CA1115024A

CA1115024A - Process of hydrometallurgical treatment for eliminating impurities from a solution containing dissolved metals

Info

Publication number: CA1115024A
Application number: CA302,859A
Authority: CA
Inventors: Jacques De Laever; Serge R. De Lockerente
Original assignee: Hollux SA
Current assignee: Hollux SA
Priority date: 1977-05-09
Filing date: 1978-05-08
Publication date: 1981-12-29
Also published as: PT68014A; BR7802920A; IE46818B1; IT7823057A0; AU3591278A; FR2390504A1; NO781608L; FI781443A; GB1597784A; PT68014B; AR219748A1; PL206693A1; BE866554A; JPS5413416A; DE2820059A1; NL7804962A; ES469620A1; IE780909L

Abstract

ABSTRACT OF THE DISCLOSURE

A process of hydrometallurgical treatment for removing metal impurities from zinc sulfate solution containing Fe+++ ions includes adding to said solution a silicate capable of forming dissolved silicic acid in situ, and a neutralizing base to provide a pH between 1.5 and 4.5. The metal impurities are precipitated as a solid residue on a base of iron silicate.

Description

This invention relates to a process of hydrometallurgical treatment for eliminating metal impuri~
ties from a solution containing dissolved metals to be ex-tracted, more particularly for eliminating iron impurities ;
from a zinc sulfate solution.
In hydrometallurgy, there are three mainindustrial processes for selectively eliminating iron as an easily decantable and filterable residue from zinc sulfate solutions issuing from attack of oxidized zinc ores, more 10 particularly roasted zinc ores. i This residue is essentially made of jaro-site, goethite or hematite according to the process involved.
These processes have, however, the draw- ~ -back that such residues have generally no commeraial or indus-trial interest and even raise important storage problems in respect to environment pollution. ;
Moreover, these processes involve expen-sive and cumbersome plants, when making allowance for flow rates and retention times involved.
One of the main purposes of this inven-tion consists of providing a process of very simple concep-tion, allowing to produce a residue containing impurities ' ', ~, .~ ~ ' '.

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to be eliminated and able to be directly used, without any further important treatment, in some inaustrial applications or as raw material in the manufacture of industrial products, for example building materials.
The present invention provides a process of hydrometallurgical treatment for removing metal impurities from zinc sulfate solution containing Fe+ + ions which comprises adding to said solution a silicate capable of forming dissolved silicic acid in situ, and a neutralizing base to provide a pH between 1.5 and 4.5 whereby the metal impurities are precipitated as a solid residue on a base of iron silicate.
This invention also relates to the residue so obtained, when used for example as embankment or as aggregate in cement or concrete.
Other details and features of this inven- -`
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tion will become apparent from the description such as he-reinafter given, by way of non limitative examples, of seve-ral particular embodiments of the invention, with reference to annexed drawings.
Figure l is a block diagram of a first embodiment of a process according to the invention.
Figure 2 is a block diagram of a variant of the embodiment of the process according to the invention illustrated by figure l.

In figures, same reference numbers rela-te to identical or similar elements.
This invention relates in general to a process of hydrometallurgical treatment for eliminating metal impurities from a solution containing dissolved metals to be extracted as metal form.
More particularly, this invention relates to a process for eliminating metal impurities, more particu-larly iron, from a solution of zinc sulfate to be submitted to an elect~ysis for extraction of zinc as a metal state.
This solution is usually obtained from an attack of a re-sidue of an oxidised zinc ore by means of sulfuric acid, mo-re particularly roasted zinc ore coming from lixiviation of this ore.
This process is characterised in that to said solution, a silicate, such as an alkaline-earth metal or zinc silicate is added, so as to form in situ dissolved silicic acid and to precipitate metal impurities at a p~ bet-ween 1.5 and 4.5, forming a well filterable and easily washa-ble silicate residue.
When a zinc silicate is used, the solu-~s~z~

