Preferential hydrometallurgical conversion of zinc sulfide to sulfate from zinc sulfide containing ores and concentrates

A hydrometallurgical process for converting zinc sulfide in an ore containing zinc sulfide, said zinc sulfide being chemically converted at elevated temperatures to ZnSO.sub.4.xH.sub.2 O which crystallizes substantially in the monohydrate formas ZnSO.sub.4.H.sub.2 O in a conversion solution having a high concentration of H.sub.2 S.sub.4. The process comprises: i) contacting the zinc sulfide of the ore with the conversion solution which comprises a concentration of sulfuric acid selected from the range of about 45% by weight up to about 70% by weight of the conversion solution and at the elevated temperature in the range of 90.degree. C. to less than boiling point of the conversion solution for the selected concentration of sulfuric acid; ii) maintaining the conversion solution at the elevated temperature and at the range of concentration of the sulfuric acid to ensure continued formation of the crystals of ZnSO.sub.4.H.sub.2 O until substantially all available ZnS is chemically converted; and iii) separating the ZnSO.sub.4.H.sub.2 O crystals and remaining solids of the ore from the conversion solution.

FIELD OF THE INVENTION 
The present invention relates to a hydrometallurgical process for 
conversion of zinc sulfide in an ore at high temperatures using high 
concentrations of sulfuric acid. 
BACKGROUND OF INVENTION 
There is a significant push to develop commercial forms of a 
hydrometallurgical process to recover various types of metal from sulfidic 
ore bodies. The significant advantage of a hydrometallurgical process over 
the standard smelting process, is the significant reduction in sulfur 
dioxide emissions. Although the chemistry might appear to be relatively 
direct in extracting zinc from sulfide ores, all known commercial 
approaches in this regard either treat only zinc concentrates containing 
less than 1% copper, or have either failed or are not economically viable. 
It is known that several of these hydrometallurgical processes for 
leaching zinc from either a concentrate or a rich ore involve the use of 
sulfuric acid and/or nitric acid and/or nitrate salts. As is appreciated, 
although sulfuric acid is very useful in removing zinc sulfides from ore 
as soluble sulfates of this metal, the resultant leach solution has to be 
electrowon to recover the zinc because there is at present no other 
economically feasible way to separate the zinc sulfates from the dilute 
H.sub.2 SO.sub.4 solution. 
U.S. Pat. No. 4,710,277 describes a process for leaching zinc from zinc 
containing ores where the zinc is removed from the ore by one or more 
leaching stages. The leached material is then purified and preferably 
subjected to electrowinning to recover zinc from the leaching solution. 
Subsequent to one or more electrowinning steps, the remaining solution may 
be evaporated to increase the acid strength until it reaches a 
concentration of about 60% to 80% H.sub.2 SO.sub.4. The solubility of zinc 
and magnesium in this composition decreases radically at acid strengths of 
this magnitude. As a result, there is precipitated a crystal mass which 
comprises mainly zinc sulfate; magnesium sulfate manganese sulfate. The 
remaining liquid is predominantly acid which can then be recycled in the 
process. The resultant crystal mass can either be discarded or dissolved 
in a small quantity of water. This redissolved solution of primarily 
magnesium sulfate, zinc sulfate and manganese sulfate can be discarded or 
recycled for further treatment. Alternatively, the zinc can be 
precipitated from the solution by neutralizing it at a high pH to 
facilitate dumping of the material. The process of evaporating and thereby 
concentrating the solution to form the crystalline mass is, however, 
expensive because of the significant fuel or energy costs for the 
evaporation step, and the need for corrosion resistant material used in 
the heat transfer evaporating process. Hence the process is not of 
commercial significance, because of the significant costs associated with 
recirculating the liquid phase and discarding the trace amount of metals 
in the liquids removed from the electrowinning stages. 
