Abstract:
A process for the production of high purity iron sulfate monohydrate from an iron sulfide mineral feed involving a leaching, purification, crystallization and dehydration procedure to recover a solution of iron sulfate containing iron sulfate polyhydrates and to convert iron sulfate polyhydrates in a series of steps to monohydrate crystals. The process can be also advantageously used with iron sulfide feed materials containing non-ferrous metals with full separation and recovery of the non-ferrous metals in the sulfide residue of the leaching and purification process of this invention. It can also be used for the purification of industrial pickle liquor solutions which can be treated with the iron sulfide mineral feed in this process leach circuit with full recovery of the ferrous and non-ferrous metals content of these solutions.

Description:
This is a continuation of application Ser. No. 410,217, filed Oct. 26, 1973, now abandoned, which was a division of application Ser. No. 113,553, filed Apr. 13, 1971, now U.S. Pat. No. 3,860,696. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the production of a high purity iron sulfate monohydrate and a separate concentrated sulfide residue containing valuable metals from complex sulfide ores and concentrates containing iron sulfide minerals and non-ferrous metals of significant value. 
     It is particularly suited to the treatment of iron sulfide ores containing base metal sulfides which because of their complex mineralogy or relatively fine grain size distribution do not lend themselves to economic processing by conventional methods of separation and beneficiation for the selective recovery of the valuable metals they contain. 
     It has, furthermore, the important advantage of recovering a substantial part of the iron sulfides sulfur in the form of hydrogen sulfide. Thus it lends itself to the adoption of pollution free flow-sheets for the complete separation and recovery of all of the valuable constituents of the sulfide ores or concentrates, including both iron and base metals while all of the sulfur can be recovered as elemental sulfur, which is the form under which it can be marketed most easily. 
     Prior art processes for the production of iron sulfate, generally have used available supplies of industrial wastes such as the acidic effluent from coal, steel, and pigment production, as feed materials, and generally have operated primarily as waste disposal processes. Most of these processes are not capable of extracting iron from iron bearing sulfide minerals. 
     Essentially, in all of them, non-ferrous impurities remain with the iron products recovered, thus adversely affecting their quality and reducing their value for use in the production of steel. Furthermore, these processes tend to consume excessive quantities of fuel to dehydrate the iron sulfate. This results in prohibitive costs for the products obtained, which cannot be marketed profitably. Under present conditions, the use of these processes would result in commercial losses and would only be justified on the basis of waste disposal considerations. 
     The prior art process of U.S. Pat. No. 3,053,651 is capable of extracting and recovering iron both from iron bearing sulfide minerals, and from waste liquors containing iron sulfate and sulfuric acid. This process also provides for purification of the solution, and the iron sulfate product. However, partially soluble sulfides of metals like zinc and/or nickel frequently are present in available feed materials, and this patent does not disclose any process for the removal of such metals from the iron sulfate solution. When present, these base metal impurities would accumulate in iron sulfate crystals, if not removed, and would generally render the iron sulfate products unacceptable as a source of iron oxide for use in the production of steel. 
     The above process, in common with other prior processes for the production of iron sulfate, discloses the use of commercial fuel as the source of heat to separate the water from the iron sulfate. This would require excessive quantities of fuel because of the large amounts of water to be removed, resulting in excessive operating costs. 
     SUMMARY OF INVENTION 
     The process of this invention provides a novel crystallization procedure that has many advantages including: 
     a. The ability to use a minimum amount of water throughout the crystallization processes, 
     b. the ability to use waste heat from the process to evaporate water, 
     c. the elimination or the substantial reduction of the consumption of fuel in the process to separate water from the iron sulfate, 
     d. significant savings in the cost of fuel, and significant improvements in the operation economy and commercial feasibility of recovering seaparately iron sulfate monohydrate and a residue containing recoverable valuable non-ferrous metals, and 
     e. the production of substantial quantities of hydrogen sulfide which can be used subsequently for the selective separation of the base metals present in the feed material and the surplus of which can lend itself to the economic recovery of elemental sulfur. 
     The invention further includes an improved selective leaching process for the production of high purity iron sulfate monohydrate from feed materials that contain partially soluble sulfides of metals like zinc and/or nickel, and an improved process for concentrating the valuable non-ferrous metal components of the feed, and for recovery of these components in a sulfide residue of high commercial value. 
     This invention includes novel crystallization and dehydration procedures for the separation of iron from the solution in form of iron sulfate monohydrate, and also includes a novel combination of such leaching and crystallization procedures to enable the use of the process waste heat from the leaching system for crystallization and dehydration of the iron sulfate product and recycle of the mother liquor from the crystallizer to the leaching system for cooling purposes and for dissolving iron sulfate crystals. 
    
