CA1136860A - Process for precipitating iron as jarosite with a low non-ferrous metal content - Google Patents

Abstract

Abstract:

A process for precipitating ferric iron as a jarosite from a sulphate solution containing ferric iron, free acid and valuable non-ferrous metals, characterised by diluting the said solution with a diluent solution of low acidity so that the ferric iron concentration of the diluted solution lies in the range 5 to 35 grams per litre and the free acid concentration lies in the range 5 to 40 grams per litre, heating to a temperature up to the boiling point at atmospheric pressure in the presence of at least one ion from the group consisting of sodium, potassium and ammonium ions, and in the presence of recycled jarosite, without the addition of any neutraliz-ing agent, so that substantially all of the ferric iron is precipitated as a jarosite, followed by separation of the jarosite from the solution, thereby producing a jarosite contaminated with only minor amounts of non-ferrous metals, and a solution containing recoverable valuable non-ferrous metals.

Description

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This is a divisional of Canadian application Serial No. 302,546, filed May 3, 1978.
This invention relates to the separation of iron from sulphate solutions as a jarosite contaminated with only minor amounts of non-ferrous metals. Jarosite com-pounds approximate to the general formula AFe3(S04)2(OH)6 where A is chosen from the group consisting of H30, Na, K and NH4. Mixed jarosites and solid solutions may also be formed containing more than one component of the afore-said group.
In hydrometallurgical processes, in which jarosite compounds are formed, such compounds usually form part of a product which may also contain other materials, which other materials may include for example zinc ferrite, gypsum, zinc sulphide. Such products are commonly referred to as "jarosite", and in this specification the word "jarosite"
is used to mean pure compounds, solid solutions and mix~
tures of these with other materials as the context may require.
The precipitation of iron as a jarosite can be represented by the following typical equation 3Fe2(S04)3 + (NH4)2S04 + 12H20 ~ 2NH4Fe3(S04)2(OH)6 +

In the hydrometallurgical recovery of valuable metals such as zinc, nickel, cobalt and copper from ores, concentrates, residues, and other raw materials, it is usuaily necessary to seParate dissol~7ed iron from .sulphate solutions of the metals, because dissolved iron lnterferes wilh .he subsequent metai recover~ steps. Although the invention has particular application to the hydrometallurgi-cal -ecovery oE zinc from zlnc sulphide concentrates or oxidised zinc ores containing soluble iron by the electro-lytic zinc process, it is not limited to zlnc bearlng solutions, but also applies to the separation of iron from sulphate solutions containing other valuable metals.
During recent years a number of processes for separating dissolved iron from sulphate soLutions by preclpi'ation have been proposed. Australian Patent No.
401,724 describes a process for treating zinc plant residues containing zinc ferrite which includes hot acid ''~ leaching of the residue in sulphuric acid solution to dissolve zinc, iron, and other valuable metals followed by heating of the clarified leach solution to a temperature above 60C in the presence of sodium, potassium or ammonium ions so that the ferric iron is precipitated as an insoluble basic iron sulphate of the jaroslte type. A
neutralising agent is required for jarosite precipitation at temperatures below the boiling point at atmospheric pressure.
A high free acidity is required in the hot acid

2', leaching step in ord-er to dissolve the zinc ~erriLe, but 17~4 G

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th~ ~xcess free acidity adve~sely affects the jaroslte ?-eci~itat~on reaction (see equatioll 1 above). ~he free acic.itl~ in the hot acid leaching step may be lot"ered to s,ome extent, but to maintain extraction of zinc from the ~?nC fe-rite the residence time in ~he said step must be increased to compensate for the lowered free acidity.
However, at low free acidities, the dissolved ferric iron becomes unstable and tends to hydrolyse prematurely and precipitate as jarosite. Thls jarosite then conta~inates 1; and downgrades the undissolved solids removed from the ho. acid leaching step. These undissolved solids are enriched with respect to lead, silver and gold, and in many electrolytic ~inc plan~s are separated for su~sequent recovery of the contained ~etal values. ~ly downsI-~ding of these solids with jarosite from premature hydro~ysis is therefore undesirable. For this reason, free acidities in the hot acid leaching step are usually held in ~cess of 4G g/l, and this level of acidity retards the precipi-tat on of jarosite at temperatures below the boiling point 2C of the solution.
It has been found however that ln order -o precipitate sufficient iron from the leach solution so that subsequent processing of the solution by established procedures is economically possible, it is necessa-y to heat the leach solution to a temperature well above the 17/~i 1 ~ 3 ~

boillng point. Such an operation requires the use of autoclaves which are relati~ely expensive to install and operate, and consequently this procedure has not yet been used commercially. Nevertheless, the jarosite preclpitate produced by this process is relatively pure and is particularly suitable for conversion into pigment grade ferric oxide or as feed to an iron blast furnace.
V.S. Patent 3,434,947 describes a process in which the iron is precipita~ed as a jarosi~e at a temper-ature up to the boiling point at atmospherlc press~re by controlling the pH of the solution at less than 1.5 by the addition of a neutralizing agent.
It will be appreciated by those skilled n the art that the neutrallzing agent used in operation of c this process is required for two duties, VlZ.:
1. to neutralize excess free acid in the solutin~
entering the jarosite precipitat,oll stage, and 2. to neutralize acid liberated accordin" to equation (1) during precipitation of jarosite.
2~ The most economical, readily obtainable, neutralizing agent available to electrolyt-c zinc producers is zinc oxide calcine, or sinter produced by roast_ng zinc sulphide concentrate. In the accompanying arawings Figure 1 illustrates a simple flowsheet for this process, in which ferrites in residues from a neutral leach~r,g '4 .

