PROCESS AND FURNACE FOR THE PRODUCTION OF TiO2 PIGMENT ACCORDING TO THE SULFATE PROCESS

Abstract: A process for the production of titanium dioxide by dissolving titanium raw materials with sulfuric acid, producing a metal sulfate-containing titanyl sulfate solution, hydrolyzing the titanyl sulfate, separating the titanium dioxide hydrate from the metal sulfate-containing sulfuric acid by filtration, optionally bleaching the titanium dioxide hydrate by reducing treatment in acidic solution and calcining the titanium dioxide hydrate optionally mixed with doping chemicals and rutile nuclei to titanium dioxide, characterized in that the sulfate-containing titanium dioxide hydrate is dried in a dryer at temperatures from 90 to 400 DEG C, preferably 90 to 200 DEG C, especially 90 to 150 DEG C, and the titanium dioxide hydrate dust from the dryer exhaust is separated by filtration, the dried titanium dioxide hydrate is calcined in a rotary kiln directly heated with flue gasses at temperatures in the range from 800 to 1100 DEG C (solids temperature) and the exhaust is de-dusted and de-sulfurized as well as a calcining furnace for the production of titanium dioxide from dried sulphate-containing titanium dioxide hydrate in the form of a rotary kiln with a length-diameter ratio in the range of 5 to 10.

Description: 1 PROCESS AMD FURNACE FOR THE PRODUCTION OF Ti0 2 PIGMENT ACCORDING TO THE SULFATE PROCESS The invention relates to a process for the production of Ti0 2 pigment from titanium dioxide hydrate and a furnace for the calcining of titanium dioxide hydrate. According to the art (Ullmanns Encyclopaedia of Technical Chemistry, 4th Edition, Vol. 18 (1979), pages 574-576), the titanium dioxide hydrate accumulating in the hydrolysis of titanyl sulfate is separated by filtration from the mother liguor, an approximately 25% metal sulfate-containing sulfuric acid. The content of coloring metal ions such as Fe, V, Cr ions is lowered to a minimum by a reducing treatment in diluted acid. After this treatment, characterized as bleaching, the titanium dioxide hydrate still contains approximately 80 kg S0 4 2" ions per t Ti0 2 which cannot be removed by washing with water. After addition of so-called doping chemicals, different amounts of alkali hydroxide, phosphoric acid and optionally rutile nuclei for controlling the pigment quality, the titanium dioxide hydrate is calcined in a directly heated rotary kiln. Flue gas with a temperature of 1100 to 1400°C and solids are moving counter-currently through the rotary kiln. In the first section of the rotary kiln, the titanium dioxide hydrate is dried. After heating it to approximately 500°C, sulfuric acid, split off by further temperature increase, is partially decomposed in the hot flue gas to S0 2 , 0 2 and H 2 0. The formation of pigment particles into the modifications anatse or rutile finally occurs at 800 to 1100°C. The rotary kilns are very large apparatuses with respect to through-put capacity, especially because in the first section, requiring approximately 50% of the furnace length, in which the drying of the titanium dioxide hydrate filter cake occurs, the introduction of heat into the product is very ineffective (Dumont, Belanger, Ind. Engn. Chem. , Process Des. Dev. 17 (1978), 107-114). The filter cake falling out of the rotary kiln in the region at the feed end must be mashed and returned for filtration. The flue gas emerging from the CONHMUiAriON COPY rotary kiln with approximately 400°C contains Ti0 2 dust which is partially converted to pigment, besides S0 2 and sulfuric acid vapor which forms mist as the gas cools. This requires large-scale facilities for dust collecting and reclaiming Ti0 2 dust as well as for removing the H 2 Sθ 4 _ mist and the S0 2 (EP-A 0 320 682) . The utilization of the heat energy of the flue gas is technically complicated, especially as a result of the content of sulfuric acid vapor (EP-A 476172, EP-A 0097259) . The customary process for the production of titanium dioxide by calcining sulfate-containing titanium dioxide hydrate in rotary kilns is characterized by several disadvantages. The individual processes cannot be optimally carried out due to the coupling of filter cake drying, solid calcination with water and sulfuric acid cleavage as well as crystal growth in one aggregate. The rotary kiln is extremely ineffective as an apparatus for filter cake drying. In order to attain an acceptable drying rate, temperature differences in the range of 300 to 600 K must be maintained between the combustion gas and the moist solid. This causes high heat losses as a result of exhaust heat content and radiation losses. The high gas temperatures required in the drying zone necessitate high gas velocities in the calcining zone of the furnace. Thereby, the dried, dust-like material is partially carried off. Therefore, the capacity of a rotary kiln is limited mainly by dust discharge which is over-proportionally increased with increasing gas velocity. The flow of the dried solids is increasingly influenced with increasing gas velocity and shooting of the product can occur when the velocity is reduced, resulting in insufficient calcined product. - Large specific gas volume per ton of Ti0 2 produced must be treated for removing Ti0 2 dust, sulfuric acid and S0 2 . It is known that the performance of the calcining furnace can be increased and the specific energy consumption can be decreased by pre-drying the charge material before a calcination (DE-A 37 25 513, US-A 4 224 288, Kirk-Othmer, Encyclopaedia of Chem. Techn., 3rd Vol. 14 (1981), pages 360- 364) . However, the described technology cannot be applied to conventional