In Canadian Patent No. 864,455 a process is disclosed to treat ores. 80% to 
100% sulfuric acid by weight of the reaction solution at temperatures 
between 160.degree. C. and 250.degree. C. are used, causing a suspension 
of solids that includes anhydrous sulfates of copper and zinc. The solids 
are washed with water so that zinc sulfate and copper sulfate dissolve 
into solution. The zinc and copper are then recovered by electrowinning 
techniques. Such extremely high concentrations of sulfuric acid and 
extremely high temperatures result in a degradation of the zinc and copper 
sulfides into anhydrous sulfates with the concurrent production of 
SO.sub.2 and a plastic form of sulfur which tends to be very gummy and 
hence hard to handle. 
U.S. Pat. Nos. 4,071,421 and 4,440,569 to Sherritt Gordon disclose a 
pressure leach system which is very effective for separating zinc from ore 
or concentrate. However, the Commercial aspect of the process requires 
that the ore or concentrate contain less than 0.5% by weight copper and 
preferably less than 0.1% by weight copper; otherwise, significant 
processing complications arise along with consequent plant shutdown and 
equipment clean out. 
Furthermore, none of the above processes work well with all types of zinc 
sulfide containing ores or concentrates. For example, other prominent 
supplies for zinc sulfide containing ore or concentrate include lead/zinc 
ores and zinc/silicate ores. Usually one or more aspects of the prior art 
processes is compromised by the presence of lead minerals or soluble 
silicates. With some of the prior art processes, the presence of soluble 
silicates forms a very gelatinous mass of hydrated silica which renders 
the leach solution unfiltrable. Soluble silicates are more basic than 
insoluble silicates. For example, the orthosilicates Zr.sub.2 SiO.sub.4 
(the mineral Willsmite) or 2ZnO.SiO.sub.2.H.sub.2 O (the mineral 
hemimorphite) are acid soluble, while the metasilicate ZnSiO.sub.3 is 
insoluble. Similarly, there are acid soluble orthosilicates of iron 
--Fe.sub.2 SiO.sub.4 (fayalite) and of magnesium (Mg.sub.2 SiO.sub.4, 
forsterite) while metasilicates FeSiO.sub.3 (gruenerite) and MgSiO.sub.3 
(dinoenstatite) are insoluble. When soluble silicates dissolve, they form 
solutions very supersaturated in quartz (SiO.sub.2), but the precipitation 
of stable quartz crystals requires geologic time frames, and so gelatinous 
silica is formed instead. This gelatinous silica is an impediment to 
liquid solids separation and a serious impurity in zinc plant 
electrolytes. Exemplary processes for the recovery of Zn from Zn silicate 
ores are described in Canadian patent 876,034 and in Kumar et al "Zing 
Recovery from Zawar Ancient Siliceous Slag" Hydrometallurgy, (1986) 
15:267-280. 
The process according to this invention overcomes several of the problems 
associated with the prior art processes in providing a process in which 
high concentrations of sulfuric acid are used to convert zinc sulfide in 
zinc sulfide containing ores. The process is operated at temperatures in 
the range of 90.degree. C. to less than boiling point of the conversion 
solution to convert the zinc sulfide into zinc sulfate crystal monohydrate 
which in the conversion solution forms crystals. Hence, the process, in 
accordance with this invention, provides a novel way to achieve separation 
of zinc sulfate from a H.sub.2 SO.sub.4 treatment solution without 
requiring an electrowinning step. 
SUMMARY OF THE INVENTION 
In accordance with an aspect of the invention, a hydrometallurgical process 
is provided for converting zinc sulfide in an ore containing zinc sulfide. 