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step are dissolved in a hot acid leaching step and the resultant dissolved iron is precipitated as a jarosite.
This process has the advantage of not requiring the use of an expensive autoclave, but produces a jarosite contaminated with non-ferrous metals contained in the residue from the neutralizing agent added to control the pH in the jarosite precipitation step. The amount of contamination will depend on the quantity and quality of the calcine or sinter used as the neutralizing agent.
Jarosite produced in the electrolytic zinc process from "normal" concentrates typically contains Zn 2 to 6%
Fe about 25 to 30%
Cu 0.1 to 0.3%
lS Cd 0.1 to 0.2%
Pb 0.2 to 2%
Ag 10 to 150 ppm Ca 0.1 to 0.8%
Mg 0.1 to 0.7%
SiO2 1 to 5%
The jarosite waste in fact often contains only 58 to 75% pure jarosite, the remainder being zinc ferrite and other contaminants.
The level of contamination of the jarosite precipitate can be reduced by leaching it with an acid as ,"

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descrl~ed in Norweglan Patent 123,248. ~owever, ~he final product still contalns significant levels OL zinc and lead, ~nlcil make the jaroslte unacceptable as a raw material for ircn manufacture or for conversion lnto plgment grade ferr c c~ide.
Ihe contamination of jarosite by '_oxic non-ferrous met~ls poses problems with the disposal of jarosite as a waste material. When it is stored on land, great care must be taken to avoid contamination of the surrounding areas.
1' Various 2rocesses have beer developed to make the contaminant non-ferrous metals inert, such as by mixing jarosite witn iime cr cement, so that jarosite can be dlsposed of without r.azards. Such processes are expenslve ana have not been entirely successful.
"Clean" neutralizing ager.ts which do not contain significant quantlties of non-ferrous metals, such as finely ground limestone, are not normally economically viabl~
alternatives to calcine or zinc ferrite bearing reaidues because of their cost, the large quantities required per 2~ unit weight of iron precipitated, and the substantial addition of gypsum to the jarosite waste which adds a rrajor burden to the disposal operations, or makes the jarosite unsuitable for pigment or iron manufacture.
It is kno~Jn from the prior art to add an 2~ additional step, termed a preneutralization step, between 17/~ G

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i-.ne~ hot acid leaching step! and the jarosite preclpitation step, in ~?hich some of the free acidity of the solution ~ a ~elltxalized by the addition of a suitable neutralizing agent, such as calcine. A typical flowsheet is shown in ~igure 2 of the accompanyir~g drawings. The level to which the free acidit~ can be lowered in the preneutralization step is typically limited to about 30 grams per litre by the fact that premature hydrolysis of dissclved ferric iron occurs to an increasing extent as the free acidity 1 is lowered. Since the undissolved solids from the pre-neutralization step are generally recycled to the hot acid leach step, any products from premature hydrolysis in the preneutralization step must be dissolved in the hot acid leach step if the recovered undissolved solids from the latter step are not to be downgraded. This is often difficult in practice.
~ie have found that by cooling t~ie solu-tlon from the hot acid leaching step it is possible to stabi..l.ize the dissolved iron and prevent substantial premature hydro-2 lysis, even when the free acid concentration is reduced to a low level during a preneutralization step.
We have also found that by raising the temperature of solutions from the preneutralization step and, ln the presence of at least one ion drawn from a ~roup containing sodium, potassium and ammonium, it is possible to 17f4 ~3~

precipitate a substantial proportion of the dissolved ferric iron as a jarosite without the need to add any neutralizing agent other than that which may be added as a source of the aforesaid ions.
For batch operation of the jarosite precipitation step recycle of jarosite seed crystals is advantageous as it eliminates the induction period during which the rate of jarosite precipitation is low. In continuous operation, we have found recycle is also advantageous in that the lQ residual ferric iron concentration in the solution leaving the last jarosite precipitation vessel is decreased by the recycle of the jarosite precipitate in one or more stages at a rate of between 50 and 400 grams of jarosite per litre of feed solution.
A process is described in application Serial No. 302,546 for precipitating iron as a jarosite from a sulphate solution containing ferric iron, free acid and valuable non-ferrous metals, characterised by the steps of:-2Q (1) cooling the solution;
(2) partially neutrali~ing the free acidity, and then clarifying the solution;
(3) heating the clarified solution to a temperature not exceeding the boiling point at atmospheric pressure, in the presence of at least one ion lh3~
selected from the group cGnsisting of sodium, potassium and ammonium ions, and in the presence of recycled jarosite, and without the addition of any further neutralizing agent, so that substan-tially all of the ferric iron is preciPitated as a jarosite; and

(4) separating precipitated jarosite from the solution;
thereby producing a jarosite contaminated with only minor amounts of non-ferrous metals, and a solution which may be further processed by established procedures for the recovery of dissolved valuable non-ferrous metals therefrom.
Another process is described for precipitating ferric iron from a sulphate solution in which the sulphate solution originates from a hot acid leaching step in the electrolytic zinc process and which after separation of undissolved solids contains 5 to 50, preferably 15 to 25, grams of ferric iron per litre, 30 to 100, preferably 35 to 70, grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre as well as non-ferrous metal impurities including cadmium, copper, and nickel, characterised by treating the said solution in a four step process comprising:-1~3~

(1 ) coolln(J ~:he ~dll sol;ltion to a temperature in the r~llq~ ' 0 to ~ ?~;
e~ti~ly tne solutior f~-o.n sce~ (1) in vne or mo~e stages ~th a neutralislng a~,ent or agents to lower the -; free _cidity oL the sclution to witrhin the r~ge ~.1 .c 25, p~eferably 2 to L5, ~rams per l~,re, and separating any residues or undlssolved neutralizing agents frcm the solution;
l_) heating the clarified solution from step (2) ln one 1' or l~ore stayes to a temperature in the range 803C to the boiling point of the solution at atmosp.,erlc pressure, preferably above 9C~C, in the presence o F at least one ion chosen from the group consisting of sodium, potassiur:l and ammonium ions, the mole ratio of the said ions to dissolved ferrlc iron lying ln the range 0.1 to lG, and in the presence of re~ycled jarosite, pr-efe~at,i,y in the range 50 to 400 grams of jarosi.e per litre of solution, and without 2~ addition of any further neutralizing agent other than ally which may be added as a source of the aforesaid ions, so that substantially alL of the rerric iron is precipitated as a j~rosite; and [4) separating the jarosite precipltated lTl step '3) from the -,oL~I.iGn b~, thic,'-~enin~, filtration al-d wasningi 17~4 G

