FI70563B - FOERFARANDE FOER FRAMSTAELLNING AV TITANDIOXID GENOM ANVAENDNING AV REDUKTIONSMEDEL - Google Patents

Description

- rn «« PATENT PUBLICATION No. / 7 ^ * 11 PATENTSKR, FT 7Ubo3 (51) Kv.lkVt.CI.21 c 01 G 23/053 (21) Patent application - Patentansökning 80251 1 V 7 (22) Application date— Aruökningsdag 08.08.80 (23) Start date - Giltlghetsdag 08.08.8 0 ^ ® M l “EN N LA ND (41) Has become public - Blivit offentlig 11.02.81 / ri \ (44) Date of display and publication— n / - q / V * · / Ansökan utlagd och utl.skriften publicerad UO.UO.oo (45) Patent granted - Patent meddelat 24.09.86

National Board of Patents and Registration (86) International application - Int.ansökan

Patent- och registerstyrelsen (32) (33) (31) Claim claimed - Begärd priority 10.08.79 30.05.80 USA (US) 65449, 151998 (73) NL Chemicals, Inc., 1230 Avenue of the Americas, New York, New York York USA (US) (72) Joseph A. Rahm, Long Branch, New Jersey, USA (US),

The present invention relates to a new process for the preparation of titanium dioxide by the preparation of titanium dioxide by a new process for the preparation of titanium dioxide. The present invention relates to a new process for the preparation of titanium dioxide. reacting the titanium-containing material with dilute sulfuric acid in the presence of a reducing agent to produce titanium saline solutions that can be hydrolyzed to provide a titanium dioxide pigment.

Titanium dioxide is a well-known material with the desired pigment properties and is useful in paint and coating preparations and plastic materials. Several different processes are known for the production of titanium dioxide, for example the sulphate process and the chloride process.

In a conventional sulfate process for the production of titanium compounds, a titanium-containing material such as ilmenite ore, including solid ilmenite and ilmenite sand, and 2,70563 titanium-containing or furnace slag are reacted with concentrated sulfuric acid (e.g., 90-96% sulfuric acid). The reaction is sometimes referred to as "extraction" or "ore extraction". The extraction reaction between titanium-containing material and concentrated sulfuric acid is exothermic and progresses very rapidly. Typically, the titanium-containing material and concentrated sulfuric acid are placed in a reaction vessel called an extraction tank. Water is usually added to the extraction tank to effect and accelerate the acid-ore reaction by the heat generated, resulting in vigorous boiling of the aqueous-acid solution at about 100-190 ° C and the development of large amounts of water vapor and vapor with entrained particulate material. As the vigorous reaction progresses, water is removed and the reaction mass becomes solid. The reaction ends in the solid phase at a temperature of about 180-215 ° C. The extraction operation is a batch process carried out in one and the same extraction tank. Use as many tanks as necessary to produce the titanium sulfate solution according to the desired capacity of the manufacturing plant.

The solid reaction mass obtained from the extraction, referred to as the "cake", is allowed to cool. The solid cake is then dissolved in water or dilute acid to provide sulfate salts of iron, titanium, and other trace metals in the titanium-containing material.

The reaction mixture is then treated to convert ferric iron to ferrous iron in a reduction step. Unless the iron in the reaction mixture is completely reduced to the ferrous state, ferric sulfate will pass through the process and possibly cause iron contamination of the final product.

In order to ensure that during the preparation of the reaction solution for hydrolysis the iron remains in the ferrous state and does not oxidize, it is preferable to allow the reduction to proceed far enough so that the titanium solution contains a small amount of trivalent titanium. In this case, it is essential that the content of trivalent titanium is within a certain range. The reduction, which has gone too far, gives a poor yield in the hydrolysis step as a result of TiCl 2 losses. Too mild a reduction, on the other hand, makes it uncertain that enough trivalent titanium is present to prevent iron moieties from oxidizing to the ferrite space.

As described in U.S. Patent 2,309,988, in a commonly used method for reducing Ferri ion, a basket containing scrap iron is immersed in a reactor vessel containing a dissolved, unreduced extraction cake. In the second method, a sufficient amount of iron is poured into the reactor and the bulk of the solution is stirred or otherwise kept in contact with the iron until reduction occurs. These methods tend to cause local overheating of the titanium solution and local decrease in acidity due to the reaction of iron and solution. This tends to provide unstable solutions that vary from batch to batch. U.S. Patent No. 1,014,793 discloses the reduction of the iron component of expression rivet solutions to a ferrous state by the addition of metallic zinc, sulfuric acid, or sodium thiosulfate. In addition, tinned iron, crystalline titanium (3) sulfate, and sulfur dioxide have also been proposed as reducing agents. These known methods, although effective when used with previously extracted ilmenite ore solutions, could not be used during the extraction reaction due to loss of solution flow and solidification. Moreover, in the subsequent dissolution of the reaction cake, such use would have resulted in an unstable solution that would self-clump in an uncontrolled hydrolysis reaction that would prevent the formation of titanium oxide in the pigment solution.

