Process for purifying a zinc sulfate solution

Claim 2

A process for the preparation of zinc sulfate solution from roasted zinc bearing ore containing iron which comprises treating said ore in a first stage with sulfuric acid to yield a solution of zinc sulfate mixed with solids containing further zinc sulfate in solution admixed with the solids, adding excess roasted zinc ore to maintain said solution at a pH of about 4 to 5.5, separating the solids from the solution in a second stage by continuously decanting one part of said solution substantially free of solids from the upper portion of a decantation vessel to yield a first solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically, continuously withdrawing the other part of solution rich in solids from the lower portion of said vessel and passing the solution rich in solids to a horizontal plane filter to obtain a filter cake consisting essentially of said solids mixed with further zinc sulfate solution, rapidly washing the filter cake successively with a plurality of continuous countercurrent washings of wash water, separating the zinc sulfate solution from said solids with substantially no hydrolysis insolubilizing zinc in the cake to solid zinc compounds in the filter cake, thus separating and recovering a further solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically.

Claim 3

Process according to claim 1, wherein the cake discharged from the filter after washing does not contain more than 1% of total zinc content and not more than 0.25% of water soluble content.

Claim 4

Process for purifying an iron containing zinc sulfate solution which comprises precipitating iron from said solution as a slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; filtering said slurry on a planar horizontal filter and forming a cake on said planar filter; washing the cake directly on the planar filter with water supplied onto the cake without forming a new slurry with washing water and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration, the thickness of said cake on the planar filter being between about 5 and about 15 mm., and the quantity of said washing water being between 1.1 and 3 m 3 per metric ton of solids, with a minimum of 125% of the total void of the cake being used.

Claim 5

Process for purifying an iron containing zinc sulfate solution which comprises precipitating iron from said solution as a slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; filtering said slurry on a planar horizontal filter and forming a cake on said planar filter; washing the cake directly on the planar filter with water supplied onto the cake without forming a new slurry with washing water and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration, the thickness of said cake on the planar filter being between about 5 and about 15 mm., and the quantity of said washing water being between 1.1 and 3 m 3 per metric ton of solids, with a minimum of 125% of the total void of the cake being used.

Claim 6

Process according to claim 3, wherein the cake discharged from the filter after washing does not contain more than 2% of total zinc content and not more than 0.5% of water soluble zinc content.

Claim 7

A process as claimed in claim 3, wherein the thickness of the cake is comprised between about 5 and about 15 mm, and wherein a quantity of wash water comprised between 1.1 and 3 m 3 per metric ton of solids, with a minimum of 125% of the total void volume of the cake, is flooded over the cake surface without damaging the surface thereof, the cake being made substantially free from zinc and copper bearing solution by a displacement washing, without any substantial mixing of the water with said solution, the washing time being kept lower than 10 minutes.

Claim 8

A process as claimed in claim 3, wherein the wash water is flooded over the cake without damaging the surface thereof.

Claim 9

A process as claimed in claim 3, wherein the filtration cake is formed on a filtering cloth having substantially following characteristics: Nature: polypropylene Weaving: Satin of 5 with the side to the cake calendered monofilament warp-threads multifilament weft-threads Number of threads: weft-threads: 1760/meter warp-threads: 4000/meter Weight: 220 g/m 2 Tensile strenght: weft sense: 215 kg.force/5 cm warp sense: 200 kg.force/5 cm Permeability: According to DIN: 220 liters air/dm 2 minute under pressure of 20 mm water column.

Claim 10

A process as claimed in claim 3, wherein the slurry is decanted prior to filtration.

