CA2072714A1 - Hydrometallurgical production of zinc oxide from roasted zinc concentrates - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process is provided for obtaining high quality high surface area zinc oxide from a roasted zinc sulphide concentrate by leaching with an ammoniacal ammonium carbonate solution. The process utilizes optional preliminary oxidation and water leaching steps to remove soluble impurities and an optional aqueous sulphur dioxide leaching step to selectively remove readily soluble zinc oxide. The process includes an improved source of roasted zinc sulphide concentrate.
The process also includes an optional reduced pressure calcining step to produce zinc oxide with further increased surface area.

Description

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HYDRONETALURGICAL PRODUCTION OF ZINC_OXIDE FROM
ROASTED ZINC CONCENTRAT~S

FIELD_OF TEE INVRNTION
Thi3 invention relates to the production of high quality zinc oxide from roasted zinc sulphide concentrates.

~ACRGROUND OF T~E INVEN~ION
~ y "roastQd zinc sulphide concentrates" is meant the product obtalned by roasting a zinc sulphide bearing concentrate. The term zinc calcine i8 al80 u~ed to refer to such products. Typically, roasted zinc sulphide concentrate~ contain zinc oxide and other components, including zinc sulphate, zinc sulphide, mixtures of metal oxides such as copper, lead, calcium, cadmium, and msgnesium, zinc ferrite, magnetite, possibly some haemetite~, and usually ~ome metal silicates, ~ulphate and sulphides.
Zinc sulphide bearing concentrates are commonly roasted in a fluid bed roaster. In the fluid bed roast~ng of zinc concentrates, the concentrates are continuously fed into the roaster chamber, wherein air blown up through the chamber converts sulphides to oxides rQleasing sulphur into the gas stream chi~fly as 2 2~7~l1 sulphur di~xide, with the development of heat. Smaller and lighter particles in the feed concentrate tend to be carried out of the chamber with the ga~ stream and are collected by downstream equipment. Larger and heavier particles tend to remain in the roaster fluidized bed through which the moving air i8 blown.
Since the bed i8 full during continuous operations, a bed overflow stream outlet is provided by which further accumulation of coarse ~olids is prevented by allowing bed material to overflow out of the chamber. This material is referred to as ~bed overflow calcine~. In some zinc refineries, calcine is compo~ed of an approximately 50:50 by weight mixture of bed overflow calcine and fine material that is recovered from the gas stream, as described above. This mixture is then used as a feed material in the production of zinc metal and iB referred to as normal calcine at least in some cases, such as at the zinc refining from which samples were used in the U. S. Patent No. 5,028,410 hereafter referred to below.
In accordance with a previous invention, U.S.
Patent No. S~028,410 aqueous ammoniacal ammonium carbonate solutions are used to selectively leach zinc from roasted zinc sulphide concentrates, leaving a substantial portion of the iron components in the leach residue.
Ammoniacal ammoniumS'carbonate solutions have been suggested for treating zinc scrap and mini-steel plant baghouse dusts to recover zinc oxide. However, it was not previously recognized that such solutions could be used to leach roasted zinc sulphide concentrates, which are dissimilar to zinc scrap and baghou6e dust, to produce such high quality, high surface area zinc oxide as are obtained by U. S. Patent 5,028,41d. Zinc 2~7~7~

oxid~ with the high specific ~urface area produced by U. S. Patent No. 5,028,410 can be classed as premium zinc oxide.
A new di~covery now reveals that by the use of S bed overflow calcine alone, rather than normal calcine as discussed above, a number of improvements result.
Thi~ is the essence of the present invention.
I~ is to be noted here that the ratio of bed overflow calcine to dust carryover calcine can vary widely at different zinc refineries. The ratio will not always be approximately 50:50. This would, in no way, detract from the practice of the pre~ent invention.

BRIFF SUMMARY OF THE INVENTION
It has now been discovered that by using bed overflow calcine as oppo~ed to normal calcine as feed material to the ammoniacal ammonium carbonate leaching pxoce~s for normal calcine, a number of very important improvements result, which should result in greater commercial utilization of the process in its improved form, as opposed to the process described in U.S.
Patent No. 5,028,410.
These will be described in the forthcoming section entitled, "Description of the Preferred Embodiment".
Reference is made t'o U. S. Patent No. 5,028,410 which is incorporated herein by reference.