The iron and titanium sulphate salt solution obtained after separate extraction and reduction steps is further treated by known measures to remove ferrous sulphate (commonly referred to as "vihtrilli") to give a titanium sulphate solution which, on hydrolysis, produces hydrated titanium dioxide. The titanium dioxide hydrate is usually subjected to a calcination treatment in a suitable incinerator to remove water of hydration and form an unhydrated titanium dioxide pigment. The above process is described in more detail in, for example, U.S. Patent Nos. 1,504,672, 3,615,305 and 3,071,439.

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The process of the present invention for producing titanium dioxide can substantially avoid or eliminate the above-mentioned disadvantages typical of a conventional sulfate process. The term "titanium sulfate" refers to sulfate salts of titanium such as titanyl sulfate and titanium (3) sulfate.

The invention relates to a process for the preparation of titanium oxide by treating a titanium-containing material with dilute sulfuric acid and reducing the ferric iron in the material to a ferrous iron with a reducing agent, hydrolyzing the titanium sulphate present in the resulting solution and separating the titanium dioxide hydrate formed from the solution. The process is characterized in that a) the amount of titanium-containing material used is about 10-400% greater than the stoichiometric amount required to react with sulfuric acid to form titanium sulfate, b) dilute sulfuric acid at a concentration of about 25-60% by weight is added, c) the reaction temperature of the titanium-containing material and sulfuric acid is adjusted to below 140 ° C, d) ferric iron is reduced to ferric iron with a reducing agent, e) the resulting reaction mixture is cooled to below about 110 ° C without precipitating reaction products, f) insoluble solids are separated from the solution, and g) the titanium dioxide hydrate formed is separated from the solution and calcined.

The accompanying drawing illustrates an embodiment of the process according to the invention using a continuous process for the production of titanium dioxide.

It has been found that a titanium-containing material can be reacted with dilute sulfuric acid completely in the liquid phase in the presence of a reducing agent to provide a stable hydrolyzable titanium sulfate solution that can be used to prepare titanium compounds and titanium oxide pigments. In particular, it has surprisingly been found that the presence of a reducing agent in the extraction step greatly accelerates the reaction rate between the titanium-containing material and dilute sulfuric acid to provide a hydrolyzable titanyl sulfate solution useful for the preparation of titanium compounds. In addition, the use of a reducing agent in the extraction step eliminates the prior art need for a separate and independent reduction step after extraction.

The extraction reaction is performed using a titanium-containing material. As used herein, the term titanium-containing material means a material that contains recoverable titanium moieties when treated in accordance with the process of the invention. Exemplary soil materials include titanium-containing slag, kiln slag, ilmenite ores such as magnetic ilmenite and massive ilmenite, and ilmenite sands.

The extraction reaction is carried out using a sufficient amount of titanium-containing material to provide an excess of said material in an amount of about 10-400% above the stoichiometric amount. This amount can also be shown to be 1.1-5 times the stoichiometric amount. The following formula shows the stoichiometry of the extraction reaction:

FeTiO, + 2H.SO. -> TiOSO. + FeSO. + 2H_0 3 2 4 4 4 2

The use of excess titanium-containing material in the extraction reaction is efficient and desirable to achieve a successful and useful process in accordance with this invention without excessive grinding of the ore. The surface area of the titanium-containing material is preferably 2 to about 0.05 to 0.6 m / cm. Ore with a larger surface area could be used, but it does not provide any benefit due to increased grinding costs. As mentioned above, an excess of titanium-containing material in the range of about 10-400% above the stoichiometric amount required to react with sulfuric acid should be used in the process extraction reaction. The use of smaller amounts of material results in unacceptably low reaction rates and long processing times, making the process economically unattractive. The use of larger than recommended excesses of material is not desirable due to the greatly reduced fluidity of the reaction mixture and the need to recycle large amounts of unreacted titanium-containing material to the extraction reactors. Unexpectedly, for example, it has been found that doubling the amount of titanium-containing material, such as Maclntyre ilmenite ore, above the stoichiometric amount to react with dilute sulfuric acid increases the reaction rate by at least an order of magnitude of 10 times in the last extractor. It should be noted that the reaction rates vary with the source of titanium-containing material used during the extraction.

