~2~
PROCESS FOR THE RECOVERY OF ZINC AND PRODUCTION
OF HEMA~:ITE FROM ZINC PLANT RESIDUES
This invention relates to a process for the recovery of zinc and production of hematite from zinc plant residues.
The principal star-ting material for the production of zinc in electrolytic plants is roasted concentrate or calcine. During roasting most of the iron in the concen-trate combines with zinc and forms zinc ferrite, irrespec-tive of the roasting procedure or the mineralogical form in which iron was originally present. Zinc ferrite is insoluble under the conditlons employed to produce pregnant electrolyte by dissolu-t~on of zinc oxide from calcine, and it thus reports to a resldue whlch contains zinc ferrite, insoluble lead and silver compounds, unroasted sulphides, silica, alumina, etc.
Zinc contained in the~above residue may be dissolved by leaching with hot strong acid~solution in a step gener-ally called hot or high acid leaching. However, a substan-tial part of the iron dissolves and must later be precipi-tated in a crystalline form that can be readily separated from the solution on an industrial scale. Prior to the devel-opment of technology for pr~eclpitation of iron in afilterable form, zinc ferrite residues containing as much .... ..
~.
~3~
as 12-20~ Zn and 25-35% Fe were ponded and the contained zinc was lost to the process.
Three processes are now in use in electrolytic zinc plants for -the precipitation of iron in crystalline form.
They have received their names (Jarosite, Goethite and Hematite processes) according -to the mineralogical compound in which iron is precipitated. In the Jarosite Process ferric iron is precipitated from solution at 95~C by neutralization and addition of a jarosite forming reagent, generally, ammonium or sodium ions. The neutralizing agent, usually calcine, is required to neutralize part of the free acid in the solution and the acid liberated during hy-drolysis. The jarosite precipitate, containing some un-leached zinc ferrite and valuable metals such as lead and silver from the calcine used in neutralization is separated from solution by thickening and/or filtration.
Several procedures (Jarosite acid wash, Stagewise Jarosite Precipitation) are used to recover the zinc contained in the zinc ferrite accompanying the jarosite residue. These are all based on the fact that jarosites, once they have formed, are difficult to redissolve. In these processes, lead and silver values are lost with the jarosite residue.
Plants that have high lead and/or silver values in calcine have installed a pre-neutralization stage to reduce the free acidity in the hot acid leach solution before it enters the jarosite precipitation stage. The residue from this pre-neutralization is returned to the hot acid leach for recovery of a lead/silver residue.
, - :
A recent process has been developed (Australian Patent No. 506691) in which the leach solution is pre-neutralized with calcine, without the premature formation of jarosite, and without any addition of calcine during the jarosite precipitation stage. It is characterized by cooling the solution from the hot acid leach stage and then partially neutralizing it with calcine. The neu-tralization residue is separated and returned to the hot acid leach stage to recover lead and/or silver.
The solution is then advanced to the jarosite stage to partially precipitate the contained iron. It has been discovered, however, that it is necessary to reduce not only the acid concentration, but also the iron concentra-tion (dilution) to achieve practical iron precipitation rates. This requirement greatly increases the volumes of solution processed in the pre-neutralization and jarosite precipitation stagesO
In the goethite process, iron dissolved from the hot acid leach of zinc ferr~te is reduced to the ferrous state using zinc sulphide as the reducing agent. The solu-tion is then pre-neutralized with calcine (a greater amount of free acid can be neutralized since the iron, being in the ferrous state, does not hydrolyze) and the neu-tralization residue is returned to the hot acid leach for recovery of lead and silver. The ferrous iron is then re-oxidized, using air (or oxygen), and precipitated from solution as goethite. As in the jarosite process, calcine must be added to neutralize the acid liberated during iron hydrolysis. It is not possible to use a process stage, similar to a jarosite acid wash, to recover zinc from the residue, because goethi-te (.unlike jarosite) would also dissolve.