DESCRIPTION OF THE PREFE M ED EMBODIMENTS
In one embodiment, the bed overflow calcine i3 fed directly to ammoniacal a~monium carbonate leaching ves6els operating continuously under similar conditions as in U. S. Patent No. 5,028,410.

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The slurry leaving the above-described leaching ves~els is then fed to a liquid-601id separation vessel as described in U. S. Patent No. 5,028,410 to separate a pregnant solution containing most of the zinc that Swa~ in the calcine from a solid residue containing nearly all of the iron thnt wss in the calcine. The solid residue i8 returned to the zinc refinery for further treatment or can be treated otherwise according to conventional means.
10The pregnant solution is then fed to a cementation step as described in U. S. Patent No.
5,028,410 for removal of impurities such as cadmium and copper where zinc dust is used for cementation purposes.
15A liquid-solid separation step as described in U. S. Patent No. 5,028,410 is then employed to separate the purified solutions from the solid residue.
The purified solution is then fed to thermal decomposition vessels as described in U. S. Patent No.
205,028,410 wherein basic zinc carbonate i8 precipitated and much of the ammonia is stripped off and sent to ammonia recovery equipment, also described in U. S.
Patent No. 5,028,410.
The solid phase containing the basic zinc 25carbonate is separated from,,the remaining liquid phase by liquid-solid separation techniques with the liquid phase being recycled to an' earlier step in the process.
The basic zinc carbonate solid is then washed to remove entrained impurities and can be marketed as 3~such or fed to a calciner where its carbon dioxide content and water content can be driven off in order to produce zinc oxide.
The calcining step is operated at a temperature between 275C and 600C to produce an essentially pure, 20727~

high surface area zinc oxide having a range of 10 to 75 m2/g~
In a second embodiment, the procedure of the first embodiment is followed except that the calcining temperature is operated at a temperature of from 600 to 1000C to produce a zinc oxide product that can be made to be essentially equivalent to French Process zinc oxide.
In the third embodiment, both the first and second embodiments would both be used so that both high surface area zinc oxide could be produced to serve certain markets and low surface area zinc oxide could b~ produced for other markets.
Some of the benefits of the improved invention over the previous invention are described below.
Details of the variou~ improvements are shown in the examples to follow. The improvements apply to both embodiments.
In the first instance, the bed overflow is greatly superior composition to the normal calcine, such as used in U. S. Patent No. 5,028,410. In addition, if the normal calcine consisted only of roaster dust carryover material, the comparison between the two materials would be substantially greater insofar as to the improvement characteristics of the bed overflow~ material are concerned.
Unfortunately, we do not have any examples yet to compare such differences.
Secondly by using bed overflow calcine as a feed material, downstream processing during continuous operations becomes greatly simplified, particularly in relation to the costs and complexities incurred in the treatment of the recycle liquor returned as a feed I

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material to the ammoniacal ammonium carbonate leaching system.
Thirdly, the leach liquor purification step i8 simplified by the reduction in cadmium be removed.
Fourthly, the chemical analyses of the intermedlate product, namely basic zinc carbonate, and the final zinc oxide product are improved.
Fifthly, there is no desire or need for a pre-- washing ~tep and subsequent liquid-solid separation step prior to feeding to the ammoniacal ammonium carbonate to leaching ~essels.
In the sixth place, the liquid-Rolid separation step following the ammoniacnl ammonium leaching step i~
improved because, of the larger particles for one thing.
There are other interesting advantages, such as improved water washing steps and other items not specifically mentioned here but will become obvious from a study of the examples.

Brief Description of Drawinq Fig. 1 provides a description of the process in the form of a block diagram.

Example 1 In this example a specimen of normal zinc calcine and a specimen of bed overflow zinc calcine were obtained and analyzed. The results of the analysis are reported in Table 1. The bed overflow calcine contained lesser amounts of arsenic and sulphur compared to the normal calcine. The bed overflow - 30 calc.~ne contained 61.2% extractable zinc, comprising 61.0% in an ammonia-soluble form and 0.2% in a water-soluble form. The normal calcine contained 57.7%

