Abstract:
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:
This is a Continuation-in-part of U.S. patent application No. 07/480,374 filed Feb. 14, 1990, now U.S. Pat. No. 5,028,410. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the production of high quality zinc oxide from roasted zinc sulphide concentrates. 
     BACKGROUND OF THE INVENTION 
     By &#34;roasted zinc sulphide concentrates&#34; is meant the product obtained by roasting a zinc sulphide bearing concentrate. The term zinc calcine is also used to refer to such products. Typically, roasted zinc sulphide concentrates contain zinc oxide and other components, including zinc sulphate, zinc sulphide, mixtures of metal oxides such as copper, lead, calcium, cadmium, and magnesium, zinc ferrite, magnetite, possibly some haemetite, and usually some metal silicates, sulphate and sulphides. 
     Zinc sulphide bearing concentrates are commonly roasted in a fluid bed roaster. In the fluid bed roasting of zinc concentrates, the concentrates are continuously fed into the roaster chamber, wherein air blown up through the chamber converts sulphides to oxides releasing sulphur into the gas stream chiefly as sulphur dioxide, with the development of heat. Smaller and lighter particles in the feed concentrate tend to be carried out of the chamber with the gas stream and are collected by downstream equipment. Larger and heavier particles tend to remain in the roaster fluidized bed through which the moving air is blown. Since the bed is full during continuous operations, a bed overflow stream outlet is provided by which further accumulation of coarse solids is prevented by allowing bed material to overflow out of the chamber. This material is referred to as &#34;bed overflow calcine&#34;. In some zinc refineries, calcine is composed 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 is 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. Pat. No. 5,028,410 hereafter referred to below. 
     In accordance with a previous invention, U.S. Pat. No. 5,028,410 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 ammonium 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 baghouse dust, to produce such high quality, high surface area zinc oxide as are obtained by U.S. Pat. No. 5,028,410. Zinc oxide with the high specific surface area produced by U.S. Pat. No. 5,028,410 can be classed as premium zinc oxide. 
     A new discovery now reveals that by the use of bed overflow calcine alone, rather than normal calcine as discussed above, a number of improvements result. This is the essence of the present invention. 
     It 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 present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     It has now been discovered that by using bed overflow calcine as opposed to normal calcine as feed material to the ammoniacal ammonium carbonate leaching process 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. Pat. No. 5,028,410. 
     These will be described in the forthcoming section entitled, &#34;Description of the Preferred Embodiment&#34;. 
     Reference is made to U.S. Pat. No. 5,028,410 which is incorporated herein by reference. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one embodiment, the bed overflow calcine is fed directly to ammoniacal ammonium carbonate leaching vessels operating continuously under similar conditions as in U.S. Pat. No. 5,028,410. 
     The slurry leaving the above-described leaching vessels is then fed to a liquid-solid separation vessel as described in U.S. Pat. No. 5,028,410 to separate a pregnant solution containing most of the zinc that was in the calcine from a solid residue containing nearly all of the iron that was in the calcine. The solid residue is returned to the zinc refinery for further treatment or can be treated otherwise according to conventional means. 
     The pregnant solution is then fed to a cementation step as described in U.S. Pat. No. 5,028,410 for removal of impurities such as cadmium and copper where zinc dust is used for cementation purposes. 
     A liquid-solid separation step as described in U.S. Pat. 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. Pat. No. 5,028,410 wherein basic zinc carbonate is precipitated and much of the ammonia is stripped off and sent to ammonia recovery equipment, also described in U.S. Pat. No. 5,028,410. 
     The solid phase containing the basic zinc carbonate 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 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 275° C. and 600° C. to produce an essentially pure, high surface area zinc oxide having a range of 10 to 75 m 2  /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 1000° C. 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 be produced for other markets. 
     Some of the benefits of the improved invention over the previous invention are described below. 
     Details of the various 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. Pat. 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 material to the ammoniacal ammonium carbonate leaching system. 
     Thirdly, the leach liquor purification step is simplified by the reduction in cadmium be removed. 
     Fourthly, the chemical analyses of the intermediate product, namely basic zinc carbonate, and the final zinc oxide product are improved. 
     Fifthly, there is no desire or need for a prewashing step and subsequent liquid-solid separation step prior to feeding to the ammoniacal ammonium carbonate to leaching vessels. 
     In the sixth place, the liquid-solid separation step following the ammoniacal ammonium leaching step is 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 DRAWING 
     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 calcine 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% extractable zinc, comprising 54.9% in an ammoniasoluble form and 2.8% in a water-soluble form. 
     A specimen 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 11.0. An amount of 450 g of calcine was leached in 1.5 liters of this aqueous ammoniacal ammonium carbonate solution with agitation at a temperature of 50° C. Leaching was conducted for 40 minutes with the occasional addition of gaseous carbon dioxide gas to maintain the pH at approximately 11.0. The resulting slurry was filtered through a filter and the resulting 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 0.5 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 zinc dust cementation for 30 minutes at a solution pH of approximately 11.0 and a solution temperature of approximately 40° C. 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. 
     The purified zinc solution obtained from the zinc dust cementation step after liquid solid separation was steam-stripped by injecting live steam into the purified zinc solution until most of the ammonia was expelled and basic zinc carbonate precipitated, approximately. Analyses of the purified zinc solution, the resulting precipitate, and the depleted solution are reported in Table 4. The pH of the final 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, 61.2% versus 57.7%. Also the total zinc is higher 67.0% versus 63.7%. 
     The cadmium content of the bed overflow calcine is very much lower than the normal calcine, thus resulting in simpler leach liquor 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 is 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 such a washing step would not be either desirable or necessary. 
     EXAMPLE 2 
     In this example a specimen 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 1,000° C. Analysis of the resulting calcine is presented in Table 5. 
     It is apparent 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 Overflow               Normal Calcine     Calcine (wt %)               (wt %)______________________________________Arsenic     0.006       0.024Sulphur     0.004       0.02Cadmium     &lt;0.0005     &lt;0.00004Iron        0.003       0.0059Silicon     &lt;0.001      0.016______________________________________ 
    