The zinc sulfide is chemically converted at elevated temperatures to 
ZnSO.sub.4.xH.sub.2 O which crystallizes substantially in the monohydrate 
form as ZnSO.sub.4.H.sub.2 O in a conversion solution having a high 
concentration of H.sub.2 SO.sub.4. The process comprises: 
contacting the zinc sulfide of the ore with the conversion solution which 
comprises a concentration of sulfuric acid selected from the range of 
about 45% by weight up to about 70% by weight of the conversion solution; 
and at a temperature in the range of about 90.degree. C. to less than 
boiling point of the conversion solution for the selected concentration of 
sulfuric acid; 
maintaining the conversion solution at the elevated temperature and at the 
concentration of the sulfuric acid to ensure continued formation of the 
crystals of ZnSO.sub.4.H.sub.2 O until substantially all available ZnS is 
chemically converted; and separating the ZnSO.sub.4.H.sub.2 O crystals and 
remaining solids of the ore from the conversion solution. 
According to another aspect of the invention, the chemical conversion 
preferentially removes zinc from ores containing other sulfides, such as 
copper sulfide, the preferential chemical conversion of zinc sulfide 
produces H.sub.2 S in the conversion solution which provides a reducing 
condition. The reducing condition in the conversion solution essentially 
precludes chemical conversion of other metal sulfides and in particular 
copper sulfide; hence separation of zinc sulfate from the zinc containing 
ores provides a copper sulfide enriched ore material. 
In accordance with another aspect of the invention, the recovered crystals 
of ZnSO.sub.4.H.sub.2 O may be dissolved in a solution having a low 
concentration of sulfuric acid where the low concentration of sulfuric 
acid may be derived from a zinc recovery electrolytic cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The process of this invention is particularly suited in the treatment of 
zinc sulfide metal ores which contain copper sulfides, lead sulfides, 
silicates or a mixture of copper sulfides, lead sulfides and/or silicates 
with zinc sulfides. The process allows for the zinc to be preferentially 
recovered therefrom without recovery of the copper, lead or interference 
by silicates in the ore. The ore may be either in a finely divided 
concentrate form, a finely divided rich ore or a combination of the two 
and hence the term ore is intended to mean anyone of these alternatives. 
Examples of such mineral bearing ores commonly include chalcopyrite, 
chalcocite, bornite, tetrahedrite, sphalerite, galena, molybdenite, 
pyrite, pyrrhotite and arsenopyrite. The ore is in particle form and is 
preferably ground such that 75% of the finest particles pass 275 mesh; 
i.e., in the range of 50 microns or less. This ensures a finely divided 
material on which the reagents used in the process of this invention 
react. Most copper and zinc ore sources normally include chalcopyrite, 
sphalerite, bornits, pyrite, galena and mixtures thereof. In a preferred 
aspect of the invention, the objective is to recover zinc in addition to 
the conversion aspect of the invention, where such recovery is in the form 
of monohydrate zinc sulfate crystals. 
It is also appreciated that such ores may include precious metals such as 
rhodium, palladium, platinum, silver and gold. Usually such constituents 
are in trace amounts and may not warrant recovery. It has been found that 
these precious metals do not present a problem with respect to the 
processing conversion conditions. Similarly, small amounts of Pb, Cd, As 
and Sb are commonly found in such ores. It has also been found that the 
presence of iron in the ore also does not present any processing problems 
and although most iron sulfide minerals are not reacted, iron in the form 
of marmalite (Zn,Fe)S or pyrrhotite (Fe.sub.1-x S) is converted into 
crystalline ferrous sulphate (FeSO.sub.4) and can be separated from zinc 
sulfate monohydrate in subsequent processing steps familiar to those 
versed in the art. 
The zinc conversion process of the present invention involves the 
production of monohydrate zinc sulfate crystals from the zinc sulfide 
fraction in the ore. Sufficiently concentrated sulfuric acid at a 
sufficiently high temperature is used to yield hydrogen sulfide and to 
convert all the available zinc sulfide. The preferred application is in 
the separation of zinc from copper containing ores and in particular ores 
containing greater than 0.5% by weight and usually greater than 1% by 
weight of copper. As previously noted, such ores are not commercially 
treatable by the Sherritt Gordon pressure leach process of U.S, Pat. Nos. 