tion is first maintained at a pH not above than 1.5 so as to allow this zinc silicate being quickly dissolved, and then an alkaline-~earth metal silicate, preferably calcium silica-te, and/or lime is added to bring up and maintain the pH
of the solution between 1.5 and 4.5 and to in~uce precipita- -tion of impurities, forming a solid silicate residue mainly made of iron silicate.
In same cases, oxidised zinc ore can al-so be used to bring the pH to the value not above than 1 5, the formation of silicic acid being then obtained by addition of an alkaline-earth metal silicate which then precipitates said impurities, formin~ said solid silicate residue, when the pH is maintained between 1.5 and 4~5 ; this may be carried out by using an basic enough silicate and/or a lime admission.
In other cases, only an alkaline-earth metal silicate may be used, more particularly calcium silica-te, since such silicates are still enouyh dissolved at a pH
between 1.5 and 4.5 and thus allow precipitation of ions to be eliminated, after dissolution of these silicates.
The real amount of silica dissolved in the solution remains always extremely reduced since as sili-cate is dissolved, it reacts in a substantially instantane-ous manner with iron and possibly other metal ions to be eli-minated, forming a solid silicate residue, when the pH is maintained between 1.5 and 4.5.
The preferential-limits so as to cause .
precipitation in the solution are in the range of 2.5 to 3.
In order to prevent a too large excess of dissolved silica after said precipitation, it is lesira-ble not to exceed the stoechiometrical amount of added sili-- 5 ~ 2f~

cate with respect to impurities to be eliminated.
Figure 1 more clearly illustrates the va-rious steps ~f a particular embodiment of the process accor-ding to the invention, such as applied to extraction of zinc from oxidised zinc ore This process consists of subjecting ore 1 to an attack by means of sulfuric acid 2, called "return acid", mainly coming from electrolysis of zinc sulfate for precipitating zinc metal at the cathode~ this electrolysis being not illustrated by the figures.
Said attack is known and may be made in one or two steps. In the ca~ of an one-step attack, such as illustrated by figure l~an ore excess is used so as to ob-tain an attack ending in neutral medium.
In II, one separates by decantation, on the one hand a solution 3 containing zinc sulfate which, af-ter a puriication step ~not shown) is sent to above-mentio-ned electrolysis, and on the other hand, a residue 4, gene-rally called "neutral residue" , having a concentration of 200 to 400 gr/l of solids for example, which is subjected to a hot acid attack at a temperature generally between 85 and 95C by means of more concentrated acid in order to main-ly dissolve zinc ferrites. This acid attac~ of the neutral residue may take place in one or more counter-current steps Figure 1 shows a two-step attack. The latter comprises attack of the neutral residue ~ in III by means of more concentrated acid 10 and a return acid admis-sion 2, separation by decantation inIV ~a dissolved iron-loaded solution 5 and submitting thickened portion 6 to a new attack in V, being generally more acidthan in the . 6 preceding step , by means of return acid 2 and a ma~e-up of fresh concentrated acid 7, for example about 98% acid The reaction products are then subjected to a filtration in VI.
The filtration cake is treated with wash water 8 and a solid residue 9, rich in lead and noble metals possibly present in ore 1 is obtained.
A portion 10 of the filtrale, forming said more concentrated abid, is recycled to attack in III, while the other portion 11, diluted with wash water 8, at the same time as solution 5, at a resulting acidity of 5 to 250 gr/
1 of H2S04, advantageousl~ of 30 to 60 gr/l of H2S04, prefera-bly of 30 to 45 gr/l of ~2S04, according to the invention, are subjected to a neutralisation in VII with a new amount 12 of oxidised zinc ore until a pH which is not a~ove 1.5, prefera-bly comprised between 0.5 and 1.5,is obtained.
By decantation, a residue 13 is separa-ted in VIII, this residue being optionally sent back to the neutral residue attack in III for solubilizing zinc ferrites~
Dissolved silica, in the silicic acid sta-te, is formed in IX, according to the invention, in the acidsolution made of the overflow 14 coming from above mentioned decantation, by addition of a hasic enough silicate 15, such as calcium silicate, for example as a slag, while main~ning the pH value between 1.5 and 4 5, preferably between 2.5 and 3, thus precipitating a silicate residue containing impurities, in particular iron.
Whèn carrying out this precipitation in oxidising medium, for example b~ admitting a~ or oxygen 16 and/
or optionally an oxidising agent 17, such as Mn 2' Fe is al-so precipi~ated,after oxidation in Fe ~ , which would be pre-. ~ .