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-;:nert~y p.oduc;ng a jarosite contamlnatecl ~Lth onl~ IllillOr amounts c- non-feL-rous meta~s, and a soint on which may `ae turt~e. ~.ocessed ~y established procedures for the recovery o/ d~sso.ved ~alua~le non-ferrous metals theref-rom.
~: ''tep (1~ Ot this ~referred form of the present nvention can be conducted in one or more stages by various p~ocedures. Thus, the solution can be cooled using heat exchangers, cooling towers, aspirators, spargers, or o~her suitable procedures, either sinqularly or together, ancl may be done in such a way as -to conserve energy. The tem~eratu~-e to which the solution is cooled lies ir. the rancle 30 _o 80 C. The stability of dissolved ferric iron increases with decreasing temperature; therefore, it is preferable to cool to a low temperature, to avoid s~bstantial 1~ precipitation of dissolved ferric iron as a jarosite due to premature hycirolysis in either step (1) or step (2). The e~e~t to which premature hydrolysis can be tolera~ed depends upon subsequent treatment of solids from step (2).
Preneutralization in step (2) call be conducted in one or more stages using various suitable neutralizing agents. For integration with the electrolitic zinc proccss, the applicants have found that thickener underflow trom the neutral leach step or calcine are the most suitable r.eutralizing ~gents and may be used separately or together.
~he presen~ ion is not limited, however, to this 1 ~ --17/~ ~

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( -p~-t:Lc~llar com',irla'-ion o: choice o. neu~-a~ in~ a~ents.
De~endin~l on the neutral.i~ing agent used, it ~ay ~e necessary to separa-te residues or undissolved neu'.l-a~L~i.nq agen hefore proceeding to step (3).
In .st2p ~3; of the process, dissolved ferric iron is substa;l~ial]n,7 precipitaled as a jarosite by heating the clarifi~d solutlon derived from step (2) in the presence of sod~ium, potassium or ammonium ions to a temperature up to the boilir.g point of the solution at atmospheric pressure.
Temperatures 1~7ing in the range 80C to the boilins point of the solution a~ atmospheric pressure are suitable, prefer-lbl~ those above 95C. The method of heating can be by live steam injection, submerged combustion, ind.rect heating using suitable heat exchangers, or other methods, ir either singularl~ or in combillation, known to thos~ skilled in the art.
The ex'ent of ferric iron precip~tation and the concClltratiorl of dissolved ferric iron in solution leaving step (3) ~ill depend on several factors. ~ome of these are 2~ related and the optimum conditions chosen will depend on how the process of the present invention can be best integ-~ated with a new or e~isting electrolytic ~inc plant circuit.
~or ma~imum precipitation of dissolved ferric iron, i.' i~ prefc-^able to operate at a hish temperature, a long esid*,cc~ t~ e, low initial ferric iron and free ac1d - l3 -:: .

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(' co-,~entr~-tions, a high concentration of sultable seed material, and a high conce~tration of sodium, potassium, 1r.r~ am~monlum ions. Concentrations of the said ions totaiiing in excess of 0.3 times stoichiGmetric to form ja~oslte a~e preferred. The aforesaid ions may be added a solu~1e basic compound at the beginning of or during ,tep ~3" so that the said basic compound will then neutralize a portion of the free acid in the solution in ;;tep (3), thereby allowing more complete precipitation of 1rJ the .er~ic iron. The residence time should preferably lie in the ranqe 5 to 24 hours.
~he initial ferric iron concentration in solution entering step (3) should preferably lie in the range 15 to 25 grams per litre and the initial free acid concentra-tion in the range 2 to 15 grams per litre.
In step (4) of the process, the ~arosite lS
separated from the solution by solids-liqu1d sepa~.tion procedures, and washed to remove most of the entralned dissolved non-ferrous metals. Various liquid-solid 2~ separation and washing procedures are well known to those skilled in the art.
The solution from step (4) may be sent tc the neutral leach step of the electrolytic zinc process or aiternatively it may be treated using known procedLres in a separa~e iro.n .~mo~al step, in which a neutralizing l7/4 G

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agent is added, any ferrous iron oxidised to ferric iron, and a major part or all the ferric iron precipit-ated to produce a solution low in iron which is then returned to the electrolytic zinc process. The solids from the separate iron removal step may be recycled as a neutralizing agent to step (2) of the process.
Alternatively, they may be recycled to the neutral leach step or dilute acid leach step of the electrolytic zinc process. Alternatively, all of the thickener underflow pulp or filter cake from the separate iron removal step can be recycled to the hot acid leach step and calcine or other suitable neutralizing agent added to the preneutralization step.
Figure 3 shows a typical non-limiting example of this preferred form of the process applied to the removal of iron from a sulphate solution containing ferric iron derived from the leaching of zinc oxide calcine. In order to conserve energy, a heat exchanger is used for cooling the solution in step l.
Under certain circumstances it is advantageous to dilute the solution by recycling a portion of the ,overflow solution from the jarosite thickener back to the preneutralization step, in order to lower the ferric iron concentration in the preneutralization step. It may also be advantageous to recycle a por-tion of the overflow from the neutral leach thickener to the preneutralization or ' , :
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3~860 ja~o~`lte prQclpltation steps, in order to dilute the sollltior.s in tne said steps. The optimum comblnation and ~h,e -x-~en~ c,f dilution desirable will depend upon many act:o;^s and can be best evaluated in plant trials. All such cperatir.g modes with recycle fall within the scope o~ _~e present invention, and the previousiy designated rar.ge cE acidl-cty of 0.1 to 25 grams per li~re inclucies the effeet of any such dilution. Dilution in the preneutraliza-~ion step is advantageous as it lowers the ~erric iron 1~ concentration, thereby reducing the possib-lity of substan- --tial premature hydrolysis. Dilution in or immediately before the jarosite precipitation step is also advantageous as it reciuces .he acldity irrespective of whether the acid was present initially or formed as a result of the jarosite precipitation reaetion. Figure 4 shows a typical non limiting example of the present invention with a recycle of r.eutral solution as a diluent in the jaro~site preci21tatlon step.
Alternatively, it may be advanta~eous to recycle 2~ a solution which is produced by partially neutralizing the free acid in all or portion of the thickener overflow solution from the jarosite precipitation step such that the acidity is reduced to within the range 0.1 to 30, preferably 0~1 to 10, grams per litre, without substantial precipita-~' -tioi~ of cne remainillg dissolved ferric iron in the solution.

