CS261617B1 - A method of treating waste in titanium dioxide production to form ferric sulfate and titanium sulfate - Google Patents

Abstract

The solution to the waste problem in the production of titanium dioxide consists in the simultaneous electrolytic reduction of the technical solution of titanium dioxide oxysulfate and the oxidation of the waste green scale solution from the production of titanium dioxide in an electrolyzer with electrode spaces separated from each other by an anion exchange membrane or diaphragm, to form ferric sulfate and titanium sulfate.

Description

The invention relates to the treatment of waste in the production of titanium dioxide to form ferrous sulfate and titanium sulfate.

The production of titanium dioxide by the sulphate process from ilmenite consists in the decomposition of ilmenite with sulfuric acid into a mixture of titanium dioxide sulphate, ferrous sulphate, ferric sulphate and other accompanying substances. These sulphates are dissolved in water, ferric sulphate is reduced by ferrous iron to ferrous sulphate and a small portion of titanium dioxide sulphate to titanium sulphate. Mechanical impurities are removed from the solution by sedimentation, part of the iron is precipitated as ferrous sulfate heptahydrate by cooling, the mother solution after separation of ferrous sulfate heptahydrate is concentrated by evaporation and hydrolyzed to titanium hydroxide. The titanium hydroxide is separated from the dilute sulfuric acid (ca. 20%) by filtration and further processed by washing with water, calcination and milling to the final product, titanium white. The washing of titanium hydroxide produces water contaminated with sulfuric acid and ferrous sulfate.

In this method of producing titanium white, a considerable amount of by-products is softened. Per 1 tonne of titanium white produced is not required

4.2 tonnes of ferrous sulphate heptahydrate (green vitriol), 1.3 to 1.6 tonnes of sulfuric acid in the form of a 20% solution and a variable amount of polluted water containing 0.4 to 0.6 tonnes of sulfuric acid.

The generation of these wastes is related to the technology of titanium dioxide production itself. Titanium dioxide, such as a high percentage of titanium dioxide, is produced from a raw material which, in addition to titanium, also contains a high percentage of iron. Additional iron in metallic form is used in production to reduce titanium solutions.

Sulfuric acid is used to decompose Ilmenite and, since it does not form part of the product, it again comes out of production either in the form of dilute solutions or bound to iron as ferrous sulfate.

Both byproducts, green vitriol and dilute sulfuric acid are contaminated with titanium dioxide sulphate and sulphates of other metals such as magnesium, manganese, aluminum, etc., originally present in ilmenite. The main impurity in dilute sulfuric acid is ferrous sulphate.

The disposal of these wastes poses considerable difficulties. Green vitriol can be used for waste water clarification, thermally processed to iron oxide and sulfuric acid, and chlorinated to chlo1. Fe ³⁺ + Θ = Fe ²⁺

2. TiO ²⁺ + 2 H + + e ~ = TP + + HsO while the oxidation of ferrous sulphate to ferric at the same time takes place in the anode compartment

Fe ²⁺ - e - = Fe ³⁺ ferric iron sulphate FeSO-iCl, useful as a coagulating agent in the clarification of drinking or cooling water, or halded. However, the widespread use of green vitriol, which falls off in the production of titanium dioxide, is prevented by the impurity content. Dilute sulfuric acid is usually thickened by evaporation and treated in this way in the production of phosphorous fertilizers. Upon thickening of the sulfuric acid, a further proportion of strongly unpurified ferrous sulphate in the form of monohydrate in an amount of 0.9 to 1.0 tons per tonne of titanium dioxide produced is omitted.

The third type of titanium dioxide waste - contaminated rinse water - is usually neutralized with some of the dilute sulfuric acid and the resulting calcium sulfate, contaminated with iron and titanium hydroxides, is separated and landfilled or dried and used as a raw material in cement production.

It follows that the problem of sulphate production of titanium dioxide from ilmenite is closely related to the problem of waste disposal and thus to the environmental protection of these plants.

The process according to the invention solves the problem of the waste produced in the production of titanium dioxide both by reducing the production of waste vitriol based on 1 tonne of produced titanium dioxide and by simultaneously processing part of the produced green vitriol waste into ferric sulphate solution.

The principle of the process according to the invention consists in the simultaneous electrolytic reduction of the technical solution of titanium dioxide sulphate and the oxidation of the solution of waste green vitriol from the production of titanium dioxide in an electrolyzer with electrode spaces separated by an ion exchange membrane or diaphragm.

The technical solution of titanium dioxide sulphate is formed by dissolving the so-called decomposition mass after the mutual reaction of ilmenite and sulfuric acid and contains besides titanium dioxide sulphate also ferric sulphate, ferrous sulphate and sulfuric acid. Before further processing of the solution to titanium white, it is necessary to reduce ferric sulfate to ferrous and a small portion of titanium dioxide sulphate to titanium sulfate. In prior art titanium dioxide, this solution is reduced by metal iron.

The following reactions take place in the cathode chamber of the electrolyzer to reduce the titanium dioxide sulphate technical solution according to the invention:

E ° = +0.77 volts

E ° = +0.10 V

In terms of the mass balance, including the sulfate anion, the following reactions should be considered:

281617 on the cathode in the first phase:

Fe2 (SO4) 3 + 2e = 2 FeSOi -I- SOd ² 'in the second phase:

2TiOSO <4 - 2 HzSO 3 - 2e - = Ti ₂ (SOi) 3 +

4- H ₂ O + SO ^{1 2} 'on the anode

2FeSOi 4-SOi ² '= Fe 2 (SOi) 3 4- 2e

The electrolytic reduction of titanium dioxide sulphate technical solution is described in a number of patents [Norwegian Pat. 4,309 (1916); U.S. Patent 310,304 (1917), French. U.S. Pat. 494,672 (1917), French. U.S. Pat. 663 063 (1928). However, it is only a reduction at the cathode, whereby an oxygen gas is produced at the anode.