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sent in the solution The attack time in IX is generally about 1 -to 4 hours and essentially depends on the presence of Fe ions.
The reaction product formed in IX is sub-jected to a decantation in X. The thickened portion 18 is then filtered in XI, followed by a treatment with wash water 19 The overflow 20 coming from decantation in X, as well as the filtrate 21 diluted with wash water 19 join the solution 3 which after a final purification known per se is submitted to electrol~is for zinc extraction.
The decantation in X could possibly be deleted in Many cases since generally the concentration in solids is relatively high. The reaction product formed in X
would be then immediately subjected to the filtration in XI.
The filtration cake 22 ormed in XI is a solid silicate residue containing substantially all the impurities to be eliminated from the zinc sulfate solution to be submitted to electrolysis, in particular iron, antimony, arsenic, alumini~am, tin and germanium which are generally present in a zinc ore.
Figure 2 relates to a variant of the em-bodiment illustrated by fig~e 1, which is dif~erent from the latter due to the fact that the hereinabove described attack IX taXes place without prior separation of a residue co-ming from a neutralisation VII.
In this va~iant, one advantageously uses in the above steps VII, IX, a basic enough silicate of an alka-line-earth metal, in particular of calcium, as a slag, or a silicated zinc ore, optionally completed with slag, zinc oxi-.... . .
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de and/or lime, so as to precipitate impurities, forming a solid silicate residue which is then separated in the same way as in the embocliment illustrated by figure l.
Hereinafter some la'^oratory tests are described, which were carried out to determine the chemical wor-king conditions of the process according to the invention, Example_l 300 ml of a lixiviation solution con-taining lO gr/l of Fe, 150 gr/l of Zn and 5.4 gr/l of Mg were added together with 29 gr of slag into a reactor con-taining 100 ml of the same lixiviation solution but without iron, The addition of reactants was regulated so as to obtain a pH ~ 5 during the reaction, The reaction mixture at 70-80C
was stirred with a helix.
The warm mixture was filtered under vacuum, The filtrate composikion was as follows : 149.8 gr/l of Zn ; 7 gr/l of Mg ; 0.05 gr/l of Al ; 0~7 gr/l of Fe, Example 2 ~ 300 ml ofalixiviation solution containing 20 10 gr of Fe, 150 gr of Zn, 8.4 gr of Mg and a trace of Cu we-re added together with 26 gr of slag into a reactor contai-ning lO0 ml of the same lixiviation solution but deprived of iron.
The addition of reactants was regulated so as to maintain a pH ~ 1.5 during the reaction. The mixtu-re was maintained in an oxidising condition by air injection, The whole was stirred at a temperature of 70-80C, The warm mixture was filtered under va-cuum, The filtrate composition was as follows : 149.8 gr/l of ,~ 'Z~L

Zn ~.3 gr/l of Mg and 0 1 gr/l of Al.
Example 3 300 ml of a lixiviation solution contai-ning 10 gr of Fe, 150 gr of Zn and 11.4 gr of Mg were added together with 28 gr of slag into a reactor containing 100 ml of the same lixiviation solution but deprived of iron The addition of reactants was regulated so as to main~ain a pH ~ 1.5 during the reaction. A small amount of organic or inorganic oxidising agent was added and the reaction medium at 70-80C was stirred with a helix. The warm mixture was fil-tered under vacuum. The filtrate composition was as follows 149.8 grjl of Zn ; 10.4 gr/l of Mg and 0.2 g/l of Al These -three examples allow to draw the following conclusions :
- substantially the whole iron~ as well divalent as trivalent, may be eliminated - the Mg concentration is maintained be-low a determined limit.
A practical example of the process accor-ding to the invention on an industrial scale, and in continu-ous, based on the block diagram of figure 2 in combination with that of figure 1, was developped as Eollows.
19.5 metrical tons/hour of roasted zinc ore having the following analysis :
Zn Fe ~b A123 60% 9% 1 77% 0.249%
were contacted in I with 79 cubic meters/hou~of return acid

2 having a concentration of 176 gr/l of H2S04.
The thickened residue in II containing 1~