The concentration of sulfuric acid used in the process of the invention should be between about 25-60% by weight based on the total weight of the acid solution. An acid concentration of less than about 25% by weight is not desirable because hydrolysis of titanium dioxide occurs during and in connection with the extraction reaction using such acids. Premature hydrolysis of titanium salt solutions prevents the formation of pigment-grade titanium dioxide at a later stage in the process. Also, the use of an acid having a concentration greater than about 60% by weight is undesirable because (1) the resulting reaction solution is more viscous and more difficult to handle, (2) the economics of recycling the used acid cannot be realized unless the spent acid is concentrated, which unnecessarily raises operating costs and (3) the higher concentration of reaction products in solution promotes the precipitation of ferrous sulfate monohydrate and recoverable titanyl sulfate dihydrate. The presence of ferrous sulphate monohydrate makes the in-weight separation inefficient and difficult to remove by filtration.

The operating conditions of the process for carrying out the extraction reaction can be easily adjusted depending on the concentration of dilute sulfuric acid and the particular amount of titanium-containing material used to achieve the optimum operation of the process. To illustrate this, the use of low concentration sulfuric acid, e.g., less than 40% by weight, initially requires the use of a process at the lower end of the recommended temperature range due to the lower boiling point of dilute sulfuric acid. It is desirable to increase the amount of titanium-containing material used to extract as much material as possible in the first extraction reactor, at which point the operating temperature and reaction rate are usually higher. As mentioned below, in subsequent extraction reactors, the temperature is kept lower than in the first reactor and, ultimately, must be lowered to prevent or avoid premature hydrolysis of the titanium brine.

770563 The temperature at which the extraction reaction takes place is below about 140 ° C and preferably from 55 ° C to the boiling point of the reaction solution, i.e. between about 55-140 ° C. The choice of a temperature that is too low in the extraction reactor should be avoided because the extraction reaction proceeds too slowly and thus requires a longer residence time of the reagents in the extraction reactor.

Prolonged residence times should also be avoided to avoid the undesirable risk of nucleation in the reaction solution due to premature hydrolysis of the titanium salt. The choice of a temperature above 140 ° C is not recommended because the titanium salt hydrolyzes at much higher rates at higher temperatures. Carrying out the extraction reaction below about 55 ° C should be avoided because the reaction products start to precipitate from solution and the viscosity of the reaction mixture increases, making the removal of unreacted solids very difficult. The recommended operating temperature for carrying out the extraction reaction is between about 70-110 ° C.

It should be noted that the extraction reaction of the process of this invention can be carried out as a batch reaction, e.g., in a reaction vessel from which the reaction mixture, after the extraction reaction has progressed to the desired extent, is removed and further processed in other vessels. A preferred embodiment of the process of the invention is one in which the extraction reaction is carried out continuously in at least two reaction vessels and in which the titanium-containing material and dilute sulfuric acid are made to flow in parallel.

When the process is carried out continuously, it is recommended that it be carried out using two or more extraction reactors. The total number of extraction equipment depends on the ease of reaction control, the production capacity of the plant and the processing of the process.

The preferred operating temperatures for carrying out the extraction reaction in the two extraction reactors or stages are those in which the first extraction apparatus is kept below about 140 ° C and preferably below about 110 ° C and the second extraction apparatus is kept below about 100 ° C and preferably below at about 75 ° C.

The preferred operating temperatures for carrying out the extraction reaction in the three extraction reactors or stages are those in which the first extraction device is kept below about 140 ° C and preferably below about 110 ° C, the second extraction device is kept below about 110 ° C and preferably below n At 100 ° C and the third extractor is kept below about 80 ° C and preferably below about 75 ° C.

s 70563

The preferred operating temperatures for carrying out the extraction reaction in the four extraction reactors or stages are those in which the first extraction device is kept below about 140 ° C and preferably below about 110 ° C, the second extraction device is kept below about 110 ° C and preferably below n At 90 ° C, the third extractor is kept below about 100 ° C and preferably at 86 ° C and the fourth extractor is kept below n, at 90 ° C and preferably below about 75 ° C.

The preferred operating temperatures for carrying out the extraction reaction in the five extraction reactors or stages are those in which the first extraction device is kept below about 140 ° C and preferably below about 110 ° C, the second extraction device is kept below about 110 ° C and preferably below n At 90 ° C, the third extractor is kept below about 100 ° C and preferably below about 85 ° C, the fourth extractor is kept below about 90 ° C and preferably below about 80 ° C. and The fifth extractor is maintained below about 85 ° C, and preferably below about 75 ° C.