The hematite process (U.S. Patent No. 4,107,265) dissolves zinc ferrite residues in a reducing leach with spent electrolyte, make-up acid and either sulphur dioxide or zinc sulphide. Since the iron is in the ferrous state, the leach solution is neutralized in two stages, at least one of which employs limestone, without precipitation of iron. (The formation of gypsum by the use of limestone maintains the plant sulphate balance and reduces the sulphate concentration in the solution from which iron is later precipitated, thus resulting in low sulphur contami-nation of the hematite residue). The solution is then heated to 200C in autoclaves and the iron oxidized with oxygen to the ferric state. As it forms, ferric iron precipitates from the solution as a fine-grained ferric oxide ~hematite) residue. After washing, impurities in the hematite are reduced to such an extent that -the residue can be sold for iron recovery or other applications (cement and pigment industries). When storage is required, hematite residues have advantages over either jarosite or goethite in that they requlre a significantly smaller storage space, have better physical properties (dewatering and structural stability) and are environmentally more stable.
All of the above mentioned processes have distinctly separate stages for leaching the zinc residue, neutralizing the leach solution and removing the iron from the solu--- 5 --tion. Only the Conversion Process (Huygare T.L., Fugleberg S. and Rastas J, ~low Outokumpu Conversion Process raises Zinc Recovery, World Mining, p. 36-42, February 1974), a variation of the jarosite process, has been developed, for the single stage leaching of zinc ferrite and precipitation of iron. The Conversion Process results in very high zinc recoveries, but requires long retention times and is not suited for the recovery of lead and silver.
The Jarosite, Goethite and Hematite processes described above have allowed zinc producers to increase recoveries from calcine to 96-99~ and produce filterable iron residues, but they involve multiple steps and are not easily adapted to the treatment of stockpiled residues produced prior to their development.
Applicant has discovered an improved process for the treatment of zinc ferrite residues by pressure leaching with sulphuric acid and oxygen at elevated temperature to dissolve zinc, present as oxides, ferrites and sulphides, and other valuable metals, and simultaneously precipitate iron as hematite.
The process, in accordance with the present invention, comprises subjecting a slurry of the zinc ferrite residue to pressure leaching at elevated temperature and low acid concentration under slightly reducing initial conditions followed by pressure leaching at elevated temperature under oxidizing conditions, such as to form a precipitate essentially consisting of hematite, and separating such hematite from the resulting zinc containing solution.
The elevated temperature is about 220~C and the initial sulphuric acid feed is such as to maintain an acidity between 10 and 40 g/l H2SO4 during pressure leaching under reducing conditions and between 20 and 60 g/l H2SO4 during pressure leaching under oxidizing conditions.
The oxygen overpressure should be less than 3.5 kg/cm2 during pressure leaching under reducing conditions and higher than 0.5 kg/cm2 during pressure leaching under oxidizing conditions.
Leaching may be carried out in a single autocla~e or in two separate autocla~es without a solid liquid separation in between, the reducing conditions being controlled in the first autoclave and the oxidizing conditions in the second autoclave.
When using a single or two autoclaves, the initial reducing conditions may be obtained by controlling the oxygen transfer into the residue slurry so that the small amount of sulphide present in the residue reduces some of the ferric iron in solution. The initial reducing conditions may also be obtained by addition of a reducing agent or ferrous iron to the solid residue or to the residue slurry.
The sulphuric acid solution may be a zinc sulphate - solution or an alkali metal bearlng zinc plant electrolyte.
With the process in accordance with the present invention, it is possible to leach not only current zinc ferrite residues but also old stockpiled zinc ferrite residues which were produced prior to the development of technology for precipitation of iron in a crystalline form. Generally, the process in accordance with the present invention permits treatment of high-iron feeds, provides flexibility in flowsheet design for calcine segregation, and results in a hematite residue having improved properties for disposal by ponding.
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure 1 shows a flowsheet of the basic process in accordance with the invention;
Figure 2 shows rates of zinc extraction from a zinc ferrite residue with and without the addition of reducing agents; and Figure 3 shows the effect of oxygen pressure during the initial acid addition period on the rates of zinc extraction.
Referring to Figure 1, a zinc ferrite residue originating from a conventlonal neutral leach zinc plant operation or from a stockpiled zinc ferrite residue ~5 is leached with a sulphuric acid solution in a pressure reactor at an elevated temperature, preferably about 220~C. Leaching is carried out under initial reducing conditions as indicated by stage 10 in the flowsheet followed by oxidizing conditions as indicated by stage 12.
