GB2102404A - Method for purifying titanyl hydrate - Google Patents

Abstract

The titanyl hydrate is slurried with water to form a titanyl hydrate slurry and treated to solubilize impurities by the addition of a trivalent titanium solution made from a clarified black liquor solution. The black liquor solution is a solution taken from a stage in a process for the production of titanium dioxide prior to the precipitation of titanyl hydrate. The titanyl hydrate is then separated from the titanyl hydrate slurry and washed to remove residual solubilized impurities.

Description

SPECIFICATION Method for purifying titanyl hydrate This invention relates to a process for the purification of Titanyl hydrate formed during the production of titanium dioxide pigments.

Titanium dioxide pigments have been produced by gaseous chloride, hydrogen chloride, and sulfate processes. In the production of titanium dioxide pigments by the sulfate process, titaniferous bearing materials, such as ilmenite and rutile ores and furnace slag, are digested with various concentrations of sulfuric acid to form a titnayl sulfate and iron sulfate solution. This solution is clarified of insoluble matter and then hydrolyzed to precipitate titanyl hydrate which is further processed to form titanium dioxide. Production of titanium dioxide by the hydrogen chloride process follows substantially the same processing steps except hydrochloric acid is used in place of sulfuric acid, During the hydrolysis process, the precipitated titanyl hydrate carries with it adsorbed impurities predominently as sulfate salts of ferric iron, chromium, and vanadium.These impurities cannot be removed even after prolonged and repeated washing operations. These impurities are originally present in the titaniferous bearing materials. For example, a typical analysis of ilmenite ores found in the State of New York is: Constituent Percent TiO2 44.4 FeO 36.7 Fe203 4.4 SiO2 3.2 Awl203 0.19 P2Os 0.07 ZrO2 0.006 MgO 0.80 MnO 0.34 CaO 1.0 V2o5 0.24 Cr203 0.001 SnO2 0.001 CuO 0.004 CeO2 0.002 Cb 0.002 U.S. Patent No. 1,885,1 87 discloses a process for purifying titanyl hydrate by treating the titanyl hydrate with a dilute acid in the presence of a reducing agent. The titanyl hydrate is suspended in a 15% acid solution, preferably hydrochloric acid or sulfuric acid.A small amount of reducing agent is added to the suspension of titanyl hydrate in dilute acid to solubilize impurities. The presence of acid in the bleaching operation solubilizes titanium values by the reaction of the titanyl hydrate with the acid and/or reducing agent. The effect of the treatment is to solubilize iron, thereby allowing removal.

When the treatment is complete, the titanium hydrate is filtered and washed. This process has potential drawbacks. Firstly, the titanyl hydrate must be digested a second time with acid. This not only requires a duplication of effort, but is wasteful of reagents. Additionally, the use of a water-insoluble reducing agent, such as zinc or aluminum, can result in the formation of a photosensitive or mixed pigment, i.e., a pigment containing anatase and rutile crystal structures, if the reducing agent is not completely reacted.

In U.S. Patent No. 2,148,283 which involves a sulfate process, a slurry of hydrous titanium oxide material, after being washed with water, is brought into contact with a water-insoluble reducing agent, such as powdered metallic zinc or aluminum, during the first repulping operation to solubilize impurities. The reducing agent is used in an amount sufficient to maintain reducing conditions throughout the washing and filtration treatment. The amount of reductant employed is dependent upon the amount of ferric iron in contact with the hydrous titanium oxide. The residual free sulfuric acid contained in the repulped hydrate is maintained, after washing, at a sufficient level that no additional mineral acid is required. This process has several drawbacks.Firstly, the use of a water-insoluble reducing agent, such as zinc or aluminum, can result in the formation of a photosensitive or mixed pigment, i.e., a pigment containing anastase and rutile crystal structures, if the reducing agent is not completely reacted and ultimately removed from the titanyl hydrate following treatment. Additionally, by not washing the hydrous titanium oxide free of the residual free sulfuric acid, other soluble impurities remain with the hydrous oxide and must be processed a second time to be removed.