zinc ferrites, at a concentration of 400 gr/l of solids, is submitted in III to a hot acid attack, at a temperature in the range of 9O to 95C where it is contacted with recycled acid lO forming the main portion of the filtrate coming from the filtration in vI,and with areturn acid admission 2, so as to obtain in III a mean acidity of about 30 gr/l. The thicke-ned residue 6 deprived of a portion of ferrites is retreated in V with a more acid solution at a rate of lOO to 500 gr/l, formed by return acid 2 and an admission 7 of about 98% concen-trated fresh sulfuric acid.
The residue 9 resultin~ from this attackis substantially deprived of the whole zinc and, on the con-trary, contains the whole lead and the noble metals which are optinaliy present in the treated ores.
The other portion 11 of the filtrate, diluted with wash water 8, is added to the overflow 5 so as to obtain an aci.cl solution containing about 3O to 40 gr/l of H2SO4-This solution which contains, in addition to æinc, a high concentration of dissolved iron and of othernoxious metals for electrolysis, is then submitted to the proper treatment according to the invention for precipita-tion of these iron and noxious metals, maintaining substan-tially the whole zinc in solution. According to the second variant illustrated by figure 2, one adds to this solution a calcium silicate 12,75, taking care to bring up and main-taln the pH between 1.5 and 4.5, if necessary by addition of -other basic substances, such as lime, and optionally anoxidi-sing agent 16, and oxidising air 17 is injected, which more-30 over improves the evaporation of the solution, thus reducing ~.
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the presence of dissolved CaSO4.
The precipitation reaction of Fe is very fast and is substantially complete after 1 hour It is, however, of interest to extend the reaction term in IX up to about 4 hours in order to oxidise ~e susceptible to is-sue for example from the presence of sulfur in the ore and the slag. As a matter of l~act, oxidation of Fe in Fe is a relatively slow phenomenon. Care has to be taken at the .
end of the iron precipitation that the pH of the obtained solu-tion is near that of the solution coming from the neutral li-.
xiviation in I, so that the solution is directly usable in the purification to which it has to be subjected before electro-lysis The relativPly low acidity, of a pH of about 3 to 4, of the solution to be filtered in XI allows to use less expensive materials than in known purification pro-cesses. The efficiency of zinc extraction is about g9 5% ``
It has been found that the silicate resi-: due so obtalned is quite will filterable and allows an inten-: 20 sive counter-current washing to be made.
Thi.s residùe is a very stable and non ~o-xlc product and fur~lermore pres~nts p-o~ties such as~according to the invention, this product can be used as embankment, as aggregate in cements and as starting material for manufactu-ring building blocks.
This residue finds a particularly inte-resting application in t~.e cement field due to the .fact that -~
it contains iron as a form allowing an easy handling on an industrial scale.

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Moreover, the presence in such a product :;
of ~ypsum coming for example from the neutralisation with slag of the sulfuric acid present in the acid solution, of silica mainly is~u.ing Erom slay but also susceptible to par-tially issue from the treated solution,and of alumina, ma~
kes the use of this product particularly useful in cement field as additive to starting materials before introduction of the latter into the furnace or after passing through the latter.
Thus the invention also includes the pro-duct formed by the dried or not filtration cake, resulting from the described process, and its valuable application, in particular in cement field and obviously in any industry susceptible to consume a product containing the above mentio-ned constituents.
It has also been found , such as resul- ~;
ting from hereinabove given laboratory tests, that the magne- .: .
sium concentration in the solution to be subjected to zinc electrolysis may be decreased by means of a coprecipitation with silicate derivatives and in particular with iron silicate As already mentioned, the silicic acid concentration in the medium is a].ways very low and is subs-tantially constant during the whole precipitation step.
Due to the high reactivity of silicakes of.alkaline-earth metals, in particular o~ the slag, it is not necessary to operate the reaction by varying the pH con-ditions and thus to work in a batchwise manner. The silicic acid such as produced is immediately precipitated by iron, .
carrying along other noxlous mekals to the later electroly-sis, such as magnesium, as also already mentioned hereinabo-ve.

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- J~ 2 The first neutralisation step till a pH
of about 1.5 may advantageous~ becarried out with an oxidised or silicated zinc ore, which thUs still increases the interest of the process. If zinc oxide is used, the ~uality of the final silicate residue is moreover considerably reduced.
Besides, the introduction of zinc silica-te in this first step is a very practical means for extrac-.in~ zinc from OreS which are treated with difficulty by `!
means of other processeswhich are specially provided for sili- :

cate ores~
As a matter of fact, it has been found that as long as the medium pH ~oes not e~rceed 1.5, the reac-t.ion of zinc silicate dissolution is fast and complete, and the residue so formed is ~uite well filterable. This is not generally the case in solutions of a higher pH. As an example, at a pH of about 3, if the solution contains aluminium, the latter will tend to precipitate as a silico-alumina form at the surface of zinc silicate grains and will lnhibit the dis- .
solution reaction of the latter. It has been thus found that it is not possi.ble to use, on an industrial scale, a silica-ted zinc ore instead of a silicate of alkaline-earth metals to cause the precipitation of metal impurities of solutions at a pH higher than 1 5 without substantially reduce the reac- ;:
tion rate and the zinc yield.
This problem does not exist with slag having a high calcium silicate content due to the high reac-tivity of this silicate at p~s between 1.4 and 4.5.