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extractable zinc ~ comprising 54.9~ in an ammonia-~oluble form and 2.8% in a watsr-soluble form.
A ~pecimen of normal and bed overflow calcine were separately processed in accordance with the process described below.
The calcines were leached in a 28% aqueous ammoniacal ammonium carbonate solution which was prepared by feeding gaseous carbon dioxide into a concentrated ammonium hydroxide solution with vigorous stirring. The final pH of this solution was 11Ø An amount of 450 g of calcine was leached in 1.5 litres of this aqueous ammoniacal ammonium carbonate solution with agitation at a temperature of 50C. Leaching was conducted for 40 minutes with the occasional addition of gaseous carbon dioxide gas to maintain the pH at approximately 11Ø Th~ resulting slurry was filtered through a filter and the resultinq filtrate and residue were analyzed. The assay of the resultant filtrate and residue is reported in Table 2.
A mixture comprising 0.5 l/minute of air and O.S
l/minute of oxygen was sparged into the leaching vessel. The leach filtrate from the bed overflow calcine contained 0.013 g/l arsenic and 0.22 g/l sulphur, whereas the filtrate from the normal calcine contained 0.052 g/l arsenic and 3.99 g/l sulphur.
The leach filtrate from the bed overflow calcine was then purified by ~inc dust cementation for 30 minutes at a solution pH of approximately 11.0 and a solution tempexature of approximately 40C. 8 g/l of fine zinc dust was used to cement out almost all the copper, cadmium, lead and cobalt in the filtrate;
arsenic and sulphur were not affected. Analysis of the feed solution, and the cementation residue and filtrate are provided in Table 3.

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The purified zinc solution obtained from the zinc dust cementation step af ter liquid xolid separation was steam-stripped by in~ecting live ~team into the purified zinc solution until most of the ammonia was 5 expelled and basic zinc carbonate precipitated, approximately. AnalyE~es of the purified zinc solution, the resulting precipitate, and the depleted solution are reported in Table 4. The p~ of the f inal solution was reduced to between approximately 8 to 8.5.
In this particular example, it is obvious that the bed overflow calcine was of superior quality to normal calcine in that the extractable zinc is substantially higher, 61.296 versus 57.796. Also the total zinc is higher 67.0% versus 63.796.
The cadmium content of the bed overflow calcine is very much lower than the normal calcine, thus resulting in simpler leach lîquor purification.
The particle size of the bed overflow is much larger, thus resulting in easier liquid-solid separation.
The zinc sulphate content of the bed overflow is essentially negligible as compared to that of normal calcine thus indicating a much simpler and less costly recycle system for the improved process.
It i8 also obvious in this example that there is no need for a pre-water-washing step and consequent liquid-solvent separation~'since this step is sometimes desirable to remove sulphates and it can be seen that the sulphate content of the bed overflow material is so low that ~uch a washing step would not be either desirable or necessary.

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Example 2 In this example a ~pecimen of normal zinc calcine and a specimen of bed overflow calcine were treated in general accordance with the process disclosed in Example 1 above.
The basic zinc carbonate was then calcined at l,000C. Analysis of the resulting calcine i~
presented in Table 5.
It is appsrent from Example 2 that bed overflow calcine produces a higher purity zinc oxide as compared to normal calcine with respect to the following elements:

Bed OverflowNormal Calcine Calcine (wt %) lwt %
Arsenic - 0.006 0.024 Sulphur - 0.004 0.02 Cadmium - ~0.0005 ~0.00004 Iron - 0.003 0-0059 Silicon - <0.001 0.016 2~727 ~r~
CALCINE ANALYSIS
Element Normal Calcine Bed Overflow N~ight ~ Weight % Calcine ~eight %
__ . ____ __ Zn 63.7 67.0 Cu 0.85 0.92 Cd 0.33 0.14 Co 0.02 0.017 Fe 10.1 9.31 Pb 0.05 0.022 Ca 0.22 0.12 Mg 0.063 0.068 Si 0.55 0.01 As 0.04 0.015 Cl 0.002 0.005 S 1.8 0.14 Mn 0.02 0.01 Zinc ComPonents Normal Bed Overflow Calcine Calcine wt % Zn wt % Zn Zn So4 (water soluble) 2.8 0.2 Zn O (ammonia soluble) 54.9 61.0 Zn SiO2 (acetic acid soluble) 1.8 2.1 Zn Fe2O4 (H3 PO4/HCL soluble) 4,4 4.2 Total 63.9 67.5 Size Analvsis Normal Bed Overflow Calcine Calcine wt % Pas~ing wt % Pas~ing 100 Mesh 94.4 1 52.4 200 Mesh 87.3 28.7 400 Mesh 68.0 2.6 7 2 7 ~