U.S. Patent No. 2,999,011 discloses a method for bleaching titanyl hydrate which comprises dividing the washed titanyl hydrate obtained from the hydrolysis of a titanium and iron sulfate solution into a major and minor portion, solubilizing most of the titanium values in the minor portion to titanous sulfate by digestion with 16% to 40% sulfuric acid in the presence of a reducing agent to produce a bleaching slurry, adding the bleaching slurry to the major portion of the separated titanyl hydrate slurry to form a bleached slurry containing 0.1% to 2% by weight sulfuric acid, the amount of said reduced titanium values in said bleached slurry being sufficient to reduce the iron in the major portion to a lower valent compound and to solubilize the same, and to maintain at least 0.05 grams/liter of trivalent titanium in said bleached slurry, and filtering and washing the bleached slurry to produce a titanyl hydrate substantially free from iron.

The aforementioned process while being used successfully on a commercial basis, has shortcomings similar to those in the prior art processes already described. Firstly, the minor portion of titanyl hydrate slurry which is removed must be digested with additional sulfuric acid values. This step not only requires a duplication of the original ore digestion step, but is wasteful of reagents. Secondly, the use of a water-insoluble reducing agent, such as zinc or aluminum, can result in the formation of a photosensitive or mixed pigment if the reducing agent is not completely reacted.

A process has been unexpectedly discovered which provides a means for removing impurities, mainly iron, from titanyl hydrate that substantially reduces the drawbacks of the prior art mentioned hereinabove while avoiding the difficulties associated with conventional techniques.

According to the present invention, there is provided a process for removing impurities from titanyl hydrate comprising: A process for removing impurities from titanyl hydrate comprising: a. slurrying an impure titanyl hydrate with water to form a titanyl hydrate slurry; b. treating the titanyl hydrate slurry to solubilize impurities by the addition of a trivalent titanium solution made from a clarified black liquor solution taken from a stage in a process for the production of titanium dioxide prior to the precipitation of titanyl hydrate; c. separating the titanyl hydrate from the treated titanyl hydrate slurry containing solubilized impurities; d. washing the separated titanyl hydrate to remove residual solubilized impurities and to produce a purified titanyl hydrate; and e. recovering the purified titanyl hydrate.

In another embodiment of the invention there is provided a process for removing impurities from a titanyl hydrate slurry comprising; a. separating the impure titanyl hydrate from the titanyl hydrate slurry to form a titanyl hydrate wet cake; b. washing the titanyl hydrate wet cake to remove residual soluble impurities; c. treating the impure titanyl hydrate to solubilize impurities with a trivalent titanium solution made from a clarified black liquor solution; d. washing the treated titanyl hydrate to remove solubilized impurities and to produce a purified a titanyl hydrate; and e. recovering the purified titanyl hydrate.

The drawing depicts an embodiment of the invention process for removing impurities from a slurry of titanyl hydrate.

In preparing titanium dioxide pigment, the titaniferous bearing material containing both soluble and insoluble impurities is digested with a mineral acid, namely sulfuric acid or hydrochloric acid, to form the titanyl and iron salts of the mineral acid. By mineral acid is meant either sulfuric or hydrochloric acid. Depending on the concentration of the mineral acid. Depending on the concentration of the mineral acid, the titanyl and iron salts of the mineral acid may be soluble in the resulting solution or form a solid mass. If a solid mass is formed, the titanyl and iron mineral acid salts must be solubilized before further processing. The solution of titanyl and iron mineral acid salts is normally clarified to remove most of the insoluble material and then hydrolyzed to produce a solid titanyl hydrate and an iron mineral acid salt solution which contains soluble impurities.The titanyl hydrate is then separated from the iron mineral acid salt solution by conventional liquid-solid separation techniques. The method for performing the digestion, clarification and hydrolysis procedures are well known in the art and do not constitute a part of this invention.

Following this initial separation to remove the iron mineral acid salt solution containing the soluble impurities from the separated solid titanyl hydrate, the titanyl hydrate cake is washed to remove residual soluble impurities. Washing may be performed with clear or acidified water on the equipment used for separation of the titanyl hydrate. However, even after copious washing, the titanium hydrate contains small amounts of impurities; predominantly iron, with minor amounts of magnesium, lead, nickel, vanadium and chromium being present.