Claim 11

Process for purifying an iron-containing zinc sulfate solution using a continuous planar filter, which comprises precipitating iron from said solution in slurry form at a pH which is maintained lower than 4, said iron being in at least one of the following forms: basic sulfate, jarosite or goethite, filtering said slurry on a planar filter and forming a cake on said planar filter, washing the cake directly on the planar filter at least twofold countercurrently without substantially forming a new slurry by mixing any portion of the cake with any of the wash liquids, and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contains substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 12

Process for purifying an iron-containing zinc sulfate solution using a continuous planar filter, which comprises precipitating iron from said solution in slurry form at a pH which is maintained lower than 4, said iron being in at least one of the following forms: basic sulfate, jarosite or goethite, filtering said slurry on a planar filter and forming a cake on said planar filter, washing the cake directly on the planar filter at least twofold countercurrently without substantially forming a new slurry by mixing any portion of the cake with any of the wash liquids, and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contains substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 13

Process for purifying an iron containing zinc sulfate solution, which comprises precipitating iron from said solution as a dilute slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; decanting the so-obtained slurry and thereby obtaining a relatively fluid slurry in the form of an underflow liquid issuing from said decanting; filter said underflow liquid on a continuous planar filter and forming a cake on said planar filter; washing the cake directly on the planar filter with water supplied onto the cake without forming a new slurry with washing water and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 14

Process for purifying an iron containing zinc sulfate solution, which comprises precipitating iron from said solution as a dilute slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; decanting the so-obtained slurry and thereby obtaining a relatively fluid slurry in the form of an underflow liquid issuing from said decanting; filter said underflow liquid on a continuous planar filter and forming a cake on said planar filter; washing the cake directly on the planar filter with water supplied onto the cake without forming a new slurry with washing water and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 15

Process for purifying an iron-containing zinc sulfate solution using a continuous planar filter, which comprises precipitating iron from said solution in dilute slurry form at a pH which is maintained lower than 4, said iron being in at least one of the following forms: basic sulfate, jarosite or goethite decanting the so-obtained slurry and thereby obtaining a relatively fluid slurry in the form of an underflow liquid issuing from the decanting, filtering said underflow liquid on a planar filter and forming a cake on said planar filter, washing the cake directly on the planar filter, at least twofold countercurrently without substantially forming a new slurry by mixing any portion of the cake with any of the wash liquids, and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contains substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 16

Process for purifying an iron-containing zinc sulfate solution using a continuous planar filter, which comprises precipitating iron from said solution in dilute slurry form at a pH which is maintained lower than 4, said iron being in at least one of the following forms: basic sulfate, jarosite or goethite decanting the so-obtained slurry and thereby obtaining a relatively fluid slurry in the form of an underflow liquid issuing from the decanting, filtering said underflow liquid on a planar filter and forming a cake on said planar filter, washing the cake directly on the planar filter, at least twofold countercurrently without substantially forming a new slurry by mixing any portion of the cake with any of the wash liquids, and thus freeing the cake from water soluble zinc and copper compounds promptly before hydrolysis thereof insolubilizes said compounds as basic sulfates, whereby the solid residue in the cake does not contains substantially more insoluble zinc content than it contained in the slurry being fed to the filtration.

Claim 17

Process for purifying an iron-containing zinc sulfate solution, comprising the consecutive steps of precipitating said iron at a pH lower than 4 as one or more of the following forms: basic sulfate jarosite or goethite; settling the resulting slurry, filtering the underflow from this settled slurry on a plane horizontal filtration surface, and washing the resulting filter cake with water on the same filtration surface by displacement effect in such a manner as to substantially avoid long contact and mixing of wash water with the liquor in the cake, whereby hydrolysis of the zinc and copper sulfates included in the liquor into insoluble compounds remaining in the cake is substantially avoided.

Claim 18

Process for purifying an iron-containing zinc sulfate solution, comprising the consecutive steps of precipitating said iron at a pH lower than 4 as one or more of the following forms: basic sulfate jarosite or goethite; settling the resulting slurry, filtering the underflow from this settled slurry on a plane horizontal filtration surface, and washing the resulting filter cake with water on the same filtration surface by displacement effect in such a manner as to substantially avoid long contact and mixing of wash water with the liquor in the cake, whereby hydrolysis of the zinc and copper sulfates included in the liquor into insoluble compounds remaining in the cake is substantially avoided.

Claim 19

A Process as claimed in claim 12, wherein the wash water is passed several times countercurrently through the cake, said amount of water being fed to the last pass and the filtrate of each pass being recycled for the feed of the upstream pass, the total washing time for all the passes being kept lower than 15 minutes.