     
                       TABLE 1______________________________________CALCINE ANALYSIS______________________________________             Normal     Bed OverflowElement           Calcine    CalcineWeight %          Weight %   Weight %______________________________________Zn                63.7       67.0Cu                0.85       0.92Cd                0.33       0.14Co                0.02       0.017Fe                10.1       9.31Pb                0.05       0.022Ca                0.22       0.12Mg                0.063      0.068Si                0.55       0.01As                0.04       0.015Cl                0.002      0.005S                 1.8        0.14Mn                0.02       0.01Zinc Components             Normal     Bed Overflow             Calcine    Calcine             wt % Zn    wt % Zn______________________________________Zn SO.sub.4 (water soluble)             2.8        0.2Zn O (ammonia soluble)             54.9       61.0Zn SiO.sub.2 (acetic acid soluble)             1.8        2.1Zn Fe.sub.2 O.sub.4 (H.sub.3 PO.sub.4 /HCL soluble)             4.4        4.2Total             63.9       67.5Size Analysis             Normal     Bed Overflow             Calcine    Calcine             wt % Passing                        wt % Passing______________________________________100 Mesh          94.4       52.4200 Mesh          87.3       28.7400 Mesh          68.0       2.6______________________________________ 
    
     
                       TABLE 2______________________________________  Normal Calcine                Bed Overflow Calcine    Filtrate  Residue   Filtrate                                ResidueElement  g/l       wt %      g/l     wt %______________________________________Zn       150       29.5      185     30.5Cu       1.58      1.01      1.58    1.53Cd       0.48      80.2      0.27    0.17Co       0.017     0.027     0.022   0.023Fe       0.008     35.2      0.035   35.5Pb       0.009     0.14      0.011   0.064Ca       0.026     0.45      0.048   0.37Mg       0.033     0.15      0.055   0.18Si       0.01      1.03      0.012   1.06As       0.052     0.085     0.013   0.043Cl       0.03      --        0.035   0.005S        3.99      0.25      0.22    0.05Mn       &lt;0.0002   0.03      0.0003  0.038______________________________________ 
    
     
                       TABLE 3______________________________________CEMENTATION OF BED OVERFLOW CALCINE LEACHLIQUOR    Leach Liquor    Feed Solution  Filtrate ResidueElement  g/l            g/l      wt %______________________________________Zn       185            190      65.6Cu       1.58           0.0002   26.7Cd       0.27           0.001    5.29Co       0.022          0.0004   0.42Fe       0.035          0.014    0.37Pb       0.011          0.0008   0.25Ca       0.048          0.051    0.005Mg       0.055          0.061    0.009Si       0.012          0.017    0.06As       0.013          0.009    0.017Cl       0.035          0.044    0.005S        0.22           0.29     0.014Mn       0.0003         0.0003   0.0______________________________________ 
    
     
                       TABLE 4______________________________________STEAM STRIPPING                  Final    Feed Solution Solution PrecipitateElement  g/l           g/l      wt %______________________________________Zn       190           0.005    60.0Cu       0.0002        0.0002   0.005Cd       0.001         0.0002   0.004Co       0.0004        0.0003   0.003Fe       0.014         0.0003   0.013Pb       0.0008        0.001    0.011Ca       0.051         0.002    0.023Mg       0.061         0.0002   0.025Si       0.017         0.003    0.05As       0.009         0.001    0.014Cl       0.044         0.036    0.005S        0.29          0.081    0.09Mn       0.0003        0.0001   0.0009NH.sub.3 190           0.46     0.24Amount   0.875 L       1.139 L  251.57 g______________________________________ 
    
     The amount of feed solution used was 0.875 l, the final solution amounted to 1.139 l, and the dried precipitate weighted 251.57 g. 
     
                       TABLE 5______________________________________                   Bed Overflow       Normal Calcine                   CalcineElement     Wt %        Wt %______________________________________Zn          81.5        81.2Cu          0.0019      0.001Cd          &lt;0.0005     &lt;0.00004Co          &lt;0.0005     0.001Fe          0.0059      0.003Pb          &lt;0.0005     0.0006Ca          0.021       0.031Mg          0.022       0.026Si          0.016       &lt;0.001As          0.024       0.006Cl          &lt;0.004      &lt;0.004S           0.02        0.004Mn          &lt;0.0002     0.0001NH.sub.3    &lt;0.0006     0.046Li          0.00003     0.00004Al          0.002       0.002______________________________________