Once separated and washed, the titanyl hydrate is slurried with water. The slurry should be capable of being handled by conventional fluid transfer equipment. Once the slurry is prepared it is treated with a trivalent titanium solution to solubilize the residual impurities. The trivalent titanium solution is preferably selected from the group consisting of titanous sulfate and titanous chloride.

The salient feature of the inventive process resides in the unexpected discovery that a trivalent titanium solution prepared from a clarified black liquor solution may be used to treat titanyl hydrate for the removal of impurities without detrimentally affecting the titanium dioxide product. A black liquor solution is any titanyl salt solution taken from a stage in a process for the production of titanium dioxide by the sulfate or chloride processes prior to precipitation of titanyl hydrate.

While the exact mechanism for the performance of the inventive process is not known, it appears as though the impurities are adsorbed by the titanyl hydrate during hydrolysis at active sites on the surface of the hydrate crystal and that the impurities, particularly iron, are removed by an exchange mechanism, wherein the trivalent titanium from the treating solution displaces and solubilizes the impurities from the titanyl hydrate sites. The solubiiized impurities can then be removed by washing the treated titanyl hydrate with water. In the exchange mechanism, trivalent titanium appears to be adsorbed from the trivalent titanium solution while the impurities are solubilized and move into the solution.The adsorption of trivalent titanium on the hydrate is evident by the iredescent blue color of the titanyl hydrate after treatment with the trivalent titanium compound. When trivalent titanium treating solution made from a clarified black liquor solution are used after the precipitation of titanyl hydrate and is the presence of a trivalent titanium compound, the impurities contained in the black liquor bleach solutions are not adsorbed by the titanyl hydrate. This is probably due to the preferential adsorption of the trivalent titanium compound contained therein.

The amount of black liquor bleach solution used for treating the titanyl hydrate is not critical, as long as there is sufficient trivalent titanium to occupy all the active sites on the surface of the titanyl hydrate being treated. The use of excessive quantities of bleach solution does not have a detrimental effect on the purity of the hydrate. The amount of sulfuric acid present during treatment appears to have an insigificant effect on impurity removal, but is preferably limited to avoid the waste of valuable acid values. The amount of sulfuric acid present during treatment should be maintained between about 0.1% and about 10% by weight, more preferably between about 0.1% and 5% by weight, and most preferably between about 0.19/0 and about 2% by weight.

It has generally been found effective to employ black liquor solutions in an amount sufficient to reduce the impurities in the titanyl hydrate to a lower valent compound to solubilize the impurities and to maintain at least 0.05 grams per liter of trivalent titanium in the bleach solution while maintaining the amount of sulfuric acid during treatment between 0.19/0 to 2.0% by weight.

When employing the conventional titanium dioxide sulfate or hydrogen chloride processes, it is preferred to use black liquor solution taken from the process immediately after crystallization and removal of iron mineral acid salts. The solutions typically have a high titanium and low free acid concentration and are free from solids. Solutions taken at earlier steps in the titanium dioxide process can be employed in the trivalent titanium treatment but must first be clarified, e.g., filtered, to remove any insoluble materials before they can be used.When employing the sulfate process for making titanium dioxide, the black liquor solution after clarification should preferably contain titanyl sulfate (measured as TiO2) in the range between about 90 grams/liter and about 250 grams/liter, iron (measured as ferrous sulfate) at less than 280 parts per 100 parts titanyl sulfate (measured as TiO2) and sulfuric acid at a weight ratio of sulfuric acid to titanyl sulfate (measured at TiO2) of between about 1.7 and about 2.2.

When making the trivalent titanium solution in a titanium dioxide sulfate process, the solution is typically made by diluting the clarified black liquor solution with water and acid and then reducing the solution with a metal reductant, such as iron, zinc, or aluminium. It has been found that under certain conditions when aluminum metal is used as a reductant for titanyl sulfate, reduction efficiencies may exceed 90%. Under general commercial practice, iron is used as the reductant for titanyl sulfate and reduction efficiencies of about 50% or less are typical. The aluminum reduction efficiency is sensitive to the amount of sulfuric acid present during the reduction reaction.In order to obtain a high reduction efficiency with the aluminum reductant, the trivalent titanium solution used in the reduction should preferably have a ratio of titanyl sulfate (measured as TiO2) to total sulfuric acid, i.e., free acid plus active acid, greater than 3.4 and a titanyl sulfate content (measured as TiO2) of about 70 grams/liter.