The advantage of the proposed solution lies in the combination of both reactions, ie reduction of the technical solution of titanium dioxide sulphate on the cathode and oxidation of the solution of ferrous sulphate on the anode in one operation, whereby both the cathode and anode reaction are used to obtain desired products.

The material balance of the reduction of the technical solution of titanium sulphate and the oxidation of the solution of ferrous sulphate shows that by replacing the reduction with metal iron by the method according to the invention the amount of iron sulphate corresponding to approx. Further, approx. 1.45 t of green vitriol is processed into 2.15 t of ferric sulphate solution (with an iron content of 180 g. I ^-1 ). t per 1 t produced titanium white.

The process of the invention has advantages for the titanium dioxide production technology itself. By introducing electrolytic reduction instead of using metallic iron to reduce titanium solutions, the amount of impurities introduced by the iron into these solutions is reduced. These are mainly metals forming color compounds, which can pass into titanium white and thus reduce its quality.

If the reduction of titanium solutions with metallic iron is carried out directly in decomposition reactors, the capacity of these reactors is increased by the introduction of the process according to the invention.

By precisely controlling the electrolytic reduction, a constant concentration of tri-titanium titanium in titanium solutions is achieved, thereby reducing the water-soluble titanium losses in the hydrolysis of the titanium solutions.

The process according to the invention can be used in titanium dioxide plants by the sulphate process.

Example 1

Solution produced by dissolving ilmenite in sulfuric acid and containing

120 g. I ¹ titanium dioxide T1O2

95.2 g. I ^-1 iron, Fe _cc i _k of which 22 g. Γ ¹ iron in trivalent form Fe ³⁺ and

410 g. I ' ¹ Sulphate SO; ² 'of which 80 g. Γ ¹ free sulfuric acid after introduction into the cathode chamber of the membrane electrolyzer, which was separated from the anode chamber by an ion-exchange membrane, was reduced on the titanium cathode at 95% current yield and current density 1 A. dur ² and its composition was adjusted to

1.8 gD ¹ titanium in trivalent Ti ³¹ form; at 72 g. I ^{-1 of} titanium (corresponding to an initial TiO 2 content of 120 g. 1 ');

95.2 g. 1 'iron only in the bivalent form of Fe ²⁺ ; and

383 gl ¹ sulphates SO1 ² ', of which 80 g. 1 &^lt; 1 & gt ^{; of} free sulfuric acid H2SO1.

Into the anode chamber was placed a solution of ferrous sulfate carbonation of crystalline ferrous sulfate heptahydrate until a total iron content of 180 g. ^I-1. When the solution was circulated through the anode chamber, all the iron was oxidized to a trivalent form on the titanium anode activated by ruthenium dioxide at a current density of 1 A.

. dm ^-2 in 82% current yield.

Electrolysis was performed by direct electric current with a ripple of max. 8%, the voltage on one electrolytic cell was 2.3 V. The linear flow rate of the solutions on the electrode surface was 3 cm.:r ^! . Example 2

Solution produced by dissolving ilmenite in sulfuric acid and containing

117 g.l ^-1 titanium dioxide TiO2,

95.6 g. I ¹ iron, of which 23 g. I * ¹ iron in the trivalent form of Fe ³⁺ a

412 g · l ^-1 of sulphates SO ^2, of which 76 gl ¹ free sulfuric acid H2SO4 after introduction into the cathode compartment of a diaphragm electrolytic cell, which has been separated from the chamber by an anode diaphragm of porous polyvinyl chloride, the cathode lead alloy with 3% antimony reduced at 94% current yield and current density of 2.5 A. dm &^lt; ^{2 &gt;} and its composition was adjusted to content as follows

1.95 g. Even ^one trivalent titanium in Ti ^3+; at 70.5 g.l ^-1 titanium (corresponding to an initial T1O2 content of 117 g. l ^-1 ;

95.6 g. I ¹ iron only in divalent form Fe ²⁺ a

381 gl “ ¹ sulphate SOd ^2- , of which 79 gl“ ¹ free sulfuric acid H2SO1.

Into the anode chamber was placed a solution of ferrous sulfate heptahydrate carbonation crystalline ferrous sulfate heptahydrate until a total iron content of 180 g. ^I-1. When the solution was circulated through the anode chamber, all the iron on the platinum anode was oxidized to a trivalent form at a current density of 2.5 A. dm ^-2 in 78% yield. Electrolysis was performed by direct current with a ripple of max. 8 why, the voltage on one electrolytic cell was 4 V. The linear flow rate of the solutions on the electrode surface was 3 cm. s ^-1 .

pííedmet

Claims (1)

Process for the treatment of waste in the production of titanium dioxide to form ferric sulphate and titanium sulphate, characterized in that, at the same time, a technical solution of titanium dioxide sulphate with a content of 50 to 180 g / l of titanium dioxide, 0.1 to 150 g / l of total iron (in divalent and trivalent form), up to 500 g / l of sulphate anion SOi ² - in the cathode cell of the electrolyser at a current density of up to 20 A. dm ^-2 and temperature up to 95 ° C the disappearance of the trivalent iron content and achieving a titanium trivalent content of at least 0.1 g / l and oxidation of the iron sulphate solution constantly saturated with the crystalline ferrous sulphate leaving the titanium dioxide sulphate process from ilmenite in the anode chamber of the electrolyzer until at least 100 g / l at a current density of up to 20 A. dm ^-2 and a temperature of up to 95 ° C.