FR2668141A1 - PROCESS FOR THE PURIFICATION OF WASTEWATER PRODUCED DURING THE PREPARATION OF TIO2, AND PROCESS FOR THE PREPARATION OF TIO2 USING SUCH A PURIFICATION PROCESS. - Google Patents

Abstract

The invention relates to a method for purifying residual solutions from the cycle for preparing titanium oxide by the sulfate process, the residual solutions (11) produced during the washing of titanium hydroxide or a part thereof being evaporated by the steam compression process, in order to form a pure condensate (13) and a residual solution (14) having a high concentration of sulfuric acid. The invention also relates to a method for preparing titanium oxide implementing this purification method.

Description

This invention relates to a method for purifying solutions.

  waste produced during the preparation of

  titanium dioxide based on a sulfate process.

  The invention also relates to a process for the preparation of titanium dioxide, using this

purification process.

  In the preparation of titanium dioxye by the sulphate process, the mineral containing titanium used as a raw material, such as ilmenite or titanium slurry, reacts on sulfuric acid, producing a solid reaction paste The reaction paste is dissolved into water and weak acids circulating in the cycle The solid material present in the solution is separated and part of the iron is separated by crystallization using ferrosulfate The purified solution is concentrated, after which the titanium contained therein is precipitated as titanium hydroxide by hydrolytic heating of the solution The precipitated slurry is filtered, and the filter cake is carefully washed in order to remove the impurities. The purified titanium hydroxide paste is calcined at a temperature of about l OOCC to form titanium oxide The calcined product is pulverized to form an uncoated titanium dioxide pigment This pigment can be coated by precipitation of various hydroxides and metal oxides on the surface of pigment crystals The coated slurry is filtered, and the water-soluble salts produced during coating are removed from the filter cake by washing using exchanged water by ion The washed pigment paste is dried and sprayed

  to form a coated titanium dioxide pigment.

  In the precipitation of titanium hydroxide, the sulfuric acid bound to titanium is released into the mother solution. The acid filtrate is called residual acid and it contains 18 to 22% by weight of sulfuric acid, 4 to 5% by weight. of iron and other metals, such as Ti, Mg, Al, Mn, Cr and V A small amount of residual acid remains in the cake obtained during filtering In the so-called prewash step, the cake is washed with filtrate obtained in the final washing step and / or with washing water, or alternatively with chemically purified water At the start of the prewash step, the residual acid present in the filter cake is moved by the washing water, the sulfuric acid concentration of the outgoing washing water being at the beginning of an importance close to that of the waste acid, so that this fraction is also called waste acid The waste acid, which typically contains about 20% by weight of sulfuric acid, can be conducted in the concentration unit of the multiphase spent acid, in which it is concentrated in sulfuric acid of about 80% by weight, which is then recycled in step

initial preparation cycle.

  As the washing progresses, the sulfuric acid concentration in the discharged washing water decreases, the quantity of washing water passing through the cake increases sharply and the viscosity decreases. The prewashed cake is diluted in the filtrate and / or the washing water obtained after the final washing, the iron present therein, oxidized to trivalence, is reduced and the slurry is conducted in the final washing step, where it is filtered and washed using chemically water purified Part of the water discharged from the final wash can be used in

  the prewash step as described above.

  The washing water containing sulfuric acid, ferric sulphates and other metallic sulphates, obtained in the prewash, can be used in the dissolution of the reaction paste, in addition to the residual acid. Both the prewash and final wash stages produce such a quantity of diluted landfill, rich in sulfuric acid and metallic sulphate, that all this quantity cannot

  not be used in the process as shown above

  above. Waste acid and landfill pose an environmental protection problem in the preparation of titanium dioxide, the solution of which

  various methods have been proposed.

  In one of these processes, the residual solution streams produced during the preparation of titanium oxide are neutralized using limestone and calcium hyrdoxide. This process has the drawback of requiring considerable quantities of neutralizing agent This results in a large amount of solid residue, the purified residual solution is not suitable for reuse in the titanium dioxide cycle, because it is saturated with calcium sulfate (Trees WE et al Journal Water

  Pollution Control, 1979, Vol 51, no 1, p 158 to 162).

  Patent application EP 0 368 203 also discloses a process, in which sulfuric acid is recovered from the residual solutions originating from a titanium dioxide plant, by implementing the electrodialysis process. present process

  the disadvantage of partial acid recovery.

  A process is also known, in which the residual solution produced in the preparation of titanium oxide is concentrated by evaporation by vacuum. This process applies to the concentration of residual acid, while its use for treating water from washing would imply investment and operating costs beyond measure (Tilsmanns, U Umwelt,

/ 88, p 510 to 511).