TA~I~ 2 NO~mA 1 Calcine sed Overfluw Calcine ~lement Filtrate Residue Filtrate Residue g~l wt % g/l wt %

zn 150 29.5 185 30.5 Cu 1.58 1.01 1.58 1.53 Cd 0.48 80.2 0.27 0.17 Co 0.017 0.027 0.022 0.023 Fe 0.008 35.2 0.035 35.5 Pb 0.009 0.14 0.011 0.064 Ca 0.026 0.45 0.048 0.37 Mg 0.033 0.15 0.055 0.18 Si 0.01 1.03 0.012 1.06 A~ O . 052 0.085 0.013 0.043 Cl 0.03 _ ~.035 0.005 S 3.99 0.25 0.22 0.05 Mn <O.0002 0.03 0.0003 0.038 12 2~7 CEMENTATION OF BED OVERFLOW CALCINE LEACH LIQUOR

Leach ~iquor Element Feed Solution Filtrate Residue g/l g/l wt %
. . . _ . . _ _ .
Zn 185 190 65.6 Cu 1.58 0.0002 26.7 Cd 0.27 0.001 5.29 Co 0.022 0.0004 0.42 Fe 0.035 0.014 0.37 Pb 0.011 0.0008 0.25 Ca 0.048 0.051 0.005 Mg 0.055 0.061 0.009 Si 0.012 0.017 0.06 As 0.013 0.009 0.017 Cl 0.035 0.044 0.005 S 0.22 0.29 0.014 Mn O.0003 0.0003 0.O

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STEAII STRIPPING
Final l~ t Feed SolutionSolution Precipitate g/l g/l w~: %
.
Zn 190 ~.005 60 r 0 Cu 0.0002 0.0002 0.005 Cd 0.001 0.0002 0.004 Co 0.~004 0.0003 0.003 Fe 0.014 0.0003 0.013 Pb o.ooO~ 0.001 0.011 Ca 0.051 0.002 Q.023 Mg 0.061 0.0002 0.025 'S~ 0.017 0.003 O.OS
As O.Oo9 0.001 0.014 Cl 0.044 0.036 0.005 S 0.29 0.081 0.09 Mn 0.0003 0.0001 0.0009 NH3 190 0.46 0.24 Amount 0.875L 1.139L 251.57g The amount of feed solution used was 0.875 1, the final solution amounted to 1.139 1, and the dried precipitate weighted 251.57 g.

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TAsL~ 5 ~ed Overf low Element Normal Calcine Calcine Rt % Wt %
. .
Zn 81.5 81.2 Cu O . 0019 0 . 001 Cd <0.0005 <0.00004 Co ~0.0005 0.001 Fe 0.0059 0.003 Pb <0.0005 0.0006 Ca 0.021 0.031 Mg 0.022 0.026 Si 0.016 <0.001 A8 0.024 0.006 Cl <0.004 <0.004 S 0.02 0.004 Mn <O .0002 0.0001 NH3 <0.0006 0.046 Li 0.00003 0.00004 Al 0.002 0.002

Claims (3)

1. A process for the production of zinc oxide comprising the steps of:
(a) roasting zinc sulphide bearing concentrates in a fluid bed roaster wherein both dust carry over calcine and bed overflow calcine are produced;
(b) slurrying a roasted zinc sulphide concentrate consisting only of bed overflow calcine in an aqueous solution of ammoniacal ammonium carbonate solution to dissolve zinc contained therein;
(c) separating a zinc bearing leach liquor from the leach slurry;
(d) purifying the zinc bearing leach liquor by cementation to remove at least one of the metals of copper and cadmium, prior to the step of steam stripping the purified zinc bearing leach liquor.
(e) steam stripping the purified zinc bearing leach liquor to remove ammonia and to precipitate basic zinc carbonate; and (f) separating the precipitated basic zinc carbonate and calcining the basic zinc carbonate to produce zinc oxide at a temperature in the range of 275°C to 1,000°C.

2. A process as claimed in claim 1 wherein the calcining temperature is in the range of 275°C to 600°C
in order to produce high surface area zinc oxide in a range of 10 m2/g to 75 m2/g

3. A process a claimed in claim 1 wherein the calcining temperature is in the range of 600°C to 1,000°C in order to produce zinc oxide near or equivalent to French process zinc oxide.