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`' ' ~ '` ' ~ '' This partial neutralization procedure is hereinafter termed the post-neutralization step. For the kest results this post-neutralized solution should be recycled to the jarosite precipitation step.
We have also found that under certain circum-stances, for example if the acidity of solution from the hot acid leach step is already at a suitably low level, satisfactory precipitation of iron as a jarosite can be achieved by adequately diluting the solution from the hot acid leach step followed by heating the diluted solution to a temperature up to the boiling point at atmospheric pressure in the presence of sodium, potassium or ammonium ions, or mixtures of these ions and in the presence of recycled jarosite, thereby making steps (1) and (2) of the previously described process unnecessary.
According to the present invention there is pro-vided a process for precipitating ferric iron as a jarosite from a sulphate solution containing ferric iron, free acid, and valuable non-ferrous metals characterised by diluting the said solution with a diluent solution of low acidity so that the ferric iron concentration of the diluted solu-tion lies in the range 5 to 35 grams per litre and the free acid concentration lies in the range 5 to 40 grams per litre, heating to a temperature up to the boiling point at 113~3613 ( ~tmos?herlc pr2ssure in t:he p-esence of at leact one ion ^.'nosen From the ~rou? consisting of sodium, potassium and a.~mos~i,im io~C. in the ?resence of recycled larosite, ~.itho~t the ~-iddi'ion of any neutraliziny a~ent so that ..,uhs.lntially al' of the ferric iron is precipitated as a J rosite, follo~ied by separatio.. of the ,arvsite from the solution which is further processed by established procedures for the recovery of dissolved valuable non-;e-rous metais, thereby producing a jarosite contaminated ith only minor amounts of non-ferrous metals.
~ccording to another preferred form of the present invention, there is provided a process for precipi-tating fer.ric iron from a sulphate solution in which the sulphate solution oriyinates from a hot acid leaching step in the~ electrolytic zinc process and which after separation o' undissol~ed solids contains 5 to 50, prefera~ly 15 to 25, ~rams of ferric iron per litre, 30 to .!)0, preierably 30 to 50, grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre as well as non-ferrous 2~ metal impurities including cadmium, copper and nickel, characterised by diluting the said solution with a diluent solution containiny 0 to 15 grams of ferric iron per litre, 0 to 30, preferably 0 to 10, grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre, the ratio OL the dil~len; soiu'ion O '_l'? said sol.litlon L~eing ( ~ .s ~-~ f~r~ t~ n ~rit~ .n~ 5 tc~ 35, pr?ferahl~ 5 to ?Ø grams of ferric iron per litre, 5 tc ~0~ pr~fer?.bl.~ 5 L? ~0, grams of free sulphuric acid per itre, an~ a~ le~st ~0 arai~s of zinc per lltre, he~=iting said di..lu~.e' solution to a temperature in the ranae 80C to the l~oiling pcint of the solution at atmospheric pressure, Dret~erablv above 95C, in the presence of at least one ion chosen from the group consisting of sodium, potassium, and am~moniu,T. ions, the mole ratlo of said ions to dissolved LO fe-ric iron lyina in the range 0.1 to 10, ln the presence of -ecyclPd jarosi.~e, preferabLy in the range 50 to 400 grams of jarosi'e per litre of diluted solution, and without addition of any neutralizing agent, other than any which may be added as a source of the aforesaid ions, so that l' substantially all of the ferric iron is precipitated as a jarosite, separating the jarosite from the solution by thic'~ening, Eiltration and washing, thereby produc~.g a solution ~.lnich i.., further processed by established procedures for ~he re~o~.~e.y of dissolved valuable non-ferrous metals ~0 and jarosite contaminated with only minor amounts of non-ferrous metals. Figure 5 shows a typical non-limiting example o:f. 'his preferred form of the process according to .he ~resent invention.
It is also advantageous if at le;ist a poI-tion or tne solltio~ m the jarosite ?~eci?itati.on step after - ln _ 17,'~

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-n~a.a'ion of ~he iarosite lS admixed in a control]ed manner wi~h a neutralizing agent, prefera~ly zinc o-~ide c-~lcirl-a, Lo proallce a solution containing O to 30, prefer-a'~ly ;3 to ld, grams of sulphuric acid per litre and after sep3ration o~ any resiclue from the neutralizing atent, at `edst a por.ion or the solution so produced is used as the diïuent solution.
Any remaining solution from the jarosite precipitation step not so treated passes advantageously to 1~ ~urther processing in the electrolytic zinc process as does any remainder of the treated solution not recycled as a diluent.
The post-neutralization step may be performed without cooling the solution, and we have found that a short residence time is preferable to avoici substantial precipitation of the remaining iron as jarosite. ~uitable neutralizing agents would include thickener underf~ow from the neutral leaching step, calcine, limestone or lime, and ~hese may be used individually or in combination.
2 Depending upon the neutralizing agent used, it may be desirable to separate residues or undissolved neutralizing agent before the solution is recycled. The post-neutraliza-tion step may be performed on part or all of the J~rosiL~-thickener overflow solution, with any e.Ycess over that needed for recy-le passing on to further processina.

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7UC'I urther processing could i~,e a neutral 7~eachlr,g step performed in a manner weli knowrl to those ~ e~ ln the art or d second stage of jarosite precipit a ~ior, ~erformed under similar conditions with : -esyec~ to tempera~ e, the presence of ions from a group _~ntaining sodium, potassium and ammonlum and in the pre7senc_ c~r recycled jarosite seed as used in the jarosite precipitation step of the present invention.
I_ will be apparent to those skilled in the art 1 ~hat the dii~tion need not occur before the star. of the jarosi-te precipltation step, but that the two solutions ~sulphate solution and diluent solution) could enter the jarosite precipitation step separately in .he proportions necessary to produce a diluted solution wlth the desired ~_ composition, so that dilution and jarosite precipitation occur within the same step.
It will also be clear that at least one ~-~f the 'wo soiu-tions makirlg up the diluted solution may be heated prior to dilution so that the temperature of the diluted solution is at or above the desired temperature for jarosite precipitation, thereby obviating ~he need to heat the diluted solution to the desired ~emperatllre.
Fi~ure 6 illustrates a non-limitlng example of this pr2ferred form of the process accordin~ to the 2_ present ir.ven,ion.