Stages 10 and 12 can be carried out continuously in a single multicompar-tment autoclave or in a two autoclave system. In the two autoclave system, there is no solid/
liquid separation between the autoclaves. The process could be carried out on a batch basis in a single auto-clave wherein slightly reducing conditions are cre~ted initi~l~y and oxidizing conditions subsequently applied.
The initial reducing conditions charactérized by high ratios of Fe(II) to Fe (III), can be met with the following measures which can be used alone or combined with one another:
a) The presence of suf~icient active sulphide/sulphur in the zinc ferrite residue.
b) Addition of a reducing reagent or ferrous ir~n to the solid residue or to the residue slurry.
c) Preventing excessive oxygen transfer into the slurry during the initial slightly reducing conditions by maintaining the oxygen overpressure at less than 3.5 kg/cm2 and/or selection of agitation conditions which minimize gas en~
trainment in the pulp~
d) The use of a two autoclave system wherein slightly reducing conditions are maintained in the first autoclave.
The second oxidizing condition is obtained by applying a sufficIent oxygen overpressure higher than 0.5 kg/cm~ to oxidize most of the Fe (II) ions passing from the first stage 10.
The solids to acid ra-tio in the feed is such as to produce a first stage concentration of 10-40 g/L HzSO4 and a second stage concentration of 20-60 g/L
H2SO4.
When a single multicompartment autoclave is used and there is sufficient active sulphide sulphur in the zinc ferrite residue, the small amount of sulphide present in the residue will reduce ferric iron at a faster rate than the oxidation of ferrous iron with oxygen. This,combined with the fact that most of any remaining ferric iron not re~uced by the sulphides hydrolyzes and precipitates as hematite results in a higher concentration of Fe(II~ than Fe(III) which maintains a reducing potential in the first stage slurry.
The combination of high temperature and low acid concentration, causes the iron in the trivalent state to precipitate mostly as hematite instead of one of the several basic iron sulphates. This in itself is an added advantage in the leaching of zinc ferrite particles since it has been found by the applicant that the precipitation of iron in any sulphate form results in longer reaction time requirements, with the zinc extraction curve showing an induction period.
The precipitate from stages 1~ and 12 is subjected to a solid/liquid separation staye identified by reference numeral 14 to separate the hematite residue which normally also contains metal values, such as lead, silver, gold, silica, from a leach solution which contains zinc and also copper and cadmium if present in the zinc ferrite residue. ~he residue is normally washed to recover the entrained metal values while the leach solution is returned to the reyular neutral leach plant operation to precipitate the remaining iron prior to the conventional stages of solution purification and metal recovery by electrolysis.
If the silver and/or lead values in the zinc ferrite residue justify their recovery, a suitable recovery process, such as a flota~ion circuit, can be installed prior to the reducing stage 10 to recover ~hese metal values fron t:he zinc ~errite residue. Such a recovery process may also be installed after the solid/
liquid separation stage 14 to recover the me-tal values from the hematite residue.
British Patent 986,909, granted to Sherritt Gordon Mines Limited, already d~scloses a two stage process for the treatment of zinc ferrite residues. The principal difference ~ith applicants process is that Sherritt precipitates iron as basic iron sulphates (FeOHSOb) in either of the options it discusses, whereas in the process no~ disclosed, iron precipitates as hematite.
As in the comparison between hematite and either , jarosite or goethite, explained above, an even more significant advantage exists between hematite and basic iron sulphates since -the weight ratio of residue/iron is double for the latter. Other disadvantages of producing S basic iron sulphate are its poor filtration characteristics and its lower environmental stability.
