FI122453B - A method for controlling impurities in the zinc manufacturing process - Google Patents

Description

METHOD FOR CONTROL OF IMPURITIES IN THE ZINC MANUFACTURING PROCESS

FIELD OF THE INVENTION

The invention relates to a process for controlling and removing impurities in the manufacture of zinc. The zinc sulphide concentrate always contains impurities, the removal of which before zinc electrolysis requires its own purification steps. When the iron in the concentrate is precipitated as goethite, some of the impurities, such as fluorine and germanium, are adsorbed there. The amount of impurity, especially fluorine, in the zinc sulphate solution to be led to the zinc electrolyte trolysis can be adjusted to a level suitable for electrolysis by precipitating the desired portion of the concentrate iron as goethite and the remaining jarosite as the easiest way to precipitate the iron.

BACKGROUND OF THE INVENTION

Zinc recovery from zinc sulphide concentrate has traditionally been accomplished by first concentrating the concentrate to roasting, where the sulphides of the concentrate are roasted into oxides. The torch (zinc oxide) is then subjected to a leaching to obtain a zinc sulphate solution which, after solution purification, is subjected to zinc-20 electrolysis. The leaching of ferrites contained in the roast is continued by strong acid polishing followed by iron precipitation. Nowadays, often part of the concentrate is leached without roasting, whereby leaching of the concentrate is usually combined with a strong acid leaching step. Most commonly, iron is precipitated from the process as jarosite. Other possible forms of precipitation are goiter and hematite.

The most well-known and used form of iron precipitation is still jarosite. The advantage of the jarosite precipitation is that the iron is removed as the basic sulfate (A [Fe3 (SO4) 2 (0H) 6]), thereby treating the sulfate balance 30 of the zinc process and reducing the need for neutralization. JAROSITE also precipitates substances harmful to zinc electrolysis, particularly arsenic, antimony and germanium. The disadvantage of the jarosite process is that the jarosite precipitates very little fluoride 2. This prevents the use of raw materials with a high fluoride content. The goiter precipitation has the advantage of its ability to remove fluoride. In particular, fluorine is a so-called. electrolytic poison because it corrodes the aluminum cathode. The disadvantage of goiter precipitation is that it always precipitates some valuable metals such as zinc and copper.

Hematite Jarositis Göteitis

The resultant compound a-Fe 2 O 3 Me-Fe 3 (SO 4) 2 (O H) 6 a-FeOOH, β-FeOOH

Fe, w-% 50 -60 30-35 40-45

Zn, wt% 0.5-1.0 3 ^ 5 2 ^ 3 Amount (dry) 1.8 t / t Fe 3.1 t / t Fe 2.4 t / t Fe Amount (wet) 2.0 t / t Fe 4.8 t / t Fe 4.0 t / t Fe

Humidity 10% 35% 40%

Today, only a few zinc plants use goutite precipitation. Hematite is the most persistent and compact form of iron precipitation, as shown in the table above, but so far only one production-scale plant has ended up in hematite precipitation.

The harmful effects of impurities, especially fluorine, in zinc electrolysis have long been recognized. In process 15 of CA 892479, fluorine is already removed from the zinc ore prior to ore enrichment by pre-leaching with a dilute sulfuric acid solution. This results in a low-fluorine zinc concentrate, which can be processed by a conventional roast-leach electrolysis process.

AU Patent Publication No. 769984 describes a direct solution of the zinc sulphide concentrate, the so-called. stand-alone, in which the entire concentrate is led directly to the leaching without roasting. The leaching is carried out in two stages, whereby the entire amount of concentrate is fed to the first stage. The leaching of the first stage precipitate is continued in the second leaching stage by introducing therein 25 parts of the first stage solution and electrolysis return acid. The precipitate from the second leaching stage contains some iron, concentrate sulfur and lead. The iron 3 can be precipitated, for example, as jarosite. The solution from the second leaching step is led to the first leaching step, and the solution from this step is mainly electrolysed through iron goiter precipitation and solution purification. It is stated in the publication that the goethite precipitate can be used to purify the zinc sulphate-5 solution not only from iron but also from arsenic, antimony and other impurities. According to Table 1, about one third of the iron entrained in the concentrate is eliminated with the goiter. About 6% of the zinc content of the concentrate is eliminated with goiter, with a maximum zinc yield of only about 94%. There is no mention of copper.