It is also to be noted that the inven-tion is not limited to the purification of zin~ sulfate so-lutions issuing from lixiviation of zinc ores or to the hereina~

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bove described variants Thus many other hydrometallurgy appli-cations could be contemp]ated for precipitation of noxious impurities as a silicate residue, optinally according to other variants of the process of the invention than those ex-plicitly described hereinbefore The acidity conditions in the precipita-tion of the silicate residue will have for example to be adap-ted to the kind of the impurities to be eliminated and to the kind of metal to be extracted by electrolysis or of the acid issued, or optionally to another extraction operation so as to obtain an as selective as possible separation.
Some steps of the variants of the pro-cess, such as illustrated by the ~lgures, could be divided in two or more stages. This could be for example the case for attack in IX where it could be first possible to essentially provide the formation of siliclc acid by addition of basic substances, such as basic enough silicates and the separation ; of the basic substance which is not completely attacked at this slag, and then, in a second stàge, the addition of - 20 excess stage for example or lime, in oxidising medium, in order to ensure a maximum iron precipitation.
Amongst some advantages of the process according to the invention, with respect to soms conventional steps preceding those of the invention, for example in the treatment of residues issuing from lixiviation of zinc ore, it is still to be noted a relative large purge of SO~ ions, improving the sulfuric acid control in the whole process and allowing concentrated solutions to be obtained with the benefit of heat produced by concentrated acid in dilution and during the attack.
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As resulting from -the prece~ing, the process according to the invention i9 more particularly use- :
ful for purification of solutions containing an amount of iron to be eliminated The invention is in fact based on the finding that iron plays an important role in the chemical and precipitation phenomenons occuring in the solution Thus the precipitation mechanism is quite different in the elimi.-nation of silica from a silicated zinc ~e, when dissolving the latter in a sol.ution at a pH lower than 1.5 and then pre-cipitating the dissolved silica by brin~ing the pH of this solution up to a value hi~her than 1.5 and by maintaining a relatively high temperature. In this case, silica solidifies by polymerisation, while in the process according to the in-vention, formation of iron silicate is more particularly con-templated, which does not suppose a hi~h temperature. It is overa~ important to note that unexpectedly it has been found that when the magnesium content in the solution to be puri- :
fied exceeds a certain limit, a portion of this magnesium is coprecipitated with iron sllicate formed In such a man-ner, presence of magnesium in the lime used to bring up the pH above 1.5 does not present any specific drawback.
Advantageously, according to the inven-tion,in some cases it could even be useful to add an amount of iron ions to mentioned solution after the pH of the lat-ter has been brought above 1.5, if for example the silicate or magnesium content of the ore is relatively high, in order to form a sufficient amount of iron silicate which is neces-sary to obtain a quite well filterable and washable residue and a sufficient coprecipitation of other impurities existing `

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in the solution. The iron amount added must preferably be subs-tantially stocchiometrical with respect to silica which is dis-solved or able to be dissolved in the solution.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process of hydrometallurgical treatment for removing metal impurities from zinc sulfate solution containing Fe+++ ions which comprises adding to said solution a silicate capable of forming dissolved silicic acid in situ, and a neutralizing base to provide a pH between 1.5 and 4.5 whereby the metal impurities are precipitated as a solid residue on a base of iron silicate.

2. The process of Claim 1 in which the silicate capable of forming dissolved silicic acid is an alkaline earth metal silicate.

3. The process of Claim 2 in which the alkaline earth metal silicate is calcium silicate.

4. The process of Claim 1 in which the silicate capable of forming dissolved silicic acid is slag.

5. The process of Claim 1 in which the neutralizing base is lime, zinc oxide or an alkaline earth metal silicate.

6. The process of Claim 1 in which the neutralizing base is added to provide a pH between 2.5 and 3.

7. The process of Claim 1 in which the metal impurities are contained in an oxidizing medium.

8. The process of Claim 1 in which the zinc sulfate solution containing metal impurities and Fe+++ ions is obtained from the acid attack of iron-containing residues.

9. The process of Claim 8 in which a first neutralizing base selected from the group consisting of oxidized zinc ore and silicated zinc ore is added to the solution to provide a pH
below 1.5 and thereafter a second neutralizing base selected from the group consisting of alkaline earth metal silicate and lime is added to the solution to provide a pH between 1.5 and 4.5.

10. The process of Claim 9 in which the first neutralizing base is added to the solution to provide a pH
of at least 0.5.

11. The process of Claim 10 in which the precipitated solid residue is introduced in a quantity of oxidized zinc ore during acid attack of the latter to form a solution of zinc sulfate.

12. The process of Claim 1 in which the metal impurities are precipitated at a temperature between 40° and 85°C.

13. The process of Claim 1 in which iron ions are added to the solution at a pH between 1.5 and 4.5 in substan-tially stoichiometric amounts to be precipitated as iron silicate.