The temperature of the reduction mixture preferably should be held between 30 C and 900 C, depending upon the titanous sulfate concentration.

The preparation of trivalent titanium solution as the bleaching solution from process solutions taken from earlier stages in the titanium dioxide manufacturing process provides substantial raw material and cost savings. Since the titanium values are already soluble as a titanyl mineral acid salt, it is not necessary to reprocess the titanyl hydrate to prepare soluble trivalent titanium mineral acid salt as performed by the prior art. Furthmore, mineral acid values are saved because additional acid is not needed for digestion of the titanyl hydrate.

In the titanium dioxide sulfate process, the black liquor solution after clarification may contain titanyl sulfate (measured as TiO2) in the range between about 90 grams/liter and about 250 grams/liter, iron (measured as ferrous sulfate) at less than about 280 parts per 100 parts titanyl sulfate (measured as TiO2), and sulfuric acid at a weight ratio of sulfuric acid to titanyl sulfate (measured as TiO2) in an amount between about 1.7 and about 2.2. The titanous sulfate solutions used for treating the titanyl hydrate slurry should have a total soluble titanium content (measured as TiO2) of between about 30 grams/liter and about 85 grams/liter, a weight ratio of ferrous sulfate to total soluble titanium (measured as TiO2) of between about 0.05:1.2 and about 1.2:1, a titanous sulfate content (measured as TiO2) of between about 30 grams/liter and about 80 grams/liter, and a weight ratio of sulfuric acid to total soluble titanium (measured as TiO2) of between about 3.4:1 and about 7.0:1.The titanous sulfate solution should preferably have a total soluble titanium content (measured as TiO2) between about 50 grams/liter and about 80 grams/liter, a weight ratio of ferrous sulfate to total soluble titanium (measured as TiO2) of between about 0.6 to 0.7:1.2, a titanous sulfate content (measured as TiO2) of between about 50 grams/liter and about 75 grams/liter, and a weight ratio of sulfuric acid to titanium (measured asTiO2) of between about 5:1 and about 7:1.

The process of the present invention is further illustrated by the accompanying figure which depicts a preferred embodiment of the process. In the figure, an impure titanyl hydrate slurry is fed to solid-liquid separator 2. The solid-liquid separator may be, for example, a vacuum filter or a pressure filter. After separation, the titanyl hydrate wet cake is washed on the separator with water to remove residual soluble impurities.

After washing, the titanyl hydrate wet cake is transferred to repulp tank 4. The titanyl hydrate wet cake is mixed with an amount of water just sufficient to form a fluid titanyl hydrate slurry. When the titanyl hydrate wet cake has been reslurried, the slurry is treated with an amount of trivalent titanium solution equal to between about 0.01 grams and about 0.70 grams trivalent titanium as TiO2 per 100 grams titanyl hydrate as calcined TiO2 in repulp tank 4.

The treated titanyl hydrate slurry is transferred to solid-liquid separator 6. The solid-liquid separator may be, for example, a rotary vacuum filter or a pressure filter. After separation, the titanyl hydrate wet cake is washed on the separator with water to remove solubilized impurities.

While the process has generally been described with regard to the sulfate process for making titanium dioxide, the process may readily be applied to use with a hydrogen chloride titanium dioxide process.

The principle and practice of the present invention is illustrated in the following examples which are exemplary only and it is not intended that the invention be limited thereto since modifications in technique and operation will be apparent to anyone skilled in the art.