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One of the advantaaes of the ?reselt in~erltion o~rer the prior art is that the precipitation of jarosite ~; Se1L -~egulatincl and no comp)ex p~l control system is re~u~red .o -egula.e the addition of a neutralizing agent d1L lr.g precipitation of the jarosite. The precipitated ~laLosite posses~ses uniEorm settling and ~iltra.ion prJper~ies all o~ wnich lead to a process which is simple to operate, and produces jarosite with uniform physical and chemical properties.
1~ The jarosite prod~lced by the process of this invention is contaminated Wl th only small amounts of non-ferrous metals and therefore it can be disposed of as a waste product with fewer en~lronmental problems.
Similarly, it is a more suitable raw material for other manufacturing processes suc~ as the production of pig iron or plgments.
For the productior- of pig iron the jarosite produced accordillg to the present invention can be sinterea or pelletised at eievated temperatures to 2C decompose the jarosite, simultaneously producing an agglomerated iron oxide. Since many non-ferrous me-tals volatilize during sintering or pelletizing at elevatod temperatures, the non-ferrous metal content of the product from the sintering of pelletizing process will be lower than that _x?ecl:~d ~ased upon the non-ferro-ls metal content - 2. -17~4 G

o~ the jarosite fed to the sinterin~ or ~elletizing ( process. Furthermore, from the prior art there are known techniques, chloridisation, for example, which induce additional volatilisation of many of the non-Eerrous S metals during sintering or pelletizing; these techniques can therefore be used to produce an ayglomerated iron oxide of even higher purity if desired.
An example of operation of the process oE the present invention with sintering of the jarosite product to form an agglomerated iron oxide is given in Example 5.
For the production of pigment grade ferric oxide, the jarosite produced by operation of the present invention can, for example, be calcined according to the procedure disclosed in Australian Patent ~o. 497,806. Because the jarosite produced by the process of the present invention is contaminated with smaller amounts of non-ferrous metals than jarosites currently produced in commercial electro-lytic zinc plants, it is obvious that the ferric oxide produced by calcination of such jarosites will be less contaminated than that currently produced by the calcina-tion of commercially produced jarosite. If the jarosite before calcination contains sodium or potassium, it may be desirable to subject the ferric oxide to a suitable washing procedure to remove any soluble salts including those of sodium and potassium which may be 1~3 ( ~rej~n~. -`he soiL1tion containing such soluble salts may, if lesi;-?.d, be re~urned to the jarosi~e precipita~lon ; L _. ~?, ^'.-_ is a1.so possible to ~ubject Ihe jarosite _ ~.o.u^t t~: ~ hydrotnermal conversion to fer.ric oxide by hoi?.t.iny ~he -jarosite ~ith water (or dilute sulphuric acid) in ~ln autoclave d ~ a tempera~ure in the ranqe l40 to 2~o3c. Vnder these conditions the jarosite is converted in.o fi.nely di.vided, high purity ferric oxide eminently l~ sui.able fol use as a pigmen~. The advantage of this ~rocedure is that during the autoclave treatment, sulphuric acid and -the cations Na , K , or Nr~4 orlginally combined in the jarosite precipitate are liberated and report in the solution which can be separated from the l. ferric oxide by standard procedures after discharge from the autocla-~e. This solution, can Wit}1 advantage be recycled, preferably bac~ to the preneutr.~1ization stage CL t'le '_. ec-rolytic zinc plant circuit. ~lere exce.;s acidity is neutralised and the cations are availab:Le for precipitaticn of more jarosite.
An example illustrating the hydrother~al conver-sion of jarosite into high purity ferric o;ide is given in Example 5.
It will be obvious to those skilled in tne art ~' t.. at bx~ 2 t'" _2r2 and the extent o.. jarosite precipitation - 2~ - -~ G

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~?~ .e ~rocess of the present invention can be further increased hy the deliberate addition of neutralizlng a?-nt~ to the ja--o~-;i'e precivitation step. lhe amount oF
ne~l~rali~ ng agen-L required to achieve the same extent ot S irc~n p.~ecipitation would be much less than would be required i.n cperation of the jarosite process according to the ~-,riox art. Tne addition of neutralizing agents to increase the precipitation of jarosite in conjunction with operatlon of the present process falls within the scope of i~ the present invention.
Neutralizing agents which could be used would depend upon whether the jarosite is to be reprocessed or disposed of as a waste product. To be suitable for reprocessing the jarosite should have a very low non-l; ferrous metal content, and neutralizing agents which leave no residue, including such neutralizing aqents as zinc oxide, basic zinc sulphate, zinc carbonate, ferric. oxide and ferric hydroxide, could be used.
If the jarosite is to be disposed of as a waste material, small amounts of neutralizing agents such as limestone or lime which contain no environmentally-harmful non-ferrous metals may be suitable. The addition ~f small amounts of neutralizing agents such as calcine whi-h leave a residue containing toxic non-ferrous metals could be made in conj~n.ctjo~ ~i.'th `he process of thls invention, arld ~he jarosite so produced would be less contamlnated w~th non-ferrous rnetals than ~ G

( ' _n2 -;aros,it~ produced accor~'incl to the prior art.
To illustrate the operation of ~he present in~-ellri~n th2 followlnq examples are quoted:
~ pie i -7 Typical high acid ieach solution contair.ing .n 78 g/l ~e + 15.2 g/l 2 4 55 g/l NH4+ 3.2 g/l t i was cooled to 50~C and neutralized with caicine.
r'ollowing separation of undissolved solids the solution analysis was as follows:
Zn 10~ g/1 Fe3 14.8 g/l i5 H2SO4 4.8 9/l Thls solution was then maintained at 50C for 20 hours and no iron was precipitated.
~t the end of this period the so]ution was heated to lOO~C and maintained at this temperature for 2r 6 hours. Ammonium hydroxide was added to maintain the ammoni~m con_entration approximately constant. Jarosite solias were precipitated and after separation and washinil, the mother liquor and dried washed solids were analysed as follows:

- 2~ -17,/4 ~, - ~
' , ' '~ ' ( Solutio~ .J ~;,;1 ~ 22._ q/l 1~14 3.3 g~l So.is Iron 32.4 f, -~ ininc 0.46 6 WatQr SGIuble ~inc 0.0045 , -Lead 0.53~ o-~iri4 2 . 8 Cu 0.016 %
Cd O.OOl O
Ca 0.05 ~J
~1g less than O.Ol %
SiO2 less than O.Ol 6 Hg O . 2 3 ppm Ag 4 ppm It wiil be apparent to those skilled in the art ~hat the precipita~ed jarosite is of much higher purity -.han ~ha. normally produced. (See the typical analysis given 011 n~ge 6).