4,071,421 and 4,440,569, while at the same time not decomposing or 
converting any sulfidic copper minerals. This absence of reaction with the 
copper sulfides is believed to be due to the presence of the reducing 
H.sub.2 S from the preferential zinc sulfide conversion reaction. 
Although the chemistry in the well known prior art leaching process 
involves the use of sulfuric acid, it is not fully understood. That 
reaction generally proceeds as follows: 
EQU ZnS+H.sub.2 SO.sub.4 (aq).revreaction.ZnSO.sub.4 (aq)+H.sub.2 S(g)(1) 
The reaction proceeds under ordinary conditions, that is at room 
temperature and at low concentrations of H.sub.2 SO.sub.4 ; e.g. 1 molar 
sulfuric acid (98 grams of H.sub.2 SO.sub.4 per liter of leach solution). 
While it has been known that the reaction equilibrium moves to the right 
with increasing acid concentration and temperature, that is, increasing 
zinc sulfate concentration and hydrogen sulfide partial pressure, we have 
discovered that the reaction will go to completion (and not merely to an 
equilibrium) when the acid concentration is high enough to salt out 
(precipitate) a lower hydrate of zinc sulfate and when the temperature is 
sufficiently high to yield a hydrogen sulfide pressure in excess of the 
ambient pressure in the reactor. Under these conditions, where the 
reaction goes to completion (as distinct from reaching an equilibrium) the 
salt produced from zinc sulfide is ZnSO.sub.4.H.sub.2 O through all the 
H.sub.2 SO.sub.4 concentration range of this invention. 
The zinc conversion is therefore believed to proceed as follows at high 
concentrations of H.sub.2 SO.sub.4 and at high temperatures: 
EQU ZnS+H.sub.2 O.revreaction.ZnSO.sub.4.H.sub.2 O+H.sub.2 S(g)(2) 
It has been found that by increasing the acid concentration and 
temperature, a point is reached where the produced zinc sulfate in its 
monohydrate form crystallizes and drops out of solution and surprisingly 
any copper sulfides are not converted, nor do copper sulfates precipitate 
out of solution. Therefore, it has been found that there is a minimum 
sulfuric acid concentration and a minimum temperature at which equilibrium 
of the above reaction exceeds the point where hydrogen sulfide partial 
pressure is i atmosphere and the solution is saturated with zinc sulfate. 
Above these minimum concentration and temperature values, sufficient 
hydrogen sulfide gas is produced and boils off and monohydrate zinc 
sulfate crystals form until all substantially available zinc sulfide in 
the ore is converted to zinc sulfate. By use of the term "substantially", 
it is intended to mean that all zinc sulfide of the ore that is available 
for conversion by the H.sub.2 SO.sub.4 solution is converted on the basis 
of a commercially viable reactor residence time and commercially viable 
extent of grinding and crushing to a sufficiently fine ore particle size. 
We have also determined that operating at extremely high acid 
concentrations and temperatures, such as with the process of 
aforementioned Canadian patent 864,455, is not acceptable because both the 
zinc and copper remain in solution as anhydrous sulfate and unacceptable 
amounts of plastic sulfur and SO.sub.2 are produced. 