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In an article by Torfs, K.J. et al., "The Union Miniere Goethite Process: Plant Practice and Future Prospects", Proceedings of the Second International Symposium on Iron Control in Hydrometallurgy, Dutrizac J.E. et al., Eds, Ottawa, Canada, October 20-23, 1996, pp. 135-146, it has been mentioned that the fluoride removal capacity of goutite 15 is up to about 10 mg per 1 g / Fe. In addition, goiter also strongly adsorbs germanium.

EP 1,597,403 discloses a zinc leaching process in which the zinc concentrate is subjected to a neutral leaching and, after leaching, the solid is led to the next leaching step, which is also fed a zinc concentrate. Zinc concentrate and roasting ferrite are dissolved in two steps, the conditions of which are such that, in addition to solubilization of the concentrate and ferrite, iron precipitates as jarosite.

PURPOSE OF THE INVENTION

The ore pre-leaching described above occurs even before the ore is enriched by pre-leaching the ore under conditions where only the fluorine content of the concentrate can be reduced. The purpose of the present process is to combine the advantages of the various iron precipitation processes such that a large portion of the iron 30 is precipitated as jarosite, but the amount of impurities is controlled by precipitating the desired portion of iron as goethite.

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SUMMARY OF THE INVENTION

The invention relates to a process for removing impurities, especially fluorine, in a precious metal-containing sulphide concentrate, whereby the iron contained in the concentrate and dissolved in the precursor is precipitated in controlled proportions both as jarosite and goitite prior to passing the precious metal solution to solution purification.

According to one embodiment of the process according to the invention, the sulphide concentrate is a zinc concentrate.

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The process according to the invention is characterized in that the first part of the iron is precipitated as jarosite and the second part as goitite.

According to one embodiment of the invention, a portion of the sulfide concentrate is roasted to oxide, whereupon it is used as a neutralizing agent in various iron precipitation steps. According to another embodiment, Waelz oxide is used as the neutralizing agent. According to another embodiment, the neutralizing agent is lime or limestone.

According to one embodiment of the invention, the solution resulting from concentrate leaching is subjected to sulfur flotation.

According to one embodiment of the invention, at least a portion of the iron is precipitated as jarosite in the concentrate leaching step.

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According to yet another embodiment of the invention, some of the iron is precipitated as jarosite after foaming of the sulfur.

It is typical of the process according to the invention that after the goiterite precipitation 30 the solution containing the precious metal is led to a neutral solution.

Typical for the process according to the invention is 10-50% iron to be precipitated as goethite.

5

According to one embodiment of the invention, in addition to fluorine, the impurity to be removed is germanium.

5 PHOTO LIST

The method according to the invention is illustrated by the following schematic diagrams, wherein Figure 1 shows a flow diagram of a leaching solution of the sulfide concentrate according to the invention, and Figure 2 shows a flow diagram of another solution of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As stated above in the prior art, iron is precipitated as jarosite in most zinc plants. If the zinc concentrate is rich in fluorine and leaching of the roast and / or concentrate has been carried out under atmospheric conditions (non-pressurized reactors and a leaching temperature of 80 to 105 ° C) and the iron is precipitated as jarosite, fluoride may not be sufficiently removed with jarosite. The object of the process of the present invention is to utilize various iron precipitation methods, in particular in the process of making zinc, by controlling the amount of substances harmful to electrolysis, such as fluorine, in the zinc sulphate solution by doping the first part of iron as jarosite and the second part as goitite. The amount of iron to be doped as goethite depends on the amount of fluorine and other impurities present in the concentrate and is in the range of 10-50%. It is not advisable to remove all iron in 25 goutite, as jarosite precipitates significantly less valuable metals such as zinc and copper than does goutite. Jarosite takes care of the sulphate balance, meaning that some of the sulfur in the concentrate leaves the process as jarosite. In addition, jarosite precipitates under more acidic conditions than goethite, thus saving on neutralization costs.