18j'~

~ 3~ 0 `; ;~;~.....
_ A high acid ]each solution after preneutlali~ation and adci ~.ion o~ ammonium hydroxide was analysed and rour.cl to contair.
Fe = 25.8 g~l SO~ = 12.9 g/l 4 5 g/l .hiC solution was divided into 6 portions, and treated in the followinq manner:
lC Portion 1 - Maintained with agitation at 100C without seed Portion 2 - Maintained with agitation at 100C wlth 50 g/l of jarosite seed Portion 3 -- Maintained with agitation at 100C with 100 g/] of jarosite seed .5 Pcrtion 4 - Maintained with agitation at 100C with 200 g/l of jarosite seed Port..ion 5 - Maint~ined with agita-tion at 100 C wlth 300 g/l of ja.rosite seed Portion 6 - Diluted with an equal volume of neutral ZnSO~
~0 solution and maintained with agitation at 100C with 150 g/1 of jaroslte seed Samples were taken at regular intervals, filtered, a:ld the ferric iron concentration in the filtrate samples d-ter~ined with the fol1.owing results.

- 2,3 -.:
- .: ~ :
- ' . ~.,'~ '~

-<~ L-_ 3 1 4 _ 5 L _ 5 __ _ I~o -,~ed ¦ G 9/1 100 g~ 200 g/1 30~ g/l¦ Diluted L 150 C~ ¦ L` `- ~ se~d_ seed_! seed_ seed ~/- 5e~d +1 ~ Fe Fe Fe Fe Fe Fe ,, ~ rs, j _Gn- conc. conc. conc. conc. conc. cnnc.
g/l g/l g/l g/l g/i of g/1 of ! ¦ diluted original . solution solution_ '5.7 18.7 18.5 17.515.0 5.511.0 17.7 16.6 15.512.7 4.38.6 .0 i'o.~ 1~.2 12.911.7 3 87 6 31 23.0 15.2 12.7 11.911.0 3.57.0 ¦ 422.2 14.5 12.0 11.210.4 N.D.N.D.
521.2 13.4 11.5 10.59.7 N.~.N.D.
, 620 1 ! 12.9 11.1 10.09.3 N.D.N.D.
__ N.~. = Not determined.

It can be seen that both dilution and the addition of jarosite seed lead to enhanced rates of precipitation of ~erric iron.

18"'-~

~3~

~ 3 .
n acid leach sol t - or r simiiar in ~:;r-c;~ n ~c ~n;at used ~n E~ample 1, ~as ccoled to 50C

-::d ex~-s a-~ .n~utralic d with calcirle. ~ollowins se~ icn ^~ und so ved sclids the solution analysis was .) 1 c ~ ~:
'~ ~. 9 7 ;~
.~e~ 16 . 4 g/l H2SO~ 4 . 5 g/
`?I' ~ . 9 ;J/l 'Lnis sOlUtlOJl WAS then pumped continuously at a rate of 2 litre/hour into the first of a series of four s-team-hea~ed, stainiess steel reaction vessels, each of ;~nich ~as fitted with baffle~s and a mechanical a~itator.
The pulp overflowed from each vessel into the next in series. The volume of each vessel was 6.0 litres, and the temperatllre of each vessel was maintained at 1()0C by injection of stea~ into the jackets of the vessels.
ïnjection of steam ~.as automatically controlled by use of O thermistors suspended in the pulp in the vessels. ~mmonium hydroxide and water were adcled to maintain the ammonium concentration approximately constant and to compensate for evaporation losses.
After the process had been operating for 12 hours, ~S an~i st-ad-i ct--e ~ ditiors ~ad been established, samples 18/~ G

:_ ' '`7i' .~'el-`? t~oil!?cted from each vessel, filtered, and ~he F~ r~,es ;_~aaiysed. The followiny resui-'-s h~ere ob'ain-d.
Ve,.ss._ 70. _ e~ gjl H~04 g/l .`!'4 q~l 1 '~1.5 9.1 , 2 10.~ 13.1 3 9.] 15.3 ~ 7.4 17.9 3.2 The test described above was repeated in a second experiment in which jarosite produced in the first test was ~0 ,-ecycled continuously back to the first reaction vessel at a ^ate equi~alent to 100 g of jarosite per litre of feed solution.
After the experiment had been operating for 12 hours and steady state cond.itions had been established samples of pulp were collected from each vessel, filtered, and the filtrates analysed. The following results were obtained~

~vTessel No. Fe g~l H S04 g/l NH~ y/l :L 6.1 16.8 2 4.7 17.6 3 4 0 18.4 4 3.5 18.9 3 ~
The test described above was repeated in a third experiment in which jarosite solids were Lecycled colltinuously ,~5 baclc to i~he Tirs~ i~e~sel at a rate of 150 n of -j~ro..i~:e i~c--18~4 G

~3~

.tre o~ feed solutionA In all other respects the expeL-iment was identical to the earlier ones.
After the experiment had been operating for 12 I'~ours and steady state conditions had been established,

5 sa.TIples of ~ulp were collected from each vessel, filtered and the filtrates analysed. The followinq results were o~tained.
Vessel No. Fe g/l H2SO4 g/l NH4 g/l 1 6.2 16.6 2 4.5 18.6 3 3.9 20.7 4 3.2 21.G 3.5 These three experiments illustrate the beneficial effect that recycli.ng of jarosite seed has on reducing the ~; final ferric iron concentration.