    
     In the annexed drawings: 
     FIG. 1 is a flow diagram of a selective iron co-current leaching process in accordance with the invention; 
     FIG. 2 is a flow diagram of a selective iron counter-current leaching process in accordance with this invention; 
     FIG. 3 is a flow diagram of a combined leaching and crystallizing process in accordance with this invention; 
     FIG. 3a is a schematic view of a crystallizer which can be used according to this invention; and 
     FIG. 4 is a graph showing the temperature and concentrations at which various forms of iron sulfate hydrates exist in equilibrium with their saturated solutions. 
    
    
     DETAILED DESCRIPTION 
     The almost saturated purified iron sulfate solution utilized in the process of this invention may be recovered from either the co-current or counter-current leach processes described in our parent application Ser. No. 133,553 filed Apr. 13, 1971, now U.S. Pat. No. 3,860,696. The solution is charged to a first stage (cooling) section of the crystallizer system as shown in FIG. 3. 
     In the first stage (cooling) section of the crystallizer system the feed solution now essentially saturated with iron sulfate at 50° to 80° C, and preferably at about 65° C is flash cooled to a temperature below about 50° C. and preferably in the range of 25° to 35° C. to precipitate crystals of FeSO 4 . 7H 2  O. It is advantageous to cool the solution to between about 10° to 30° C. to obtain an effective precipitation. As a result of cooling, a fraction of the iron sulfate is precipitated in the form of high purity crystals of FeSO 4  .7H 2  O, or commercial copperas. The precipitation of the copperas may be accomplished in three stages as follows: 
     a. cooling the solution to temperatures below 50° C. and preferably below 35° C. to precipitate high purity crystals of copperas (FeSO 4 .7H 2  O), 
     b. subjecting the solution to reduced pressure to evaporate and remove water, and to precipitate additional crystals of copperas, 
     c. reheating the solution to evaporate and remove additional water at reduced pressure, and to produce additional crystals of copperas, and 
     d. separating the crystals from the solution at temperatures below 50° C. and preferably below 35° C. The crystals are separated from the mother liquor on screens, or other conventional equipment as a major product from this stage of the crystallizer system. The mother liquor, now cold and partially depleted in content or iron, is recycled either to the gas cleaning system and/or to the last stages of the co-current  leach system or to the last stages of the neutral and/or acid leach circuits of the counter-current leach system, as a solvent for crystals of iron sulfate, an absorbent for waste heat, or a coolant for leach pulps or slurries as previously described. 
     The cold oversize crystals of FeSO 4  .7H 2  O produced on the screens of the first stage crystallizer, as discussed above, are charged to a saturated solution of iron sulfate in the second stage vessels of the crystallizer system. The solution is heated with waste heat from the gases generated in the leach system, as described above. The temperature in this stage of the crystallizer is maintained between about 56.5° and 64.8° C., a range of temperatures in which the stable hydrate of iron sulfate is FeSO 4  .4H 2  O as indicated on FIG. 4. The iron sulfate is converted from FeSO 4  .7H 2  O in the feed, to FeSO 4  .4H 2  O in the product from this crystallizer system. 
     The existence of iron sulfate as FeSO 4  .4H 2  O as an exact hydrate has been questioned by some scientists. This iron sulfate hydrate probably exists as indefinite composition of a mixture of various iron sulfate hydrates averaging about FeSO 4  .4H 2  O. The formula FeSO 4  .4H 2  O is used herein in this context. 