All of the above extractor temperatures can be varied depending on the desired yield and reaction times at each step. One of the essential and striking features of the invention in providing a useful process is that the temperature of the extraction reaction is lowered as the reaction progresses to prevent or avoid premature hydrolysis of the titanium salt solutions obtained. Premature hydrolysis of the titanium salt solution prevents the formation of a grade or grade of titanium dioxide.

The duration of the extraction reaction in the extraction reactor is adjusted by the optimum degree of conversion or extraction of the titanium-containing material. stage. In general, it is recommended to extract or react as much titanium-containing material as possible in the first extraction reactor or stage, where the temperature is maintained at the highest level to prevent hydrolysis of the titanium sulfate in solution. For example, in a continuous multi-stage system using McIntyre ore as a source of titanium-containing material, it is sometimes possible to extract up to n in the first stage.

90% by weight of the stoichiometric amount of ore charged to the process, excluding excess ore. Preferably about 30-80% and most preferably 60-80% by weight of the stoichiometric amount of ore is extracted in the first step, except for excess ore. The conversion is measured by the stoichiometric amount of titanium-containing material from the amount of reaction.

970563 The temperature is used to control the extraction reaction, preferably by controlling the ratio of active acid to titanium in the reaction solution. This ratio is an indication of the degree of conversion or extraction. The phrase "active acid" means the total amount of free acid in the reaction solution plus the acid combined with the titanium in the reaction solution. The ratio of active acid to titanium dioxide (active acid: titanium dioxide) is calculated as the sum of both the free acid in solution and the advanced acid in the titanium in solution divided by the titanium in solution (calculated as TiO2). The active acid content of the solution can be determined, for example, by titrating a selected sample (by weighing or pipetting technique) with a 0.5 N base solution (NaOH) to pH 4.0 in a barium chloride / ammonium chloride buffered solution. The titration gives the content of the free acid plus the combined acid of TiO2, which is called the active acid. To illustrate this, 60 ml of a buffer solution containing 75 g / l barium chloride and 250 g / l ammonium chloride is added to a beaker containing the sample in question and diluted to 250 ml with water and titrated with 0.5 N base to the methyl orange end point.

In the batch process, the concentration of active acid can vary widely and is not critical except to the extent that the extraction and reduction take place in the liquid phase. In a continuous process, the active acid is allowed to drop from infinity at the beginning of the reaction to 1.50 to 7.0 at the end of the reaction, depending on the extraction conditions. Typically, the level of the ratio of active acid to TiO 2 ranges from 2.0 to 3.5. As the level of active acid decreases, the stability of the titanyl sulfate solution against hydrolysis deteriorates. In general, the temperature of the reaction solution should be kept below about 140 ° C, and preferably below about 110 ° C, when the ratio of active acid to titanium (calculated as titanium dioxide) drops to about 2.0. To illustrate this in a two-step extraction process, the temperature of the reaction solution in the first stage of the extraction reaction or in the extraction apparatus should be kept below about 140 ° C, e.g. 110 ° C until the ratio of active acid to titanium dioxide in the reaction solution drops to about 3.0. to about 100 ° C, such as 70 ° C. In contrast, in the three-step extraction process, the temperature of the first step is maintained at about 110 ° C to obtain a reaction mixture in which the ratio of active acid to titanium dioxide in the reaction solution is between about 2.5 and 3.0, and then the reaction is carried out in a second step at about 100 ° C. At a temperature of C to obtain a reaction mixture in which the ratio of active acid to titanium dioxide in the reaction solution is between about 2.2 and 2.5. The reaction can then be completed in a third step at a temperature below about 80 ° C to provide a reaction mixture in which the ratio of active acid to titanium dioxide in the reaction solution is about 2.0.

The reducing agent can be added at any point in the extraction process. In a multiple extraction process, it is recommended to add a reducing agent to the first step to maximize ore extraction and ferric iron reduction initially.

Typically, the reduction of ferric iron in the extraction mixture can be achieved using conventional sulfate process reducing agents. Such substances may be gaseous reducing agents, such as sulfur dioxide, which may be used alone or in combination with activated carbon. Other typical reducing agents include sulfuric acid, sodium thiosulfate, titanium (3) sulfate, reduced ilmenite, and mixtures thereof, as well as metallic materials such as tin, iron, zinc, zirconium, titanium, aluminum, magnesium, and mixtures thereof, including alloys containing them. These exemplary reducing agents are not intended to exclude other substances known in the art. These substances can be used in the form of solutions or solids. The preferred solid reducing agent is in the form of granules or powder.