The theoretical minimum sulfuric acid concentrations and minimum 
temperature can be calculated empirically using reported data. Theoretical 
data, as applied to the equilibrium of equation (1) in a commercial 
recovery environment are not available, but may be extrapolated from 
measured data reported--L. T. Romankiw and P. L. DeBruyn, "Kinetics of 
Dissolution of Zinc Sulfide in Sulfuric Acid", in Unit processes in 
Hydrometallurqy, (eds. Wadsworth and Davis), Gordon & Breach Science 
Publishers, N.Y. (1964), pp 45-65. It is important to understand, however, 
that these measured data were made on synthetic zinc sulfide precipitates, 
and that natural zinc sulfides are up to 20 KJ per mole more stable. Data 
from Bard, Parsons, and Jordan "Standard Potentials in Aqueous solution" 
published by the International Union of Pure and Applied Chemistry (Marcel 
Dokker, New York and Basel, 1985), pp. 252-253 give the following 
thermodynamic values for zinc sulfide phases: 
______________________________________ 
Phase .DELTA.H.degree..sub.298 (Kj/Mole) 
.DELTA.G.degree..sub.298 (Kj/Mole) 
______________________________________ 
ZnS, sphalerite 
-206.0 -201.3 
ZnS, wurtzite 
-192.6 -185 
ZnS, Precipitate 
-185 -181 
______________________________________ 
The theoretical calculations on precipitated ZnS would indicate that as 
little 20% by weight sulfuric acid at 130.degree. C. and a minimum of 35% 
by weight sulfuric acid at 70.degree. C. would effect such conversion. 
These theoretical calculations are based on solubility data of zinc 
sulfate in sulfuric acid. Based on analysis of this data, it would appear 
that at sulfuric concentrations of approximately 20% by weight and a 
temperature of about 130.degree. C., or approximately 35% by weight 
sulfuric acid at a temperature of 70.degree. C., would convert zinc 
sulfide into zinc sulfate monohydrate which should have presumably 
crystallized and dropped out of the conversion solution. Quite 
surprisingly however, at these lower concentrations of H.sub.2 SO.sub.4, 
no zinc sulfate monohydrate was formed. Any zinc sulfate formed in the 
solution was not enough to saturate the acid conversion solution, so that 
no crystals of zinc sulfate monohydrate appeared in conversion solutions 
of that lower concentration. It would appear that these theoretical 
calculations were not accurate in respect of what we have found is 
required in terms of the minimum concentration of sulfuric acid and 
minimum temperatures to achieve production of the zinc sulfate monohydrate 
which would crystallize in the conversion solution. These differences 
appear to be due to the thermodynamic calculations being somewhat askew 
because the reaction was not as favourable as the theoretical data would 
indicate. The natural ore is far more stable and hence less apt to be 
converted compared to the materials reacted with sulfuric acid on which 
the theoretical calculations were based. The zinc sulfide was made 
synthetically, where the material contained less than 0.006% iron and was 
of size in the range of 0.1 to 0.3 microns. On the other hand, actual ores 
to be treated, in accordance with this process, may be of the above noted 
types and in particular marmatite containing approximately 5% to 10% iron 
and having a particle size of 50 microns or greater. 
Higher concentrations of sulfuric acid and higher temperatures for the 
conversion solution were investigated in order to achieve the process 
conditions of equation (2). By various tests carried out in accordance 
with this invention and as described in the accompanying examples, it has 
been determined that at a temperature as low as about 90.degree. C. and at 
approximately 70% by weight of sulfuric acid in the conversion solution, 
sufficient zinc sulfate is formed which drops out of the conversion 
solution in crystalline form as zinc sulfate monohydrate. At a 
concentration of sulfuric acid of approximately 45% by weight in the 
conversion solution, a temperature of about 130.degree. C. provides 
sufficient zinc sulfate monohydrate which crystallizes and drops out of 
the conversion solution. Hence the process of this invention has an 
operable concentration of sulfuric acid and temperature well above that 
predicted by the theoretical values. Furthermore, it has been found that 
increasing beyond approximately 75% by weight of sulfuric acid also 
results in a commercially inoperable processes, because of the formation 
of plastic sulfur and SO.sub.2 and the conversion of copper into solution. 
Hence the extremely high concentrations and temperatures employed, in 
accordance with the aforementioned prior art, such as in Canadian patent 
864,455, are not applicable in respect of this invention. 