In the present invention at the elevated temperature involved, iron in tha zinc ferrite structure is either . transformed in the solid state through a topotactic reaction to hematite, so that the only acid involved is used to leach the zinc out as:
ZnFe~04 (ZnO-Fe203 )+ H2S04__~ Fe203 +ZnS04 +H20 (1) or, if it dissolves, part of the ferric iron immediately hydrolyses and precipitates as hematite, liberating acid which is consumed in the leaching reaction, so that the overall reaction is the same as (1) and the concentration of acid remains low. This mechanism can be explained as f OllGWS:
Dissolution: ZnFez04(s) + 4H2S04(l) ~ ZnSO~ Fez(SO4)3(l)+4H O
(2) Iron hydrolysis and precipitation:
' Fe2 (Su4) 3 (1) + 3H20(.1) ~~~~ Fez03(s)+3H2SO4(l) (3) Overall reaction (2+3):
ZnFe204 +H2S04 (1~ ZnSO4(1)+ Fe203(s) + H20(1) (4) The simultaneous occurrence of these reactions at reasonable rates is possible on]y at elevated temperature low acid concentration (20-60 g/L H2S04) and when there is an initial period under slightly reducing conditions~
U.S. Patent 4,362,7Q2 granted to Outokumpu Oy, also describes a two stage process for the treatment of zinc ferrite residues in which iron is finally precipitated as hematite. This process is a modification of Outokumpu Conversion process which introduces an improvement consisting of the production of hematite instead of jarosite. The first stage consists of an atmospheric oxidizing leach in which zinc ferrite residue and solution and a jarosite forming reagent are reacted at 80-105C, zinc ferrite is dissolved and iron is precipitated simultaneously as jarosite. This first stage is controlled so that only approximately half of the zinc ferrite residue is reacted and the solids residue which emerges from this step consists of jarosite and unreacted zinc ferrite. This residue is then transferred to a second stage at 220-250DC and oxygen pressure of 1-2 bars to complete the extraction of zinc and to convert the iron in the solid residue to hematite. This process depends on the hydrothermal decomposition of jarosite ~which must be added in greater proportion than zinc ferrite) to hematite, which produces the acid required to leach the zinc ferrite and since this reaction requires a higher temperature (about 240C) than what is necessary for the reaction of zinc-ferrite with sulphuric acid, it thus ':, - ~3 -controls the overall process which must be operated at a temperature higher than 220C, preferably at about 240C.
The invention will now be further described in S the following illustrative examples:
Example 1 A series of tests were run to determine the importance of the initial reducing period and the effect of reducing species (i.e. sulphides) in the residue feed. A zinc ferrite residue (13.6% Zn, 33.4% Fe, 0.88% Cu, 5.1% S04=/5 and 0.68% S=/S) was submitted to a sulphide flotation step using normal sulphide collectors. This produced a residue (tailings) that assayed 13.9% Zn, 3502% Fe, 0.85% Cu, 4.78~ S04=/S~ 0.34% S /S and resulted in the effective removal of 48% of the zinc ferrite residue S=/S.
The sulphide depleated residue was reacted in a 2-L autoclave at 220~C and 15% solids with a mixture of synthetic and actuaL zinc plant spen electrolyte (180-190 g/L H2SO4), sufficient to maintain an acidity of 30-60 g/L H2SO4in the reactor during the tests, with and without the addition of reducing reagents to the solid or liquid phase. For comparison, a sample of the original residue, without flotation treatment and without additives, was tested under the same conditions. In all tests, the zinc ferrite residue was heated in an aqueous zinc sulphate solution to avoid any reactions during the heat-up period. Zinc plant spent electrolyte, fortified with sulphuric acid, was added at temperature over a period of approximately 10 minutes, during which time the reactor was maintained oxygen free.
(The quantity of acid added to -the spent electrolyte was such that the "instantaneous" acid concentration after dilution of the fortified electrolyte with zinc sulphate solution used to slurry residue was equivalent to that in the original spent electrolyte). After completion of the acid addition, an oxygen overpressure of 3.5 kg/cmz was applied for the duration of the test. The reducing agent additions to the solid or li~uid feed in each test are described in Table I while Figure 2 ~hows the rates of zinc extraction.
- :L5 -_ BLE I
ADDITIONS TO HIGH TEMPERATURE CONVERSION TESTS
TEST Fig. 2 ~o REDUCING RE~GENTCurve OBJECTIVE
100 None E Leaching of as-received zinc ferrite residue.
` 104 None A Determine the effect of leaching a zinc ferrite residue treated to r~move active sulphides by flotation.
110 Equivalent amount B Reconstruct the original zinc of sulphide concen- ferrite residue (i.e. return the trate removed in the depleated sulphides) to determine previous flotation if there are hindering effects step caused by the reagents used in flotation.