Examples 1 to 1 6 are presented to illustrate the effectiveness of impurity removal by the inventive process in comparison with a commercial bleach process. In each example a sample of the same titanyl hydrate taken from a commercial titanium dioxide process was treated to remove impurities by a commercial bleach process and the inventive process at different acid concentrations using trivalent titanium solutions made from black liquor solutions which were produced by the digestion of different titaniferous bearing materials, namely Maclntyre ilmenite, Q.l.T. slag and Richard Bay slag.In the commercial bleach process 1000 g. of washed titanyl hydrate, the equivalent of about 335 g titanyl hydrate asTiO2, was repulped with 610 ml of water and 80 ml concentrated H2SO4to form a hydrate slurry containing 100 g/l H2SO4. Then 0.2 g of powdered aluminum was added to the hydrate slurry and reacted at between about 60"C and 80"C for one-half hour. It was then.deliquored in a 1 5 cm Buechner funnel and washed with 1400 ml of water. When testing the inventive process, 1000 g of washed titanyl hydrate, the equivalent of about 335 g titanyl hydrate as calcined TiO2, was repulped with 350 ml of water and the appropriate quantity of concentrated H2SO4 to form a fluid hydrate slurry containing 20 g/l 40 g/l or 80 g/l H2SO4.The slurry was treated with 5 ml of a titanous sulfate solution, deliquored in a 1 5 cm Buechner funnel, and then washed with 1 500 ml of water. The titanyl hydrate after treatment by either process was calcined at 9000C and then analyzed. Analyses of the calcined hydrate impurities are presented as parts per million in Table I.

Analyses of the clarified black liquor solutions and the trivalent titanium solutions used for treatment in the instant examples are given in Table II and Table Ill, respectively.

Comparing the calcined hydrate impurity analyses of the commercial bleach process with the inventive process in Table II clearly shows the impurity removal of the inventive process to be equivalent or superior to that of the commercial bleach process.

The invention being thus described, it will be obvious that the same may be varied in many ways, such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be within the scope of the following claims.

Table 1 Hydrate impurity analysis (PPM) Commercial bleach Inventive process 100 g/l H2SO4 20 g/l H2SO4 40 g/l H2SO4 80 g/l H2SO4 Type of Ti3+ Fe Cr V Ni Cu Example Fe Cr V Ni Cu Fe Cr V Ni Cu Fe Cr V Ni Cu solution 23 2.2 8 1 0.9 1 24 1.8 6 6 0.4 19 1.7 7 2 0.1 16 1.5 6 5 0.2 14 1.8 7 2 0.6 2 23 2.0 7 5 0.2 18 1.7 7 3 0.6 17 1.8 7 6 0.3 12 1.4 5 5 0.1 3 13 1.0 5 0.5 0.1 11 0.7 4 0.5 0.1 17 0.7 2 1 0.1 16 1.2 5 2 0.1 4 17 1.1 6 2 0.1 16 1.2 5 6 0.1 16 1.3 5 0.5 0.1 Maclntyre ore 10 1.6 6 5 1.4 5 13 1.6 5 4 1.1 19 1.7 6 6 1.5 12 1.1 4 4 1.5 12 1.4 5 8 1.1 6 11 1.2 5 5 1.4 14 1.5 5 3 7.6 16 1.7 6 4 1.4 10 1.6 7 0.5 0.1 7 15 1.5 6 0.5 0.1 12 2.0 7 0.5 0.1 15 2.3 8 0.5 0.1 10 0.7 5 0.5 0.1 8 12 1.7 7 2 0.1 9 0.7 4 4 0.1 9 1.0 5 3 0.1 3 2.2 8 8 0.1 9 17 2.5 6 10 0.1 9 2.7 9 3 0.1 11 1.8 7 4 0.8 5 2.3 9 10 0.2 10 22 1.8 8 6 0.1 6 1.2 5 3 0.1 12 1.4 8 8 0.3 R.B.S.Slag 8 0.9 4 0.5 0.9 11 7 0.4 1 0.5 0.1 6 0.7 2 0.5 0.1 6 0.1 2 0.5 0.1 33 0.2 2 0.5 0.9 12 5 0.1 2 0.5 0.1 6 0.2 3 0.5 0.1 5 0.1 1 0.5 0.2 17 1.2 5 0.5 4.7 13 20 1.6 6 3 3.9 12 1.0 5 3 4.0 16 0.4 2 1 4.4 13 1.5 5 0.5 3.3 14 23 1.1 4 0.5 3.7 21 1.5 6 5 3.8 15 1.4 5 4 3.7 O.I.T. Slag 12 1.9 6 4 0.1 15 15 2.1 7 2 0.1 18 1.6 6 4 0.1 11 1.3 5 0.5 0.1 13 1.8 5 4 0.1 16 17 1.2 4 4 0.1 17 2.5 8 5 0.1 12 1.6 6 0.5 0.1 Table II Clarified black liquor solutions %TiO2 g/lTiQ2 %H2SO4 %FeSO4 A/R* Maclntyre Ore 11.27 165.1 20.97 9.82 1.86 Richards Bay Slag 15.82 238 32.72 6.6 2.07 O.l.T. Slag 13.28 - 27.44 4.96 2.07 *Ratio of sulfuric acid to titanyl sulfate (measured as TiO2).