The amount of reducing agent used is that amount which is at least stoichiometric to the amount of ferric iron present. A small excess of reducing agent, typically no more than about 10% above that required to reduce all ferric iron, can also be used to reduce a small amount, typically less than 1% of tetravalent titanium to trivalent titanium. The following chemical oxidation-reduction reaction equation shows the stoichiometry of the system using aluminum and iron reducing agents: a) Al ° + 3Fe + 3 = Al + 3 + 3Fe + 2 b) Fe ° + 2Fe + 3 = 3Fe + 2 When a reducing agent is used, such as powdered iron, the amount of reducing agent added to the reaction solution in the extraction equipment depends on the amount of ferric iron in the titanium-containing feedstock. In general, about 3 to 8% by weight of reducing agent, based on the total weight of the reacted titanium-containing material, is sufficient to provide satisfactory results in a process using a titanium-containing material, such as ilmenite ore containing 5 to 13% Fe 2 O 3. The addition of a reducing agent such as powdered iron has another advantageous effect in that it accelerates the rate of the extraction reaction.

Coincidentally, the accelerating effect of iron on the rate of the extraction reaction increases as the particle size of the iron decreases.

Upon completion of the extraction reaction, the resulting reaction mixture containing titanium sulfate, ferrous sulfate, and trace elements derived from the titanium-containing material can be treated to recover a titanium sulfate solution for the preparation of titanium compounds or treated according to conventional sulfate process techniques to produce a titanium dioxide pigment.

Referring to the diagram shown in the appended figure for the production of titanium dioxide in a multi-stage reactor system, reference numeral 10 shows an extraction reactor. A titanium-containing material such as ilmenite is adapted to be fed to the extraction reactor 10 from an ore storage tank 11. Dilute sulfuric acid having a concentration of 25-60% by weight based on the total weight of the acid solution is fed from a strong acid mixture (96% by weight) from a fresh acid source 12 acid (15-45% by weight 9 or water, directly into the extraction reactor 10. The ilmenite ore in the extraction reactor 10 and dilute sulfuric acid are continuously stirred at a temperature not exceeding the boiling point of the reaction solution in the reactor.

The reagents in the extraction reactor 10 are maintained at a temperature below about 140 ° C and preferably between about 55-140 ° C. More specifically, the reagents in the extraction reactor 10 are preferably kept at 110 ° C. The extraction reactor 10 can be maintained at any suitable pressure; normal pressure is recommended for economic reasons.

When operating continuously in the three-stage extraction systems shown, the reaction mixture is transferred from the extraction reactor 10 to a conventional separation device 13, e.g. a filter or cyclone separator, where part or all of the 5 6 3 from the trough 10 to the extraction reactor 15, in which case it does not involve the recycling of any unreacted ilmenite ore to the extraction reactor 10.

It is preferable to keep the reaction solution in the extraction reactor 15 at a somewhat lower temperature than the temperature in the extraction reactor 10. The reaction mixture in the extraction reactor 15 is maintained, for example, at about 100 ° C. Temperature control in the extraction reactor 15 can be achieved by adding recycled acid or water. The pressure in the extraction reactor 15 is preferably normal pressure, but higher pressures may be used if desired.

The reaction mixture can be continuously transferred from the extraction reactor 15 to a conventional extraction device 16, e.g. recycling to the extraction reactor 15.

It is recommended that the reaction mixture in the extraction reactor 18 be maintained at about 70 ° C and normal pressure.

The reaction mixture from the extraction reactor 18 is continuously fed to a suitable separation device 19, e.g. a filter or a self-weight separator (or multiples thereof arranged in series and / or in parallel), in which the unreacted ilmenite ore is separated from the liquid reaction product. Excess or unreacted ilmenite is recycled through lines 21 and 21 'to one or both of the extraction reactors 18 and / or 10. The liquid reaction product from the separation device 19 is passed to a settling device 20, e.g. a conventional settling device or a LAMELLA device, where a side stone device or other undesirable solid material is removed from the reaction product. The use of a LAMELLA landing gear is not considered part of this invention.

As mentioned above, the extraction reaction is carried out in extraction reactors 10, 15 and 18. It is not essential that the extraction reaction be carried out in three extraction reactors. In fact, the process can be carried out batchwise using only one extraction reactor. However, it is recommended to use two or more extraction reactors to carry out the process continuously. When only two extraction reactors are used, the temperature of the second extraction reactor, such as the extraction reactor 15, can be adjusted to a lower temperature, such as e.g. 70 ° C.