'-''`~;~IP' I` '}
~ soiution containing l ++
Fe = 6.6 g/l H2SO4 = 32.7 g/l 4 = 3 g/l was reacted with zinc oxide calcine at 90C. The residue was separated off and the resulting solution was analysed and found to contain ~e+++ = 6.5 g/l H2SO4 = 3.8 g/l 500 ml of this solution was then mixed with 500 ml of t~ypical solution from the hot acid leaching step of a zinc plant, containing Fe = 21.1 g/l ;5 H2SO4 = 40.4 ~/l NH4OH solution was then added to the combined solutions tc make up for the ammonia which would be remo~-ed in jarosite precipitation. The resulting solution contained Fe = 13.8 g/l ~ H2SO4 = 20.1 g/l 4 4 g/l This solution was heated to 100C and 120 grams of jarosite was added as seed. After 5 hours the solution was separated from the jarosite, analysed and found to contain 18,i4 G

: : ;

Ff~ 7 . 0 5 04 = 31.7~ g/
~ his solu~ion was then xeacted with calcine at 90C, 'he residue separated oEf and the solution 5 ar,al~sed alld found to contain r e - 7 05 g~1 H S04 5 . 8 g/l This solution was heated to 100C, and seed jarosite was added at the rate of 120 g/l to precipitate iron as jarosite in a second stage. After 3 hours the solution was separated from the jarosite, analysed, and found to contain Fe = 2.0 g/l H2S04 = 14 3 g/l ~5 Thus the ferric iron content of the original solution was reduced from 20 g/l -to 2.0 g/], a 90~ overall removal oE dissolved ferric iron.

18'` ~, : ~ , , . .

!PLE 5 Typical high acid leach solution similar i,n composition to that used in ~xample 1 was cooled to 55C
and excess free acidity neutralized with calcine.
Following separation of the undissolved solids the solution was stored in a 15 m3 holding tank.
Analysis of the solution was as follows;
Zn 97.6 g/l H25O4 6.3 g/l Fe3' 18.0 g/l NH~ 3.3 g/l This solution was then pumped continuously at a rate of 1.7 litre/minute into the first of three baffled, mechanically agitated, lead lined, wood stave reaction ~5 vessels arranged in series so that pulp overflowed rrom each vessel into the next in series. The volume of each vessel was 200 litres. Stainless steel steam coils immersed in each tank were used to maintain a temperature of 100C in each vessel. Ammonium hydroxide and water were `0 added to the three vessels to maintain the ammonium ion concentration approximately constant and to compensate for evaporation losses.
Overflow from the third vessel was collected in a small scale thickener from which settled jarosite solids were removed, su~e~ted to a two stage decantation wash with water, f,iltered and stockpiled.

i8/'4 G

This e~p~ Oci~ r~
four days at the end of ~hich period, a stocKpile oE
seve.-ai kilograms of jarosite had been accu~,ulated.
Typical analyses of the jarosite produced and of solu,ion ieaving the last reaction vessel are listed below.
Solution Zn ~7.2 g/l H2SO4 21.8 g/l Fe3+ 7 ' 6 g/l NH 3.4 g/l ] Jarosite Solids: Total ~n o .75?6 ~7ater Soluble Zn 0.49%
CaO 0.34%
MgO 0.04%
A12~3 0.008%
S102 0 ~ 2 5?o Fe 31.9%
SO~ 40.35?o ~H4 2.S'~
Pb 0.21 ?o Cd 0. 002o As 0.14%
Cu 0.007O
Cl 0.0015%
Mn 0.05 ~5 ~o Ni <0.01~
I'g 0 46 p~n ~3~

The jarosite prodllced in this run, and con~ain-8.3% moisture, was then subjected to a series of pot sintering tests, using either a 0.04 m2 or a O.l m~ surface area suction pot. The jarosite was premixed with limestone, -~ coke and return fines derived from preliminary tests, and isnited by means of charcoal in the small pot and by an oil fired ignition hood in the larger pot. In all tests the sinter was allowed to cool in the pot and then dropped five times from a 2 m height onto a steel plate and different screen fractions collected. The +6 mm screen fraction, less the hearth layer quantity, was taken to be the sinter product, while the -6 mm screen fraction was used as recycled fines in later runs.
A typical result obtained using a 300% of recycled fines, 36~ of coke, 8% of limestone and a bed height of 30 cms was the production of a fine grained, hard, non dusting, granular sinter at a rate equivalent to a specifie- sinter output of 7.2 tonnes of sinter per m2 of grate area per 24 I-ours.

13,'~ G

~3~

A typical analysis of the sinte~ was as follows.
Fe 5~-59 Total '; 0.14 - 0.23 %
Ni 0.0045 %
Zn l.U - 1.4 %
Pb 0.05 - 0.08 %
As 0.03 - 0.05 %
Sb 0.01 %
The analysis indicates that such a sinter could O be blended wlth high grade ore for the production of pig iron.

~:

11 ~3 (: "
,i.~'~PLE 6 In yet another variant of the process of the present invention, a sample of jarosite produced in a batch e~periment identical to that described in Example 1 was heated with water in a stainless steel autoclave for 4 hours at 235C. After discharge from the autoclave the finely divided ferric oxide product was separated from the solution by filtration, washed, dried and analysed.
Analyses of the original jarosite and of the ferric oxide product are given below, together with the analysis of a typical pigment terric oxide produced from jarosite.
Percentage Composition Element FeTotal Zn _b Cu Jarosite 32.5 0.24 0.04 <0.01 Ferric Oxide 64.0 0.02 0.12 0.002 Pigment Fe2O3 60.0 0.5 >0.5 0.1 The average particle size of the ferric oxide was 0.2 micrometres, which, together with the low level of impurities, makes this material eminently suitable as a ~o pigment.