The method according to the invention will be described in more detail with reference to the flow diagrams below. Flow diagrams show the treatment of zinc sulphide concentrate 30 6, but the method is also applicable to other precious metal sulphide concentrates which require iron precipitation in the treatment of which impurities such as fluorine present further problems. If part of the concentrate has been roasted to zinc oxide, this roast is preferably used as a neutralizing agent in iron precipitation steps. Waelz oxide is a preferred neutralizing agent because it does not contain roast ferrites. In the case of leaching of the sulphide concentrate alone, without being combined with the leaching of the roasted material (the so-called stand-alone process), another suitable substance, such as Waelz oxide, lime or 10 limestone, must be used as the neutralizing agent.

The flow diagram of Fig. 1 illustrates a process in which part of the concentrate is leached directly without roasting and part is roasted. The leaching of concentrate and roast is part of the overall zinc recovery process, which, after 15 leaching, involves solution cleaning and electrolytic zinc recovery. In the leaching step of the sulfide concentrate, the leaching is carried out with the aid of the acid coming from the electrolysis. Also fed to the same leaching step is a slurry of the neutralizing leaching step 4, i.e. the undissolved precipitate, which mainly contains the ferrites of the roasting and iron hydroxide precipitated from the solution. Oxygen-containing gas is also introduced into the concentrate leaching step. When the conditions of the leaching step are adjusted, the iron in both the concentrate and the ferrites, as well as the iron hydroxide supplied with the neutral leaching leach, dissolve, but with dissolution, the iron begins to precipitate as jarosite. In the conventional jarosite process, iron is precipitated at a temperature near the boiling point of the solution. The free acid is neutralized to 3-5 g / l H2SO4 (optimum pH 1.5). The iron content of the zinc sulphate solution is 20-35 g / l. In order to obtain a substantially crystalline form of jarosite with advantageous settling properties, potassium, sodium or ammonium ions are also introduced into the solution. The precipitation of iron as jarosite 30 occurs according to the following reaction, wherein A may be an alkali or ammonium ion: 7 3 Fe 2 (SO 4) 3 + A 2 SO 4 + 12 H 2 O 2 A [Fe 3 (SO 4) 2 (O) 6] + 6 H 2 SO 4 (1)

The acid formed in the jarosite precipitation reaction is consumed by concentrate leaching.

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Each leaching step usually includes several mixing reactors, and the slurry suspension is transferred from one reactor to another in an overflow. Thus, it is possible within the step to adjust the dissolution conditions in the first reactors to favorable conditions and in the latter to favor the precipitation of jarosite, although the dissolution reactions still continue there. When the reactor already has jarosite deposition cores, it further facilitates precipitation.

After the concentrate leaching step 1, the slurry is led to a flotation 2, where the sulfur in the concentrate is foamed to a sulfur concentrate, and the flotation residue contains an isosite precipitate. The zinc sulfate-containing solution, with a further part of the iron present, is fed to the second iron precipitation step 3, where the remainder of the concentrate iron is precipitated. The iron is precipitated at this stage as goethite and is carried out by using zinc pellet or Waelz oxide as the solution's neutralizing agent and supplying the solution with an oxygen-containing gas. A precondition for goutite precipitation is that the amount of trivalent iron (Fe 3+) in the solution is low, in the order of 1-5 g / L. In direct concentrate leaching, it is possible to change the Fe 2+ / Fe 3+ ratio of the solution by adjusting the driving mode and this is utilized in the process of the invention. Adjustment of the driving mode provides a solution suitable for goutite precipitation without a separate iron reduction step or dilution of the leaching solution, the mother liquor, to obtain a suitable concentration of Fe 3+. The precipitation of goiter is carried out at a pH of from 2 to 4, preferably from 2.5 to 3.8, and at a temperature of 70 to 90 ° C, whereby no iron hydroxide precipitate is formed substantially.

30 The precipitation of goiter occurs according to the following reaction: 2FeSO 4 + 0.5 O 2 + H 2 O + 2ZnO 2 FeOOH + 2 ZnSO 4 (2) 8

The precipitation results in a goethite precipitate which is co-precipitated with substances harmful to zinc electrolysis such as fluorine, arsenic, antimony and germanium. If desired, some of these metals may be recovered from the precipitate in some manner known per se. The formed iron deposits, jarosite and goethite 5 can be combined with each other and lead to disposal. Between the steps, liquid-solid separation is always performed, but is not described in more detail in the diagram.