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A suitable reducing agent from tank 22 is added to the extraction reactor 10 or the extraction reactor 15 or both reactors in order to reduce the trivalent ferric iron in the extraction solution to the divalent ferrous iron. The presence of the reduced state prevents the contamination of the titanium hydrate obtained later with ferric salts. In the extraction equipment, the amount of reducing agent added to the reaction solution depends on the amount of Ferri iron in the ilmenite feed ore, as described above.

With this measure, it is possible to avoid a separate reduction step for the extraction solution, which would otherwise be necessary. The cutters can be added at any point in the extraction operation. The amount of reducing agent used is selected so that not only all the trivalent iron in the ilmenite ore is reduced to the divalent state, but also part of the titanium in the reaction solution is reduced to the trivalent state to obtain a titanium sulfate solution for hydrolysis containing sufficient trivalent titanium. The presence of trivalent titanium prevents the formation of ferric iron that would be adsorbed on the titanium dioxide particles at a later stage of the hydrolysis process.

A certain amount of metal sulfates, i.e. ferrous sulfate monohydrate, usually precipitates during the extraction reaction without any noticeable deterioration in the fluidity of the reaction mixture. The ferrous sulfate monohydrate can be easily dissolved at the end of the extraction reaction by adding water. At least a portion of the water can be replaced with a titanium sulfate solution from which much of the ferrous sulfate has been removed (by crystallization and separation of the ferrous sulfate heptahydrate at a later stage in the process described below). By this measure, the addition of excess water to the system can be minimized or avoided. Usually the excess water has to be removed at a later stage of the process, e.g. by evaporation.

Water or a solution of water and titanium sulfate can be added to the reaction solution in the last extraction reactor or at a suitable point between the last extraction reactor and the separation device 19 to provide cooling. The addition of water or a solution of water and titanium sulfate is not considered part of this invention.

The resulting solution containing titanium sulfate, ferrous sulfate, and trace elements derived from ilmenite ore can be recovered and processed to produce titanium compounds. Alternatively, the solution can be processed to produce titanium dioxide pigment i4 70563, whereby the reaction solution is passed to a settling device 20 to remove solids from the solution.

When titanium dioxide is prepared, the reaction solution is then passed from the settling apparatus 20 to the crystallization apparatus 23, where vihtrill (i.e., ferrous sulfate heptahydrate) is crystallized and removed by known process steps. For example, the solution is cooled in a continuous or batch vacuum crystallizer to about 10-20 ° C by subjecting SS to a vacuum of 23.5 mmHg to form large crystals of Vihtrill (FeSO 4 .71 ^ 0) that can be easily filtered with a drum or table filter. Vihtrill cake can be washed to recover soluble titanium parts. The reaction solution from the filter can be concentrated by known measures, e.g. by evaporation before it is subjected to hydrolysis. The reaction solution can also be clarified either before or after crystallization and removal of ferrous sulfate heptahydrate. The clarification step before crystallization is preferred if it is desired to obtain highly pure ferrous sulfate heptahydrate, which can be further processed, e.g., to prepare reagents used for water and sewage purification.

It is recommended that the reaction solution be subjected to a fine filtration step prior to hydrolysis. After removal of the ferrous sulphate heptahydrate and, if necessary, clarification and fine filtration, a titanium sulphate solution is obtained which has a preferred Fe: TiO2 ratio and which can be directly hydrolysed or optionally evaporated by known means in a vacuum evaporator to the desired TiO2 concentration, followed by hydrolysis.

The reaction solution from the crystallizer 23 consists of a titanyl sulfate (TiOSC> 4) solution which is fed to a hydrolysis apparatus 24, where the titanyl sulfate is hydrolyzed by known process steps to obtain titanium dioxide hydrate. In particular, the titanyl sulfate solution is hydrolyzed to obtain insoluble titanium dioxide hydrate by diluting the titanyl sulfate solution with water at elevated temperatures. For example, a predetermined amount of titanyl sulfate solution, preferably with a titanium dioxide content of more than 200 g / l, is preheated to more than 90 ° C and added with stirring to clear water at substantially the same temperature and in a ratio of 3-4 1/2 parts of solution per part of water. The solution is brought to a boil and titanium dioxide in the form of colloidal particles is precipitated, the colloidal particles flocculating to produce filterable titanium dioxide hydrate. The method and means for performing the hydrolysis step are well known in the art and are described, for example, in U.S. Patent Nos. 1,851,487 and 3,071,439.

After hydrolysis, the titanium dioxide hydrate is filtered through a filter device 25, such as a Moore filter, and the resulting filter cake is fed to a calciner 26, where it is heated in a known manner to remove hydration water and adsorbed sulfuric acid to give titanium dioxide of suitable pigment quality.