  A vapor compression method is also known, using the heat of evaporation contained in the evaporated vapor, and thus considerably reducing the energy consumption. This method is used in the

  preparation of pure water from sea water (Chem Eng.

  Handbook, 5 th edition, paragraph 11, page 32) In order to obtain significant energy savings by this process, the increase in the boiling point of the concentrated solution must remain as low as possible,

about 1 CC at most.

  The object of the present invention is to obtain a process by which the residual solutions produced in the preparation of titanium dioxide can be purified while saving energy costs, more particularly washing water having a low concentration of sulfuric acid. , produced in the washing of titanium oxide, so as to eliminate the abovementioned drawbacks and to allow either the reuse of purified water in the preparation of titanium dioxide, or its riskless discharge into the water system, and his

economic recycling.

  The object of the invention is also to obtain a process for preparing titanium oxide, in which the residual solutions produced in the washing of titanium hydroxide are capable of being purified so as to allow the reuse of the purified water in the preparation of titanium oxide, and also recycling

  economical of the acid separated during the purification.

  The main features of the invention are

  presented in the appended claims.

  In a particularly preferred embodiment of the invention, the residual solutions rich in sulfuric acid produced in the washing of titanium hydroxide, or a part thereof, are evaporated by the steam compression process, resulting in a condensate very pure, which is then recycled to be used as washing water in the first place, and in a residual solution having a high concentration of sulfuric acid, which is further concentrated together with the residual acid and is then recycled for use as l sulfuric acid required in the preparation cycle. When determining the dependence relationship between on the one hand, the boiling point of the residual solution produced in the preparation of titanium oxide and on the other hand, the sulfuric acid concentration of the solution, we have observed that the boiling point was as follows under normal pressure: Concentration of sulfuric acid in the residual solution 3.2%, 1%

12.5%

14.7%

17.1%

19.8%

21.0%

  The results show that the boiling point remains so low that it is to use the boiling point compression method YC lorc 102 C 'C

104 'C

  106 C O C from the advantageous point steam to

  level of an acid concentration of -21% by weight.

  In the process according to the invention, the washing waters from washing the titanium hydroxide typically contain 1 to 200 g / i of sulfuric acid, 0.01 to 4 g / i of iron, and in addition to other metallic sulphates, and these washing waters are evaporated to the concentration of 2 to 21% by weight, in a preferred manner to 15 to 20% by weight, using the steam compression process. in sulfuric acid of the residual solution thus concentrated is equivalent to that of the residual acid, thus making it possible to direct it together with the residual acid in the unit for concentrating the residual acid with a view to an additional concentration According to the invention , evaporation can be carried out in a step or in

  several stages, preferably combined in series.

  The principle of the vapor compression process is based on bringing the pressure of the evaporated vapor to a higher level than that which prevails on the evaporation side, using a blower or any other medium The evaporated vapor, the pressure of which has been increased in the aforementioned manner, is conducted in the form of heating vapor on the heating side of the evaporator heat exchanger, where, during its condensation, it gives off a quantity of heat equivalent to the heat of condensation This released heat passes through the thermal surface of the heat exchanger on the evaporation side The pressure prevailing on the boiling side of the evaporator can be between 0.05 to 1 .00 bar The increase in vapor pressure required is a function of the evaporation pressure, the increase in the boiling point and the temperature difference in the heat exchanger.

  heat, and it constitutes a duty of optimization.

  The invention is described below in detail with reference to the accompanying drawings, of which Figure 1 is a schematic view of a device suitable for carrying out the method according to the present invention, and Figure 2 is a schematic view of installation for

  carry out the method according to the present invention.

  In figure 1, the evaporator is indicated by the reference 1 The evaporator 1 comprises an evaporation side and a condensation side, separated by a heat exchange surface The residual solution 11 coming from the titanium dioxide cycle is led through the devices preheating 3 and 4 on the evaporation side of the evaporator 1 If necessary, the residual solution 11 can be filtered by the filter 10 in order to separate the solid material. The pressure of the evaporated vapor is increased by means of the blower 2 and is then conducted to the condensation side of the evaporator 1, where it is condensed The condensed steam 13 is very pure and it is evacuated by means of the pump 6 on the condensation side by the preheating device 3, where it gives off heat The solution concentrate 14 is evacuated in a similar manner by the pump 7 through the preheating device 4 The concentrated solution 12 is recycled to the surface of the shell evaporator 1 heat hanger by means of the pump 8 The solution 12 to be recycled can if necessary be filtered by the filter 5 in order to prevent soiling of the thermal surface A vacuum is maintained in the system by means of a vacuum pump 9. The water evaporated from the residual solution, obtained in the form of condensate 13 in the evaporator, represents a high degree of purity, being applicable for a large number of purposes of use Such as for example its use in the conditioning washing stage in replacement of the water exchanged by the ion or else its use as boiler water for electric power stations, after having undergone additional purification, if need be, of course, such water can serve as a replacement for chemically purified water in various applications, or can lead to

  in the water system without any risk.