FIG. 1 is a plot of the experimental test results which clearly indicate 
the region in terms of temperature versus concentration of sulfuric acid 
in which zinc extractions greater than 50% can be achieved in 
approximately one to three hours with minimal, if any, generation of 
sulfur. The experimental test results are based on the conversion of ores 
and ore concentrates so that it is believed that the parameters in respect 
of temperature and sulfuric acid concentration can be extrapolated to a 
commercial process to achieve the preferential removal of zinc from zinc 
sulfide containing ores, where other sulfides may be present including 
copper sulfide which is not affected by the conversion process and is not 
crystallized out with the zinc. This processing condition, in accordance 
with this invention, provides a significant advance in the 
hydrometallurgical treatment of ores to remove zinc sulfides for recovery 
and hence provide a treated ore which is now enriched in copper sulfide 
for treatment by other processes, such as that described in applicant's 
copending U.S. patent application Ser. No. 009,844 filed Jan. 27, 1993. 
Based on the region identified in FIG. 1, it is apparent that, at any 
temperature above approximately 90.degree. C. and for a selected sulfuric 
acid concentration in excess of about 60% by weight, conversion of zinc 
can be achieved and for temperatures up to approximately the boiling point 
of the conversion solution for weaker Sulfuric acid concentrations, such 
as in the range of 45% to 55%, conversion can also be achieved. It is also 
understood that the rate of reaction increases measurably if at the higher 
concentrations of sulfuric acid, either approaching the boiling point of 
the conversion solution or in the range of about 130.degree. C. to 
140.degree. C., excellent preferential conversion of the zinc sulfide is 
achieved without impacting on the copper sulfides remaining in the ore. 
It is also apparent that concentrations of sulfuric acid above 80% by 
weight or less than 40% by weight do not produce any commercially 
significant result, either by virtue of poor zinc extractions at less than 
40% H.sub.2 SO.sub.4 or by virtue of generating SO.sub.2 and plastic 
sulfur at greater than 80% H.sub.2 SO.sub.4. Region B is indicated on FIG. 
1 to identify the predominate production of SO.sub.2 which is undesirable. 
Region A indicates the process parameters of the aforementioned Canadian 
Patent 864,455 to Treadwell Corporation which results in the unacceptable 
production of SO.sub.2 and the gummy deposit of sulphur. 
Therefore in accordance with the preferred aspect of the invention, 
practising any of the conditions as set out in FIG. 1, which are within 
the region identified as the zinc extraction region, generates a 
sufficiently high yield of the zinc sulfate monohydrate at equilibrium 
such that the conversion solution becomes saturated with the monohydrate 
form, whereby the zinc sulfate monohydrate commences to crystallize and 
fall out of solution. Providing fresh ore is continuously introduced to 
the conversion solution and the concentrations of sulfuric acid and 
temperature for the conversion solutions are maintained, the conversion of 
zinc sulfide to zinc sulfate monohydrate will continue and provide on a 
continuous basis salt containing the zinc sulfate monohydrate which can be 
later processed for recovery of the zinc. 
It is believed that, due to the presence of hydrogen sulfide gas which 
boils off the conversion solution during the conversion process, the 
conversion of copper minerals and, in particular, copper sulfide is 
prevented by a far poorer equilibrium between copper ions in solution, 
hydrogen sulfide gas, and sulfuric acid. Indeed, any copper ions initially 
present in the solution would be precipitated as copper sulfides. Hence 
the process provides an excellent commercial zinc-copper separation, 
particularly with ores or concentrates containing more than 0.5% by weight 
copper and usually in excess of 1% by weight copper in the form of copper 
sulfides. It is expected that some of the iron, particularly in the form 
of (Zn, Fe)S and Fe.sub.0.88 S might react with the conversion solution. 