A significant advantage of the process of this invention is that it can reduce or even eliminate the serious "spent acid" disposal problem characteristic of the conventional sulfate process for the production of titanium dioxide pigment. In particular, the spent acid from the extraction, crystallization and hydrolysis steps of the process is reprocessed or recycled for use in performing an extraction reaction with ilmenite ore. Thus, the process of the present invention may be free or substantially free of spent waste acid.

To illustrate this, the spent acid from the filtered 25 is passed through a tube 27 and returned to the extraction reactor 10. If desired, the spent acid from the filtered 25 can be concentrated, e.g., by evaporation in a known manner in the concentrator 28 before returning to the extraction reactor 10.

Another and significant advantage of the process of this invention is that the recycled spent acid can be fed directly to any or all of the extraction reactors to control the temperature in each extraction reactor. The above provides a suitable and efficient way to balance and control the reaction temperature between the reactors.

The principle and practice of the present invention are described in the following examples, which are illustrative only and are not intended to limit the invention thereto, as changes in the art and procedures will be apparent to those skilled in the art. All parts and percentages defined herein are by weight unless otherwise indicated. Conversions are measured by the amount of reaction to the stoichiometric amount of treated ore.

i6 70563 The procedures and tests defined in this and the examples below were performed as follows:

Area was measured by the sedimentation method described by Jacobsen A.E. and Sullivan, W.F. "Method for Particle Size Distribution for the Entire Subsieve Range" Vol 19, page 855 in Analytical Chemistry (November, 1947).

Example 1 800 g of ilmenite ore (Maclntyre ore) with an area of 2 3 0.39 m / cm was charged to an extraction reactor vessel. 1.16 liters of 43% by weight sulfuric acid was added to the reaction vessel. The temperature of the reagents was raised to 108 ° C by heating under constant stirring with a stirrer made of TEFLON-3 material. After 50 minutes, a 15 cm sample of the reaction mixture was filtered by gravity through glass filter paper into a 100 ml polypropylene laboratory glass. The filtrate was analyzed for active acid content and titanium content (expressed as TiCl 4). The active acid content was 430 g / l, giving a ratio of active acid to titanium of 7.1.

The conversion of the reaction was determined after about 1 1/4 hours by analyzing a sample of the reaction solution. The analyzed acid had an active acid content of 396.9 g / l and a titanium content of 78.5 g / l (TiCl 2) ·

After about 1 1/4 hours, 17 g of powdered iron was added to the reaction vessel to provide a reducing agent to the ferric iron content of the reaction mixture.

After about 1 3/4 hours, the temperature of the reaction mixture was lowered to 70 ° C by placing the reaction vessel in a cooling water tank. Analysis of a portion of the reaction solution after cooling and removal of insoluble solids showed an active acid content of 353.3 g / l I 2 SO 4 and a titanium content of 89.25 g / l (TiCl 4), giving a ratio of active acid to titanium of 3.96.

The reaction mixture was maintained at 70-74 ° C for about 15 hours. The reaction mixture was cooled to about 50 ° C to quench the reaction, filtered to remove insoluble solids, and analyzed for active acid and titanium content. The active acid content was 275.8 g / l and the titanium content was 136.2 g / l (TiO 2), giving a ratio of active acid to titanium of 2.025.

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The reaction solution was stable and suitable for hydrolysis to prepare the titanium dioxide pigment. The titanium dioxide pigment can be prepared from the reaction solution according to conventional manufacturing techniques.

Example 2

A two-stage system was constructed consisting of a heated, stirred five-liter first stage reactor with an overflow to a heated, stirred 25 liter second stage reactor. Maclntyre ilmenite ore with the following particle size distribution (U.S. Standard sieves)

Mesh weight% +100 1.2 + 200-100 35.8 + 325-200 23.0 + 400-325 6.0 -400 34.0 and containing 46.8% TiO 2 were continuously fed to the first stage at a rate of 3 .78 g / min. A dilute sulfuric acid solution having the following analysis: 29.9% free H 2 SO 4 1.4% titanium (3) sulfate (TiCl 2) 3.3% soluble titanium (as TiO 2) was also fed to the first stage at a rate of 12, 5 ml / min. Titanium (3) sulfate was added to the reaction vessel to provide a reducing agent to the ferric iron content of the reaction mixture. Both stages were initially charged with sufficient ore to achieve a 100% excess over the stoichiometric requirement. The unreacted ore flowing over the second stage was recycled to the first stage to maintain this excess ore in the system. The first stage reactor was set at 106 ° C, while the second stage reactor was set at 71 ° C. The residence times of the first and second phases were approx.