     FIG. 3a shows an example of equipment and a process for recovery of FeSO 4  .7H 2  O crystals and for the conversion thereof to FeSO 4  .4H 2  O. The FeSO 4  .7H 2  O crystals are separated from the liquid on the screens as shown in FIG. 3a, causing the crystals to be deposited by gravity in the vessel 1 containing a saturated solution of iron sulfate maintained advantageously at a temperature between 56.5° and 64.8° C. They are gradually converted to the stable FeSO 4  .4H 2  O hydrated form and since these crystals are heavier than the 7 hydrated crystals they fall to the bottom of the vessel and can be removed through line 2 and discharged to the third stage crystallizer which can utilize process and equipment similar to that described above for the second stage crystallizer with the exception of the solution heater. 
     The saturated solution of iron sulfate is removed from the top of the vessel 1 through line 3, to the heater 4, which can be a direct contact heater but is advantageously a surface heater, and charged to the bottom of the vessel 1 through line 5 as shown in the drawing by means of a suitable pump to maintain the temperature in the vessel at between about 56.5° and 64.8° C. 
     The water generated from the above conversion of FeSO 4  .7H 2  O to FeSO 4  .4H 2  O, accumulates in the form of an increasing supply of saturated solution in the second stage of this crystallization process. This excess solution may be either recycled to the first stage (cooling) crystallizer, or the excess water may be evaporated in the second stage of the crystallizer, and no excess solution produced. 
     In the combined leach and crystallization process of this invention, the waste heat available from the leach system generally will be sufficient both to dehydrate the FeSO 4  .7H 2  O to FeSO 4  .4H 2  O, and to evaporate the excess water from the system. Consequently, evaporation rather than recycle, will be the preferred method for removal of the excess water from the system. The recycle method, however, will be used when waste heat is deficient. 
     The product crystals from the second stage of the crystallizer system (FeSO 4  .4H 2  O) are separated from the solution, at temperatures between 45° and 65° C on conventional equipment such as screens, and are repulped in a solution saturated with iron sulfate, at temperatures above 65° C., in the manner described above for conversion of FeSO 4  .7H 2  O to FeSO 4  .4H 2  O. The resulting slurry is heated to temperatures above 65° C., and advantageously in the range of 75° to 90° C., in direct contact with hot gasses containing water vapor. At temperatures above 65° C., the stable hydrated iron sulfate is FeSO 4  .H 2  O, and the FeSO 4 .4H 2  O in the feed slurry to this section of the crystallizer is converted to FeSO 4 .H 2  O in the third stage crystallizer. The FeSO 4 .H 2  O being heavier settles to the bottom and can readily be recovered as a major product of the invention in the same manner as the FeSO 4 .4H 2  O is recovered. Solution heater 5 is a direct contact heater in which the solution is heated in direct contact with hot gases. When heating the iron sulfate solution to a temperature above 65° C. a direct contact heater must be used from a practical and economical standpoint. The monohydrate crystals can then be dewatered by centrifuge, for example, and recovered. 
     In the combined process of this invention, the above slurry of FeSO 4  .4H 2  O in a saturated solution of FeSO 4  is contacted with hot gases generated in the process and heated to temperatures above 65° C. with waste heat generated in the leach section and/or in other sections of the process. Furthermore, the combined process of this invention generally will provide sufficient waste heat to evaporate all water that is produced in conversion of FeSO 4  .4H 2  O to produce monohydrate (FeSO 4  .H 2  O) as the major product of the combined process. 
     EXAMPLE I 
     Co-current Leach 
     This example utilizes a co-current leach procedure that can be carried out illustrating both the selective extraction of iron and concentration of the non-ferrous metals from a sulfide mineral calcine that contains pyrite as a major component, and sulfide minerals of zinc, lead, and copper as minor components. 
     The original concentrate of sulfide minerals is roasted to remove a fraction of the sulfur content and to render the residual iron sulfide reactive with sulfuric acid to generate H 2  S gas and iron sulfate. The assay and composition of the calcine feed to this leach is approximately as follows: 
     