It is very doubtful, however, that other types of iron, such as FeS.sub.2 
(pyrite) and FeAsS (arsenopyrite) would be attacked by the conversion 
solution. It is also doubtful that arsenic or antimony would enter the 
conversion solution. Certainly mercury, silver and gold would not enter 
the conversion solution. However, magnesium and calcium minerals would be 
converted and enter the conversion solution, but unlikely any highly 
silicious minerals or quartz. Silicious zinc sulfide ores presented a 
significant prior art processing problem, because of the conversion of 
soluble silicates into gelatinous hydrated silicate substituents which 
interferes or prevents filtration to separate leached zinc from the 
treated ore or concentrate. The process, in accordance with this 
invention, overcomes this problem because in treating silicate/zinc ores 
at the elevated temperature and prescribed range of sulfuric acid 
concentrations, the silicates are marginally hydrated so that the 
silicates remain solid rather than forming a gelatinous mass. Such solid 
form of silicates does not, then, appreciably interfere with the process 
of the zinc sulfide conversion and the falling out of the zinc sulfate 
monohydrate crystals. 
Hence in removal of the crystalline zinc sulfate monohydrate from the 
conversion solution, there may be trace amounts of iron, magnesium and 
calcium, but these minerals can be readily separated from the zinc sulfate 
monohydrate material during the recovery of the zinc from the crystalline 
material. Ideally, the recovered crystalline material, once separated from 
the conversion solution, can be treated with either water or dilute acid 
solution to dissolve the zinc sulfate monohydrate in the form of 
ZnS.sub.4.xH.sub.2 O. The remaining constituents in the crystalline 
material may be insoluble in the dilute acid mixture or water; hence 
providing a further purification of the zinc sulfate before carrying out 
electrowinning or the like to remove or recover zinc from the composition. 
The reaction of equation (2) is endothermic and hence requires the input of 
heat during the conversion which may either be carried out on a batch or 
continuous basis. On a continuous basis or batch basis, heat may be 
introduced to the reactor by various types of heat exchange devices, 
although in view of the very high concentration of sulfuric acid, the 
preferred way of heating the reaction is by submerged combustion. 
The amount of heat needed for this endothermic reaction is far smaller than 
that necessary for boiling down a 15% sulfuric acid solution to 60 to 80% 
sulfuric acid, as previously described with respect to U.S. Pat. No. 
4,710,277. 
The zinc sulfide containing ore may be in the form of a concentrate, a 
finely divided ore or the like. The particle size of the finely divided 
ore is normally in the range of 50 microns to 100 microns. It is 
appreciated that the process will work equally well on various particle 
sizes for the ore and ore concentrate. However as is understood, the finer 
the division in the ore, the faster the rate of reaction in converting the 
available zinc sulfide and as well, the less residence time to achieve 
greater than 50% conversion of the zinc sulfide. Under optimum conditions, 
it is expected that conversions in the range of 80% to 90% can be achieved 
with sufficiently fine ore, temperature and sulfuric acid concentration 
selection. The selection of the upper range of temperature is, of course, 
determined by the boiling point of the conversion solution for a selected 
concentration of sulfuric acid. It is appreciated that, as the sulfuric 
acid concentration increases, so does the boiling point of the conversion 
solution. Conversion solutions having a concentration of sulfuric acid in 
the range of 40% to 50% by weight boil at approximately 120.degree. C. to 
140.degree. C., whereas at sulfuric acid concentrations of 70% to 80% by 
weight, the conversion solution boils in the range of 165.degree. C. to 
about 195.degree. C. It is appreciated, however, that, in achieving 
equilibrium for the reaction of equation (2), sufficient hydrogen sulfide 
is produced that it will tend to bubble off at temperatures below the 
boiling point of the conversion solution. Preferably the hydrogen sulfide 
is removed from the reactor so that the reaction is carried out at 
approximately atmospheric pressure. The reaction could be expedited by 
enhancing the removal of H.sub.2 S from the reaction solution by applying 
a vacuum or using a flushing gas. Lower concentrations of sulfuric acid 
and/or temperature might then be possible. However, the application of a 
vacuum or the addition of a flushing gas to the reactor, which has such a 
high concentration of sulfuric acid, would dramatically increase the 
overall costs in the process and are believed to render it economically 
unviable. 