Claim 20

A process for the preparation of zinc sulfate solutions from roasted zinc bearing ore containing iron which comprises leaching said ore in a first stage with sulfuric acid to yield a solution of zinc sulfate mixed with the undissolved solids adding excess roasted zinc ore to maintain said solution at a pH of about 4 to 5.5, separating at least a major portion of the solution so obtained, substantially free of solids to yield a first solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically, withdrawing the solids with any remaining portion of the solution and submitting said solids to a further leaching treatment by sulfuric acid in such conditions that at least a major part of both the remaining zinc and iron of the solids are dissolved, thereafter precipitating the iron from the zinc iron sulfate solution at a pH comprised between 1 to 4, in at least one of the following forms: basic sulfate, jarosite or goethite, separating thereafter the solids by at least passing in a final step a slurry of these solids with at least a part of the sulfate solution to a horizontal plane filter to obtain a thin filter cake consisting essentially of said solids mixed with some remaining zinc sulfate solution, rapidly washing the filter cake with wash water, displacing the zinc sulfate solution from said solids by gravity and differential pressure with substantially no hydrolysis insolubilizing zinc in the cake to solid zinc compounds in the filter cake, thus separating and recovering a further solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically.

Claim 21

A process for the preparation of zinc sulfate solutions from roasted zinc bearing ore containing iron which comprises leaching said ore in a first stage with sulfuric acid to yield a solution of zinc sulfate mixed with the undissolved solids adding excess roasted zinc ore to maintain said solution at a pH of about 4 to 5.5, separating at least a major portion of the solution so obtained, substantially free of solids to yield a first solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically, withdrawing the solids with any remaining portion of the solution and submitting said solids to a further leaching treatment by sulfuric acid in such conditions that at least a major part of both the remaining zinc and iron of the solids are dissolved, thereafter precipitating the iron from the zinc iron sulfate solution at a pH comprised between 1 to 4, in at least one of the following forms: basic sulfate, jarosite or goethite, separating thereafter the solids by at least passing in a final step a slurry of these solids with at least a part of the sulfate solution to a horizontal plane filter to obtain a thin filter cake consisting essentially of said solids mixed with some remaining zinc sulfate solution, rapidly washing the filter cake with wash water, displacing the zinc sulfate solution from said solids by gravity and differential pressure with substantially no hydrolysis insolubilizing zinc in the cake to solid zinc compounds in the filter cake, thus separating and recovering a further solution of zinc sulfate from which zinc is recoverable by further treatment electrolytically.

Claim 22

Process for purifying an iron containing zinc sulfate solution which comprises precipitating iron from said solution as a slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; filtering said slurry on a planar horizontal filter and forming a cake on said planar filter; rapidly washing the cake directly on the planar filter with water supplied onto the cake surface without damaging the cake surface and without forming a new slurry with wash water, and thus freeing the cake from water soluble zinc and copper compounds promptly within a time short enough to prevent hydrolysis thereof and insolubilization of said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration and the total zinc recovered as water soluble zinc is increased by at least about 1.5% of the zinc content in the ore processed.

Claim 23

Process for purifying an iron containing zinc sulfate solution which comprises precipitating iron from said solution as a slurry in at least one of the following forms at a pH which is maintained lower than 4: basic sulfate, jarosite or goethite; filtering said slurry on a planar horizontal filter and forming a cake on said planar filter; rapidly washing the cake directly on the planar filter with water supplied onto the cake surface without damaging the cake surface and without forming a new slurry with wash water, and thus freeing the cake from water soluble zinc and copper compounds promptly within a time short enough to prevent hydrolysis thereof and insolubilization of said compounds as basic sulfates, whereby the solid residue in the cake does not contain substantially more insoluble zinc content than it contained in the slurry being fed to the filtration and the total zinc recovered as water soluble zinc is increased by at least about 1.5% of the zinc content in the ore processed.

Claim 24

A process as claimed in claim 15, wherein the thickness of the cake on the filtration surface is between about 5 and about 15 mm; wherein the quantity of wash water is between about 1.1 and about 3 m 3 per metric ton of solids, with a minimum of 125% of the total void volume of the cake; and wherein the cake is made substantially free from zinc and copper bearing solution without any substantial mixing of the water with said solution, and the washing time is kept lower than 10 minutes.