.7 82.1 96.0 98.0

10 6.8 17.3 24.0 92.0

11 21.7 33.2 44.3 46.4

12 14.5 38.5 72.5 73.6 From a comparison of Table 2 and Table 3, it will be observed that whereas a very high proportion of the zinc had been extracted after 4 hours employing hydrogen peroxide and a temperature of 70"C, only a very small proportion of the copper had been extracted.

WHAT WE CLAIM IS: 1. A process for the extraction of zinc from a zinc sulphide-containing ore, comprising the step of agitating the ore in ground form with an aqueous sulphuric acid leach liquor containing as the primary oxidising agent one or both of hydrogen peroxide and monoperoxysulphuric acid at an elevated temperature up to the boiling point of the leaching liquor.
2. A process as claimed in claim 1 wherein the pH of the leach liquor is maintained at below pH 1.5 during the leaching.
3. A process as claimed in any preceding claim wherein the initial sulphuric acid

concentration in the leach liquor is in the range of 170 to 250 gpl.
4. A process as claimed in any of claims 1 to 3 wherein the leach liquor is maintained at a temperature in the range of 70 to 1000C during the leaching.
5. A process as claimed in claim 4 wherein the temperature of the leach liquor is maintained in the range of 90 to 100"C.
6. A process as claimed in any preceding claim wherein at least a part of the primary oxidant is introduced into the leach liquor during the extraction period.
7. A process as claimed in claim 6 wherein the rate of introduction of the primary oxidant is regulated to maintain the electrochemical potential of the leach liquor within a predetermined range.
8. A process as claimed in claim 7 wherein the predetermined range is from 450 to 650 mV with respect to a standard calomel electrode.
9. A process as claimed in claim 7 wherein the leach liquor has its electrochemical potential maintained at 450 mV or higher with respect to a standard colomel electrode its particle size of below 100 mesh (BSS) and its acidity below pH 1.2.
10. A process as claimed in any preceding claim wherein the ore is ground to a particle size of below 200 mesh BSS.
11. A process for the extraction of zinc from sulphide ores containing as impurity iron compounds as claimed in any preceding claim wherein the acidity of leach liquor is subsequently adjusted to within the range of pH 2.5 to 4.0 to effect preferential precipitation of ferric salts.
12. A process for preferentially extracting zinc from sphalerite and chalcopyritecontaining ores comprising the steps of i) agitating the ore in ground form with a sulphuric acid leach liquor having a pH of below 1.5 and an oxidation potential maintained at above 450 mV with respect to a standard calomel electrode and containing either hydrogen peroxide and maintaining a temperature of 65"C to 750C, or containing peroxymonosulphuric acid and maintaining a temperature of at least 70"C, and ii) separating the liquor from the ore when a predetermined proportion of the zinc has been extracted.
13. A process as claimed in claim 12 wherein the predetermined proportion is in the range of 80 to 95% when hydrogen peroxide is used and 75 to 90% when peroxymonosulphuric acid is used.
14. A process for extracting zinc from sulphide containing ore substantially as described herein with respect to any of Examples El and E12.
15. Zinc-containing solutions whenever obtained by a process claimed in any preceding claim.
16. Zinc metal whenever obtained from zinc containing solutions claimed in claim 15.