6.8 hours and 34.2 hours, respectively. When sufficient time had elapsed for the equilibrium to stabilize, it was found that 54.2% of the TiO 2 in the ore feed had reacted in the first stage and 28.2% had reacted in the second stage. In these two steps, an overall conversion of 82.4% was achieved.

18 70563

Analysis of the final product was: 9.4% soluble titanium (TiCl 2) 9.0% free H 2 SO 3 0.3% titanium (3) sulfate (TiCl 3)

Example 3

The system described in Example 2 was operated at the following feed rates to the first stage reactor: 3.27 g / min ilmenite ore (46.8% TiCl 4) 12.28 g / min dilute acid solution containing 42.9% free H 2 O 4: * -kci no titanium (3) sulfate.

Powdered iron was also fed to the first stage at a rate of 0.19 g / min. Powdered iron was added to the reaction vessel to provide a reducing agent to the ferric iron content of the reaction mixture.

A 100% excess of ore over the stoichiometric requirement was maintained in the system as in Example 2. The reactor of the first stage was adjusted to 106 ° C and the second stage to 72 ° C. The residence times of the first and second phases were about 9.4 hours and 47.1 hours, respectively. When equilibrium was reached, it was found that 73.9% of the TiO 2 in the ore feed had reacted in the first stage and 20.9% in the second stage. With these two steps, an overall conversion of 94.9% was achieved. Analysis of the final product was: 8.9% soluble titanium (as TiO 2) 8.8% free 1 ^ SO 2 and 0.1% titanium (3) sulfate (TiCl 3)

Example 4

A reaction acid solution containing 41.6% by weight of sulfuric acid was prepared by combining 1,385 g of 96.5% sulfuric acid, 2,087.5 spent acid containing 16.32% by weight of sulfuric acid and 515 g of water in a reaction vessel. The reaction acid was heated to 100 ° C with constant stirring. 2,130 g of ilmenite ore, twice a stoichiometric amount, was heated to 100 ° C and charged to a reaction vessel. The temperature of the reaction mixture was then raised to about 108 ° C and held for 10.5 hours. Samples of the reaction mixture were taken periodically and analyzed. After about 8.5 hours, no further increase in soluble titanium levels was observed, indicating that the reaction had reached equilibrium.

19 70563

After about 10.5 hours, the temperature of the reaction mixture was lowered to 104 ° C and 10 g of granular aluminum was added. The reaction mixture was allowed to react at 104 ° C for approximately two hours. Analysis of the reaction mixture after the addition of aluminum showed an increase in the content of soluble titanium, which indicated that an additional reaction had taken place due to the presence of the aluminum reducing agent.

After about two hours, the reaction temperature was lowered to 74 ° C. The reaction mixture was maintained at about 74 ° C for approximately six hours.

The reaction mixture was then cooled to about 50 ° C to quench the reaction, filtered to remove insoluble solids, and analyzed for active acid and titanium content. The active acid content was 18.11% and the titanium content was 9.04% (TiCl 2), giving a ratio of active acid to titanium of 2.00.

Claims (7)

A process for the preparation of titanium dioxide by treating titanium-containing material with dilute sulfuric acid and reducing the ferric iron in the material to ferrous iron with a reducing agent, hydrolyzing the titanium sulfate in the solution formed and separating the titanium dioxide hydrate formed from the solution, characterized therein. , that a) the amount of titanium-containing material used is about 10-400% greater than the stoichiometric amount needed for reaction with sulfuric acid to form titanium sulfate; b) dilute sulfuric acid having a concentration of about 25-60% by weight is added , c) the reaction temperature of the titanium-containing material and sulfuric acid is adjusted to below 140 ° C; (d) the ferric iron is reduced to ferrous iron with a reducing agent; the insoluble solids are separated from the solution, and g) the titanium dioxide hydrate formed during the hydrolysis is separated from the sol sewing and calcined. Process according to Claim 1, characterized in that the reaction mixture formed is cooled to a temperature below about 75 ° C. 3. Process according to claim 1, characterized in that the insoluble solids are returned to the process step where the titanium-containing material is reacted with sulfuric acid and where the reducing agent is added. 4. A process according to claim 1, characterized in that the sulfuric acid formed during the hydrolysis of the titanium sulfate is returned to the process step where the titanium-containing material is reacted with sulfuric acid. Process according to claim 1, characterized in that iron, titanium (3) sulfate, reduced ilmenite, sulfur dioxide, sodium thiosulfate or a mixture thereof are used as reducing agents. Il