The hydrogen sulfide gas removed from the reactor may be treated by various 
techniques to either convert the hydrogen sulfide into sulfur or sulfuric 
acid. If converted into sulfuric acid, it can be used to replenish the 
conversion solution. 
The various tests, as carried out in establishing the operable region of 
this process, establish several factors which include laboratory tests and 
indicate that for an economical zinc extraction the zinc should be 
converted by at least 50% within one hour of being subjected to the 
conversion solution. Amounts of sulfur generated, normally in excess of 
0.5 to 1 gram based on the quantities used in the laboratory tests, would 
predict an uneconomic process because of excessive generation of sulfur. 
Experimental Tests 
The following laboratory scale experiments demonstrate the useful region of 
the process parameters involving sulfuric acid concentration and 
temperature. The experimental tests were carried out principally as 
follows. A suitable zinc sulfide ore or concentrate was selected and 
finely divided to approximately 50 microns size. The suitable zinc sulfide 
ore may be sphalerite or bulk concentrates made from zinc copper sulfide 
ores. Copper in the ore may be in equal amounts compared to the weight of 
zinc in the ore and may be less than weights of iron in the ore. For 
example, the ratios of zinc, copper to iron may be 2:2:3. 
Approximately 100 grams of the mineral in 150 mls of Water is placed in the 
reaction flask. Approximately 100 mls of the acid solution of the selected 
sulfuric acid concentration is slowly added to the mixture while mixing. 
The conversion solution was allowed to react with the ore from 1 to 3 
hours, where the temperature of the reaction was maintained at the 
selected temperature. At the end of the selected period of reaction, any 
crystalline material was filtered from the conversion solution and an 
analysis carried out with respect to amount of zinc and other components 
in the conversion solution in the crystalline material and in any other 
solids. The results, in terms of temperature, concentration of sulfuric 
acid and percent conversion is set out in Table 1. From these results, it 
is apparent that acceptable conversions in excess of 50% and minimal 
production of sulfur are identified. 
TABLE 1 
______________________________________ 
ZnS Conversion - Acid Concentration Effect 
Temperature % Zn converted 
Reaction 
Exp. # .degree.C. from Ore Time/Hours 
______________________________________ 
20% Sulfuric Acid 
1 70 5.0 1 
2 100 22.5 3 
30% Sulfuric Acid 
3 70 12.4 2 
4 100 8.0 2 
5 114 19.8 1 
40% Sulfuric Acid 
6 70 7.9 1 
7 100 18.0 1 
8 114 22.5 1 
9 120 23.7 1 
10 120 24.2 1 
45% Sulfuric Acid 
11 70 2.0 3 
12 100 23.5 1 
13 127 51.2 3 
14 124 76.0 1 
55% Sulfuric Acid 
15 70 13.8 1 
16 100 47.7 1 
17 132 89.0 1 
60% Sulfuric Acid 
18 138 97.4 1 
65% Sulfuric Acid 
19 70 35.6 1 
20 100 76.9 1 
70% Sulfuric Acid 
21* 136 91.5 1 
75% Sulfuric Acid 
22* 70 20.6 
23* 100 77.7 1 
24* 134 91.8 1 
______________________________________ 
*Excessive amount of sulfur produced in excess of 0.5 to 1 gram. 
In accordance with these experimental results, the process parameters for 
an economically viable process have been defined which surprisingly and 
with repeatable success provide a system for recovering zinc from zinc 
sulfide ores, which may include copper sulfide, where the resultant 
material can be solubilized to provide a solution from which zinc may be 
electrowon. When the ore includes copper sulfides, the process provides 
ore now enriched in copper sulfide which may be processed to recover 
copper therefrom. 
Although preferred embodiments of the invention are described herein in 
detail, it will be understood by those skilled in the art that variations 
may be made thereto without departing from the spirit of the invention or 
the scope of the appended claims.