         ______________________________________Component    Wt.%        Grams        Gm. Mols______________________________________Fe       49.40       494.0        8.850S        27.70       277.0        8.650Cu       0.68        6.8          0.107Pb       1.74        17.4         0.057Zn       3.10        31.0         0.474 Other   17.38       173.8        --Total    100.00      1000.0______________________________________ 
    
     Approximately 850 grams of the above calcine are mixed with about 500 grams of wash solution to produce a high density pulp in closed vessel leach equipment that simulates the first stage of the selective co-current leach process of the invention. The entire charge, approximately 760 grams of concentrated sulfuric acid are mixed and allowed to react with the above slurry for a period of 1 hour. During the entire period of the leach reactions, streams of both additional wash solutions and H 2  S gas are injected to control both the density and the circulation of the pulp within the leach vessel. 
     The temperature of the pulp during the first stage high density leach described above, can be varied between 95° and 105° C, and the composition of the product slurry at 95° C. is approximately as follows: 
     
         ______________________________________Component      Wt.%     Grams      Gm. Mols______________________________________Unreacted Solids        285        --Crystals (FeSO.sub.4 . H.sub.2 O)                   935        5.5 Total Solids  35.      1120Soluble FeSO.sub.4      520        3.4H.sub.2 SO.sub.4        110        1.1H.sub.2 O               1620       90.0 Total Liquids 65.      2250 Total Slurry  100.     3470______________________________________ 
    
     The above slurry is diluted and cooled to about 80° C with additional wash solution, and an additional 100 gram fraction of the calcine mineral feed is mixed with the diluted slurry in the second stage of the co-current leach process of this invention. 
     The new slurry is agitated by a stream of H 2  S gas injected at the bottom of the leach vessel, and allowed to react for approximately 1 hour at a temperature of 80° to 85° C. The composition of the reacted slurry from the second stage of the leach is approximately as follows: 
     
         ______________________________________Component      Wt.%     Grams      Gm. Mols______________________________________Unreacted Solids        305        --Crystals (FeSO.sub.4 . H.sub.2 O)                   750        4.4 Total Solids  27.2     1055Soluble FeSO.sub.4      840        5.5H.sub.2 SO.sub.4        10.        0.1H.sub.2 O               1980       110.0 Total Liquid  72.8     2830 Total Slurry  100.0    3885______________________________________ 
    
     The above product slurry from the second stage is diluted and cooled to about 65° C with a solution of FeSO 4  from the recycle from the cooler stage of the crystallization process of this invention. The remaining 50 gram fraction of the calcine mineral feed is mixed with this diluted slurry in the third stage of the co-current leach process of this invention. This new slurry is agitated by a stream of H 2  S gas injected at the bottom of the leach vessel, and allowed to react for approximately one hour at a temperature of 60°-65° C. At the end of this leach the pH of the solution is about 1.8, and the composition of the slurry is approximately as follows: 
     
         ______________________________________Component      Wt.%      Grams     Gm. Mols______________________________________Unreacted Solids          320      --Crystals                 None Total Solids   3.7       320FeSO.sub.4 from Calcine  1180       7.78FeSO.sub.4 from Recycles 1560      10.22 FeSO.sub.4 Total        2740      19.00Zn in Solution Trace*    Trace*    Trace*H.sub.2 O                5660      314.0 Total Solution          96.3      8400 Total Slurry  100.0     8720 ##STR1##______________________________________The above slurry is filtered at about 60° C, the residue is washed with water, and both the residue and filtrate are accumulated as products of the co-current leach process of this invention. The composition of the washed residue is approximately as follows: 
    