111 6 g Technical grade D Determine the effect of adding ZnS chemical grade sulphide.
113 3 g Zinc concentrate C Determine the effect of adling (46% Zn, 11.0~ Fe, zinc concentrate to the zinc 25.1~ 5--/S) ferrite residue.
112 5 g/L Fe(II) ions F To create an immediate reducing potential in the zinc ferrite residue slurry during the initia]L
leaching stage.
The rssults of the above tests may be summarized as follows:
1. The absence of active reducing species in the zinc ferrite residue results in a ~rolonged induction period of the zinc extraction curve as indicated by curve A.
2. A comparison between curves A and B shows that;
the slow reaction rate in curve A is principally caused by the absence of sulphide and not by the presence of remaining reagents rom the flotation stage~
3. A comparison between curves B and E indicates that the effect of active sulphides was slightly diminished during the flotation operation, possibly by oxidation.
4. Curves D, C and F indicate that leaching of zinc ferrite can be greatly enhanced by the addition of any reducing species to the zinc ferrite residue or to the residue slurry.
5. The greater extraction rate observed upon addition of Fe(II) ions, curve F is attributed to the existence of an immediate reducing potential as opposed to the addition of solid sulphides which must react in a solid/liquid reaction with Fe(III) ions previously leached from the zinc ferrite residue. The condition of a large concentration of Fe(II) ions will occur in practice in a continuous system if the leaching stages are clearly separated into an initial reducing one followed by an oxidizing oneO
P3~
Example 2 Two comparative tests were run at 200 and 220C
(other variables identical) in a batch 2-L autoclave with a zinc ferrite residue assaying 12.3% Zn, 34.3% Fe, 0.56% Pb, 5.77% S, 4.85% sio2. The procedure employed 5 was identical to that described in example 1, except that the % solids and the quantity of acid used to fortify the spent electrolyte was such as to give an instantaneous acid concentration, after dilution, of 110 g/L. The conditions and results of both tests are 10 presented in llable II.
TABLE II
HIGH TEMPERATURE CONVERSION AT 200 and 220C
T~E Zn E~traction Resid~e Ccmposition S~MPT~ % FeS
h 200OC 220C 200C 220C 200C 220C
37 50.9 36 ~3.8 6.72 1.69 0.5 48.8 93 7 35.3 50.2 8.72 2.97 1.0 51.6 96.9 34.9 53.3 8.91 2.02 1.5 97.0 54.0 1.65 2.0 53.5 98.0 :35 54.6 9.40 1.43 3.0 55.5 35.2 9.98 The results of the above test indicate that a higher zinc recovery is obtained at 220C than at 200C.
Furthermore, sulphur precipitation is lower at 220 C, indicating that a higher iron grade hematite is obtainedO
Example 3 Samples of stockpiled zinc ferrite residue (12.3% Zn, 34.3% Fe, 0.56% Pb, 4.85% sio2 ~ 5.25% S04 /S, 0.71% S /S, 0.9% NH4+) were leached in a 2-L glass lined titanium autoclave at 15% and 30% solids with equivalent instanta-neous acid concentrations of 110 and 170 g/L H2SO4. The test procedure was as employed previously, except that oxygen overpressure was set at 0, 1.75, or 3.5 kg/cm2 during the period of acid addition and fixed at 3.5 kg/cm2 immediately after acid addition for the remaining part of the run.
Table III summarizes the test data.
TABLE III
H2SO4Equivalent H2SO4 Oxygen Test % Instantaneous Consu~ion Gverpressure-N ~ er Solids Concen~ration, g/L kg/kg residue during Acid _ AddKg/cmn2 67 15 110 0.22 0 30 170 0.26 1.75 78 15 110 0.22 3.5 Zinc extraction rates (Figure 3) clearly indicate the advantage of decreasing or eliminating the oxygen overpressure during the initial leaching period. An extraction level of 95% was reached in just over 0.5 h, 1.0 h and 1.5 h at 0,1.75 and 3.5 kg/cmZ oxygen over-pressure respectively.