Table Ill Trivalent titanium solutions Total g/l Specific % g/l Tl > + gravity TiO2 TiO2 as TiO2 Macintyre Ore 1.424@310C 6.37 90.71 84.2 Richards Bay Slag 1.36a,(2 Z3 C 6.09 83.07 79.8 Q.l.T. Slag 1.366C50 C 5.34 73 67

Claims (8)

Claims

1. A process for removing impurities from titanyl hydrate comprising: a. slurrying an impure titanyl hydrate with water to form a titanyl hydrate slurry: b. treating the titanyl hydrate slurry to solubilize impurities by the addition of a trivalent titanium solution made from a clarified black liquor solution taken from a stage in a process for the production of titanium dioxide prior to the precipitation of titanyl hydrate; c. separating the titanyl hydrate from the treated titanyl hydrate slurry containing solubilized impurities; d. washing the separated titanyl hydrate to remove residual solubilized impurities and to produce a purified titanyl hydrate; and e. recovering the purified titanyl hydrate.

2. A process for removing impurities from a titanyl hydrate slurry comprising: a. separating the impure titanyl hydrate from the titanyl hydrate slurry to form a titanyl hydrate wet cake; b. washing the titanyl hydrate wet cake to remove residual soluble impurities; c. treating the impure titanyl hydrate to solubiliza impurities with a trivalent titanium solution made from a clarified black liquor solution; d. washing the treated titanyl hydrate to remove solubilized impurities and to produce a purified titanyl hydrate; e. recovering the purified titanyl hydrate.

3. The process of claims 1 or 2 wherein the clarified black liquor solution contains titanyl sulfate (measured as TiO2) in the range between about 90 grams/liter and about 250 grams/liter, iron (measured as ferrous sulfate) at less than about 280 parts per 1 00 parts titanyl sulfate (measured as TiO2), and sulfuric acid at a weight ratio of sulfuric acid to titanyl sulfate (measured as TiO2) in an amount between about 1.7 and about 2.2.

4. The process of claim 1 or 2 wherein the trivalent titanium solution is a titanous sulfate solution containing a total soluble titanium content (measured as TiO2) of between about 30 grams/liter and about 85 grams/liter, a weight ratio of ferrous sulfate to total soluble titanium (measured as TiO2) of betweeen about 0.05:1.2 and about 1.2:1, a titanous sulfate content (measured as TiO2) of between about 30 grams/liter and about 80 grams/liter and a weight ratio of sulfuric acid to total soluble titanium (measured as TiO2) of between about 3.4:1 and about 7,0:1.

5. The process of claim 1 or 2 wherein the trivalent titanium solution is a titanous sulfate solution containing a total soluble titanium content (measured as Tit2, of between about 50 grams/liter and about 80 grams/liter, a weight ratio of ferrous sulfate to total soluble titanium (measured as TiO2) of between about 0.6 to 0.7:1.2, a titanous sulfate content (measured as TiO2) of between about 50 grams/liter and about 75 grams/liter, and a weight ratio of sulfuric acid to total soluble titanium (measured as TO2) of between about 5:1 and about 7:1.

6. The process of claim 1 or 2 wherein the trivalent titanium solution is selected from the group consisting of titanous sulfate and titanous chloride.

7. Purified titanyl hydrate obtained by the process as claimed in any one of claims 1 to 6.

8. Titanium dioxide obtained by calcining the purified titanyi hydrate claimed in claim 7.