     
         ______________________________________Component Wt.%       Grams       Gm. Mols______________________________________Fe        18.7       60.0        1.070S         17.2       55.0        1.708Cu        2.1        6.8         0.107Pb        5.4        17.4        0.084Zn        9.7        31.0        0.474Balance   46.9       149.8       --Total     100.0      320.0______________________________________Assumed Mineral Composition:FeS       29.4       94.2        1.070CuS       3.2        10.2        0.107PbS       6.3        20.2        0.084ZnS       14.5       46.3        0.474Insol.    46.6       149.1Total     100.0      320.0______________________________________ 
    
     EXAMPLE II 
     Crystallization and Dehydration of FeSO 4   
     The purified leach solution from Example I, which is available at about 60° C, is cooled to about 30° C. by the (flash) evaporation of water at reduced pressure, in the cooling stage of the crystallization and dehydration process of this invention. As a result of both the evaporation of water from the solution, and the reduced solubility of FeSO 4  at the lower temperature, crystals of FeSO 4 .7 H 2  O (copperas) are precipitated and a slurry is obtained of the following approximate composition at 30° C.: 
     
         ______________________________________Component       Wt.%     Grams     Gm. Mols______________________________________Crystals (FeSO.sub.4 . 7H.sub.2 O)           31.2     2500      9.0Soluble FeSO.sub.4           --       1360      9.0H.sub.2 O       --       4170      226.0Total Solution  68.8     5530Total Slurry    100.0    8030Crystals from Calcine           77.2     1930      7.8Crystals fromImport Solutions           22.8      570      1.22______________________________________Crystals Total (Above)           100.0    2500      9.00______________________________________ 
    
     The above slurry is processed on screens at 30° C to separate the crystals, and the solution is accumulated for recycle to the third stage of the leach process of this invention, as described in Example I. The cold crystals of FeSO 4 .7H 2  O are repulped in a solution that is essentially saturated with FeSO 4  at ±60° C, and this solution is heated to about 60° C by the transfer of heat through the walls of the equipment in the second stage of the crystallization process. As a result of the higher temperature, the crystals of FeSO 4 .7H 2  O are converted to crystals of FeSO 4 .4H 2  O in a slurry of the following approximate composition at 60° to 65° C.: 
     
         ______________________________________Component      Wt.%     Grams     Gm. Mols______________________________________Crystals (FeSO.sub.4 . 4H.sub.2 O)          25.8     2080      9.0Soluble (FeSO.sub.4)    2050      13.5H.sub.2 O               3950Total Solution*          74.2     6000Total Slurry   100.0    8080______________________________________ 
    
     The above product slurry from the second stage of this crystallizer process is subjected to a conventional liquid-solid separation procedure at 60° C, and the solution is recycled to the repulp procedure of the second stage of the process. The crystals of FeSO 4 .4H 2  O at about 60° C are repulped in a solution recycled from the third stage of the process and saturated with FeSO 4  at about 80° C, the resulting slurry is contacted with hot gases containing water vapor in the third stage of the crystallizer process of this invention. As a result of the above contacting, the slurry is heated to about 80° C, the crystals of FeSO 4 .4H 2  O are converted to crystals of FeSO 4 .H 2  O, and water is evaporated from the slurry equivalent in quantity to the production of water in conversion of the FeSO 4 .4H 2  O to FeSO 4 .H 2  O. The composition of the product slurry from the third stage of this crystallizer process at the temperature of about 80° C is approximately as follows: 
     
         ______________________________________Component      Wt.%     Grams     Gm. Mols______________________________________Crystals (FeSO.sub.4 . H.sub.2 O)          24.2     1530       9.0Soluble FeSO.sub.4      1550      10.2H.sub.2 O               3250Total Solution 75.8     4800Total Slurry   100.0    6330______________________________________ 
    
     The above product slurry from the third stage of this crystallizer process is subjected to a conventional liquid-solid separation procedure, and the solution is recycled to the repulp procedure of this stage of the process. 
     The monohydrate product crystals (FeSO 4 .H 2  O) from this stage of the process, are accumulated as the major product of the crystallization and dehydration process of this invention.