~xample ~
A zinc ferrite residue (13.3% Zn, 35.8% Fe, 1.1% Cu, 5.8% S, 0.71% Pb, 2.7% sio2 ~ 170 Ag/tonne) obtained from a residue pond stockpile was reacted on a continuous basis in a cascade autoclave system, consisting of four 38L vertical titanium vessels provided with transfer tubes between the vessels so as to permit gravity flow of slurry and a common gas phase in all the system. This simulates a multicompartment horizontal autoclave which is normally used for hydrometallurgical industrial processes. The residue was slurried with water and reacted with zinc plant spent electrolyte (58 g/L Zn, 185 g/L H2SO4, 3g/L NH4+) at 220C under conditions which maintained a reducing potential, as reflected by high Fe(II) to Fe (III) ratios in the first autoclave solution. The oxygen overpressure was 0.6-0.7 kg/cm2, and the acid/residue ratio was adjusted to give a solution acidity of 10-25 g/L H2SO4 in the first autoclave.
At steady state conditions, final e~tractlons of 96% Zn, 92% Cu and 90% C~ were obtained with an overall residence time of 2.3-2.4 h, as indicated in Tables IV and V, and hematite residue assaying 0.7% Zn was produced.
TA~LE IV
TEST CO:NDIl7IONS
~utoclàve Fee~l Residue SlurrySFent hlectrolyte Autoclave Oxygen kg/h % T Volume T TemFeraturePressure Solids C L/h oC oC ~/cmz _ _ 22 36 160 19 90 220 0.6 T~BLE V
RESULTS AT STEADY STATE
. . .
Autoclave ¦ `~iqui~ g/L I Extraction, ~
Zn Fe~7) Fe(II) H2SO4 -~F: Zn Cu Cd , _ _ 1 6.2 26.4 26.3 23.5 1.0 62.8 2 2.6 0.6 0.1 37.2 0.17 84.0 3 1.3 0.7 0.1 40.9 0.14 92.5 4 ~.~ 0.8 0.1 46.6 0.13 96 2 92 90 Example 5 The same residue was tested as in example 4 with the exception that the acid/solids ratio was increased so that the acidity in the first autoclave was 42-46 g/L H2 S04 . As observed in Tables VI and VII this reduced the zinc extraction and resulted in significant precipitation of iron as a basic sulphate instead of hematite.
TABLE VI
TEST C O ND I~ IONS
Autoclave Feed _ _ _ _ R esidue Slurry Spent ElectrolyteAu.toclave Oxygen kg/h % TVolume T Temperature Pressure Solids CL/h C C kg/Gm2 160 1 27 90 220 0.6 TABLE VI I
RESULTS AT STEADY STATE
~clave _ C ~?Sition NoSolids (%) Liquid (g/L) Zinc Zn Fe - S Fe ~ Fe(II) H2SO4 F-F~Extraction, %
. .. . .
1 9.9 37.46.7~10.6 7.6 42.8 0.72 29 2 8.1 36.4 8.1 4.2 0.7 49.9 0.17 40 3 7.5 36.7 8.6 3.9 0.3 53.3 0.08 45 4 6.1 36.9 9.3 4.2 0.2 52.4 0.05 56 Exam~le 6 A similar residue (16.0% Zn, 37.2% Fe, 1.3% Cu, 4.2% S, 0.71% Pb, 43% sio2 ~ 180 Ag/tonne) was reacted with spent electrolyte (54 ~/L Zn, 189 g/L H2SO4) under similar conditions as in example 4, except that the oxygen overpressure was raised to 3.1 kg/cm2. As observed in TABLE VIII and IX this caused slightly less reducing conditions in the first autoclave which resulted in lower rates zinc extraction TABLE VIII
TEST C~NDITIONS
. _ Autoclave Feed _ Residue Slurry Spent Electrol~te Autoclave Oxygen kg/h % T ~olume T Temperature Pressure : Solids C L/h C C kg/cm2 34 41 130 150 21 88 220 3.1 TABLE_IX
RESULTS AT STEADY STATE
Composition I
~utoclave Solids (%) Liquid (g/L) ! FeII Zinc Extraction, No. Zn Fe(T). Fe(II) H2SO4 Fe(T~ %
_ _ . .
1 9.5 1~ 8.3 19.70.69 44 2 7.3 1.3 0.3 3n.60.23 63 3 5.9 1.3 0.3 31.10.23 72 . 4 2.6 0.9 30-9 0.35 79