  The acid solution 14 concentrated in the evaporator can be led together with the residual acid in the installation for concentrating the residual acid in

  view of additional concentration.

  FIG. 2 represents a preferred embodiment of the invention, in which the evaporation is carried out in several, in this case, three evaporation units 15, 16 and 17, associated in series The operating principle of each of the units d evaporation is identical to that of the steam condensation installation shown in FIG. 1 The residual solution 11 coming from the titanium dioxide cycle is led by the preheating devices 3 and 4 in the first evaporation unit 15 The concentrate 14 a leaving the first evaporation unit is led into the second evaporation unit 16, from which the outgoing concentrate 14 b is led into the third evaporation unit 17, from which the concentrate 14 is evacuated by the preheating device 4 The condensed vapors 13 a, 13 b and 13 c, obtained from the respective evaporation unit, are combined and discharged by the preheating device 3 in the form of c ondensat 13 This embodiment has the advantage of not causing a significant increase in the boiling point from the first evaporation unit, which would increase the costs of evaporation, and

  thus reducing overall costs.

  The invention is described in more detail below in

by way of an example.

Example

  In a titanium dioxide plant, preparing titanium dioxide pigment by the sulphate process at rates of 230 t per day, a waste water stream is produced in the quantity and with the composition mentioned below, in addition of residual acid: quantity 5000 t / d density 1 kg / dd HSO 4 1.5% by weight Fe 0.1% by weight solid matter 0.02% by weight other metals t / d of these waste waters were conducted in the three-phase steam compression installation described above (Figure 2) 45 t / d of purified concentrate and 5 t / d of concentrated sulfuric acid solution having a supluric acid concentration of% by weight were obtained The condensate was conveyed in the conditioning wash of the plant as washing water, and the concentrated acid solution was conveyed in the unit for concentrating the residual acid, where it was subjected to an additional concentration up to 70% and was then recycled

in the factory.

Claims (8)

Claims   1 Process for purifying waste water solutions from the titanium dioxide preparation cycle, characterized in that the residual solutions rich in sulfuric acid, produced during   washing of titanium hydroxide, or a part thereof   ci, are evaporated by the steam compression process to form a pure condensate and a residual solution having a high concentration of sulfuric acid. 2 Method according to claim 1, characterized in that the condensate thus obtained is recycled to be reused as washing water.   3 Method according to claim 1 or 2, characterized in that the residual solution having a high concentration of sulfuric acid is subjected to an additional concentration in the concentration unit of the residual acid, and is then recycled to be used as l 'sulfuric acid   required in the preparation cycle.   4 Method according to one of the preceding claims,   characterized in that the sulfuric acid concentration of the residual solutions to be purified is between 1 and 200 g / l.   Method according to any of the claims   above, characterized in that the sulfuric acid concentration of the residual solution obtained in evaporation is between 2 and 21% by weight,   preferably between 15 and 20% by weight.   6 Method according to any one of the claims   previous, characterized in that the evaporation is carried out in several stages, associated in series of preference -   7 Process for the preparation of titanium dioxide by the sulfate process, in which the mineral or the titanium-rich concentrate is treated with sulfuric acid, the product obtained is dissolved in water, the solid material is separated, part iron is separated by crystallization in the form of ferrosulfate, after which the titanium present in the solution is precipitated as titanium hydroxide by hydraulic heating of the solution, the precipitated slurry is filtered, giving the residual acid as filtrate, the paste of titanium hydroxide is washed with washing water several times in order to remove the impurities, and then the purified titanium hydroxide paste is calcined into titanium oxide, characterized in that the residual solutions rich in sulfuric acid produced during   washing of titanium hydroxide, or a part thereof   ci, are evaporated by the vapor compression process, resulting in a pure condensate, which is recycled to be reused as washing water, and in a residual solution having a high concentration of sulfuric acid.   8 Method according to claim 7, characterized in that the residual solution thus obtained, having a high concentration of sulfuric acid, is concentrated together with the residual acid in the unit for concentrating the residual acid, and is then recycled to be reused as sulfuric acid   required in the preparation cycle.   9 Method according to claim 7 or 8, characterized in that the concentration of sulfuric acid in the solutions   waste to be purified is between 1 to 200 g / l.   Method according to any of claims 7 to   9, characterized in that the sulfuric acid concentration of the residual solution obtained in the evaporation is between 2 and 21% by weight,   preferably between 15 and 20% by weight.   11 Method according to any one of claims 7 to   , characterized in that evaporation is carried out   in several stages, preferably in series.