The solution from the gothite precipitation step 3 is led to the neutral leaching step ίο 4, where the solution is further neutralized with zinc pellet, whereupon the remaining iron is precipitated as Fe (OH) 3 as iron hydroxide. As the trumpet dissolves, the zinc content of the solution increases accordingly. The so-called zinc sulfate solution from the neutral leaching step. a crude solution which is passed through solution purification to electrolysis. The neutral leaching step ensures that the goethite precipitation step can be carried out under conditions where iron is not significantly precipitated as ferric hydroxide. The precipitate of the leaching step comprises a small amount of ferric hydroxide and the insoluble ferrites and is passed to the leaching leaching step 1 as described above. The zinc sulphate solution to be electrolysed must have an iron content of less than 20 20 mg / L and to achieve this the pH at the end of the leaching step must be adjusted to more than 4, resulting in a small amount of ferric hydroxide.

Flow diagram 2 shows a process similar to that of Figure 1, but in this case the conditions of concentrate leaching step 5 are set (acid content 20-30 g / L) so that jarosite does not yet precipitate in the leaching step. After the leaching step, the slurry is led to a flotation 6 to obtain a sulfur concentrate and a lead containing fraction and a solution which is first led to the jarosite precipitation step 7 and then to the goitite precipitation step 8.

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EXAMPLE

In the experiments performed, the iron was precipitated from the zinc sulphate solution as either jarosite or goethite.

In the first experiment, iron was continuously precipitated as jarosite during the concentrate leaching step. The sulfuric acid content of the solution was 15 g / L at the beginning of the six-day test period and was reduced to 10 g / L after two days. The solution had a fluorine concentration of 80 mg / L at the beginning of the precipitation and after a six day precipitation period the fluorine content had dropped to 60 mg / L. Thus, 25% of the fluorine had precipitated with the jarosite from the solution.

In another experiment, iron was precipitated from a zinc sulphate solution in a batch experiment as goethite, whereby the pH of the solution was adjusted to 3.7. The iron content of the solution was 217 mg / L at the beginning of the experiment and 0.7 mg / L at the end. At the onset of precipitation, the solution had a fluorine content of 40 mg / L and after a seven hour precipitation period, 5 mg / L, or 87% of the fluorine, had been precipitated from the solution.

From the above experiments, it is seen that the amount of fluorine to be removed from the zinc sulphate solution can be controlled by selecting the proportion of iron to be precipitated as jarosite and goethite.

Claims (12)

A process for removing impurities, in particular fluorine, from a value-metal-containing sulfide concentrate and for controlling its amount in connection with the dissolution of the concentrate, characterized in that the iron contained in the concentrate and which has dissolved in the solution is precipitated both as jarosite and which poured in regulated condition before the value-metal-containing solution is led to solution purification. Process according to claim 1, characterized in that the sulfide concentrate is a zinc concentrate. Method according to Claim 1 or 2, characterized in that a first part of the iron is precipitated as jarosite and a second part is cast. 4. A process according to claim 1 or 2, characterized in that part of the sulphide concentrate has been roasted to oxide, whereby it is used as a neutralizing agent in the various precipitation phases of the iron. 5. A process according to claim 1 or 2, characterized in that Waelz oxide is used as a neutralizing agent in the various precipitation phases of the iron. Process according to Claim 1 or 2, characterized in that lime or limestone is used as neutralizing agent in the various precipitation phases of the iron. Process according to claim 1 or 2, characterized in that flotation of sulfur is carried out on the sludge from the dissolution of the concentrate. Method according to claim 1 or 2, characterized in that at least part of the iron is precipitated as jarosite in the dissolution stage of the concentrate. Method according to claims 1 and 7, characterized in that part of the iron is precipitated as jarosite after the flotation of sulfur. Process according to Claim 1 or 2, characterized in that, after the precipitation of the ingot, the valuable metal-containing solution is led to a neutral solution. Method according to Claim 1 or 2, characterized in that the amount of iron to be precipitated as cast is 10 - 50%. 12. A process according to claim 1, characterized in that in addition to fluorine, germanium is removed as a contaminant.