. G

Claims (15)

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

1. A process for precipitating ferric iron as a jarosite from a sulphate solution containing ferric iron, free acid and valuable non-ferrous metals, characterised by diluting the said solution with a diluent solution of low acidity so that the ferric iron concentration of the diluted solution lies in the range 5 to 35 grams per litre and the free acid concentration lies in the range 5 to 40 grams per litre, heating to a temperature up to the boiling point at atmospheric pressure in the presence of at least one ion from the group consisting of sodium, potassium and ammonium ions, and in the presence of recycled jarosite, without the addition of any neutraliz-ing agent, so that substantially all of the ferric iron is precipitated as a jarosite, followed by separation of the jarosite from the solution, thereby producing a jarosite contaminated with only minor amounts of non-ferrous metals, and a solution containing recoverable valuable non-ferrous metals.

2. A process according to claim 1 in which the sulphate solution originates from a hot acid leaching step in the electrolytic zinc process and which after separation of undissolved solids contains 5 to 50 grams of ferric iron per litre, 30 to 100 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre as well as non-ferrous metal impurities including cadmium, copper and nickel, characterised by diluting the said solution with a diluent solution containing 0 to 15 grams of ferric iron per litre, 0 to 30 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre, the ratio of the diluent solution to the said solution being such as to form a diluted solution containing 5 to 35 grams of ferric iron per litre, 5 to 40 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre, heating said diluted solution to a temperature in the range 80°C
to the boiling point of the solution at atmospheric pres-sure in the presence of at least one ion chosen from the group consisting of sodium, potassium and ammonium ions, the mole ratio of said ions to dissolved ferric iron lying in the range 0.1 to 10, in the presence of recycled jarosite and without addition of any neutralizing agent, other than any which may be added as a source of the aforesaid ions, so that substantially all of the ferric iron is precipitated as a jarosite, and separating the jarosite from the solution by thickening, filtration and washing, thereby producing a jarosite contaminated with only minor amounts of non-ferrous metals, and a solution containing recoverable valuable non-ferrous metals.

3. A process according to claim 1 in which the sulphate solution originates from a hot acid leaching step in the electrolytic zinc process and which after separation of undissolved solids contains 15 to 25 grams of ferric iron per litre, 30 to 50 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre as well as non-ferrous metal impurities including cadmium, copper and nickel, characterised by diluting the said solution with a diluent solution containing 0 to 15 grams of ferric iron per litre, 0 to 10 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre, the ratio of the diluent solution to the said solution being such as to form a diluted solution containing 5 to 20 grams of ferric iron per litre, 5 to 20 grams of free sulphuric acid per litre, and at least 20 grams of zinc per litre, heating said diluted solution to a temperature in the range 95°C
to the boiling point of the solution at atmospheric pres-sure in the presence of at least one ion chosen from the group consisting of sodium, potassium and ammonium ions, the mole ratio of said ions to dissolved ferric iron lying in the range 0.1 to 10, in the presence of recycled jaro-site in the range 50 to 400 grams of jarosite per litre of diluted solution and without addition of any neutralizing agent, other than any which may be added as a source of the aforesaid ions, so that substantially all of the ferric iron is precipitated as a jarosite, and separating the jarosite from the solution by thickening, filtration and washing, thereby producing a jarosite contaminated with only minor amounts of non-ferrous metals, and a solution containing recoverable valuable non-ferrous metals.

4. A process according to Claim 1, 2 or 3 in which at least a portion of the solution from the jarosite precipitation step after separation of the jarosite is admixed in a controlled manner with a neutralizing agent to produce a solution containing 0 to 30 grams of sulphuric acid per litre and after separation of residue from the neutralizing agent, at least a portion of the solution so produced is used as the diluent solution.

5. A process according to Claim 1, 2 or 3 in which at least a portion of the solution from the jarosite precipitation step after separation of the jarosite is admixed in a controlled manner with zinc oxide calcine to produce a solution containing 0 to 10 grams of sulphuric acid per litre and after separation of residue from the neutralizing agent, at least a portion of the solution so produced is used as the diluent solution.

6. A process according to Claim 1, 2 or 3 in which the diluent solution is added directly to the jarosite precip-itation step so that dilution and jarosite precipitation occur within the same step.

7. A process according to Claim 1, 2 or 3 in which at least one of the two solutions making up the said diluted solution is heated before dilution to a temperature such that the temperature of the combined solutions is at least the temperature required in the jarosite precipitation step.

8. A process according to Claim 1, 2 or 3 in which a portion of the solution from the jarosite precipitation step, after separation of the jarosite is recycled back to an earlier step of the process.

9. A process according to Claim 1, 2 or 3 in which a solution containing dissolved valuable non-ferrous metals and a low free acid concentration is recycled to an earlier step of the process.

10. A process substantially according to Claim 1, 2 or 3 in which at least a portion of the solution from the jarosite precipitation step, after separation from the jarosite, is treated with a neutralizing agent to reduce the free acidity to within the range 0.1 to 30 grams of sulphuric acid per litre, separated from any residue formed from the neutralizing agent, and at a temperature up to the boiling point at atmospheric pressure in the presence of at least one ion chosen from the group consist-ing of sodium, potassium and ammonium ions, and in the presence of recycled jarosite, additional ferric iron is precipitated as a jarosite without the addition of neutral-izing agents, followed by separation of the jarosite from the solution, said solution containinq recoverable valuable non-ferrous metals.

11. A process according to Claim 1, 2 or 3 in which the extent of jarosite precipitation is increased by the deliberate addition of a small amount of neutralizing agent to the jarosite precipitation step, the choice of neutralizing agent being dependent upon the method of disposing of the jarosite.

12. A process in which jarosite is produced substantially according to Claim 1, 2 or 3 and is converted by calcina-tion into high grade ferric oxide suitable for use as a pigment, followed by washing to remove water soluble salts with recycle of the wash liquor to an earlier step of the process.

13. A process in which jarosite is produced substantially according to Claim 1, 2 or 3, and is subjected to hydro-thermal conversion by heating the jarosite in an autoclave to a temperature in the range 140 - 260°C, followed by separation of the solid and solution products with recycle of the solution back to an earlier step of the process and production of high grade iron oxide.

14. A process in which jarosite is produced substantially according to Claim 1, 2 or 3, mixed with appropriate reagents and sintered to produce a product suitable for the production of pig iron.

15. A process according to Claim 1, 2 or 3 in which the jarosite precipitate is disposed of as a waste material.