Ti0 2 calcination because the sulfuric acid vapor released in the calcining furnace would be condensed in the pre-drier. An internal sulfuric acid circulation with continually increasing amounts of acid would result thereby. A certain increase in the capacity of the facilities can be obtained according to EP-A 0406 605 in that a portion of the titanium dioxide hydrate filter cake to be calcined is pre- dried with separately produced H 2 S0 4 _ free flue gas and the dried titanium dioxide hydrate is mixed with the remaining filter cake before charging in the calcining rotary kiln. Narrow limits are set thereby for the pre-drying by the required crumbly consistency of the mixture. Therefore, the performance of a conventional rotary kiln for Ti0 2 calcination with a length to diameter ratio of approximately 20:1 can only be increased within certain limits due to the already mentioned strong influence of the flow behavior of the solids and the increase of dust discharge when increasing the gas velocity. An object of the present invention is to provide a process and a plant for the production of titanium dioxide by drying and calcining of titanium dioxide hydrate with a high feed rate and lowered costs by avoiding the disadvantages of the art. This object was solved according to the invention in that the entire sulfate-containing titanium dioxide hydrate filter cake is dried in any suitable directly or indirectly heated drier at product temperatures up to a maximum of 400°C, preferably a maximum of 200°C, most preferably 90 to 150°C and the dried sulfate-containing titanium dioxide hydrate is calcined in a suitable rotary kiln and the exhaust is de- dusted. The subject matter of the invention is a method for the production of titanium dioxide by dissolving titanium raw materials with sulfuric acid, producing a metal sulfate- containing titanyl sulfate solution, hydrolysing the titanyl sulfate, separating the titanium dioxide hydrate from the metal sulfate-containing sulfuric acid by filtration, optionally bleaching the titanium dioxide hydrate by reducing treatment in acidic solution and calcining the titanium dioxide hydrate optionally mixed with doping chemicals and rutile nuclei to titanium dioxide, characterized in that the sulfate-containing titanium dioxide hydrate is dried in a dryer at temperatures from 90 to 400°C, preferably 90 to 200°C, especially 90 to 150°C, and the titanium dioxide hydrate dust from the dryer exhaust is separated by filtration, the dried titanium dioxide hydrate is calcined in a directly heated rotary kiln with counter-current flowing gasses and solids at temperatures in the range from 800 to 1100°C (solids temperature) and the exhaust is de-dusted and de-sul urized. In principal, a conventional Ti0 2 calcining furnace can be used as a rotary kiln especially if the dry material is compacted. However, in this connection, the advantages of the process according to the invention can only be partially used. Therefore, subject matter of the invention is also a calcining furnace for carrying out the process according to the invention. This calcining furnace is a rotary kiln with a length to diameter ratio in the range of 5:1 to 10:1. In addition to the ring shaped weirs at the kiln ends, further weirs are preferably installed over the length of the furnace; 1 to 5 additional weirs are particularly preferred. In order to ensure an optimal adjustment of the operating parameters with respect to the quality of the titanium dioxide and the fuel demand at different production rates, the rotary kiln is preferably equipped according to the invention with a device for changing the speed of rotation which is preferably variable in the range of 0.2 to 1 min "1 . A tighter residence time spectrum of the solids in the rotary kiln results due to the additional weirs. The speed of rotation of the furnace can be relatively high without substantially shortening the product residence time. At the same time, the product surface is more frequently renewed, and as a result, the temperature difference between the combustion gas and the product can be reduced. This also leads to a more uniform product quality. Changes of the gas velocity in the furnace have a considerably smaller influence on the product residence time than is the case with conventional Ti0 2 calcining furnaces. The de-dusting of H 2 S0 4 and S0 2 containing exhaust from the calcining furnace preferably occurs with electrostatic precipitators from which the separated dust is led back into the rotary kiln again either directly or after compacting with the dry material. However, in principle, a de-dusting of the calcining exhausts by filtration or wet washing is also possible before these are treated to remove H 2 S0 4 and S0 2 in a conventional manner. Preferably, the dust separated in the filter, optionally together with coarser dry material taken from the drier, is directly fed into the rotary kiln or advantageously fed into the furnace after compacting. A considerable advantage of the process according to the invention is that the specific volume of the calcining exhausts loaded with H 2 S0 4 and S0 2 is only 30 to 50 % in comparison to the art. As a result, the costs for de- sulphurizing the exhausts are decisively lower. Since filter cake drying and dry material calcination can each be operated under optimized conditions, the process according to the invention leads to an energy saving of 15 to 25 % as compared to titanium dioxide production according to the art. The execution of the process according to the invention in a plant according to the invention is described for a preferred embodiment via example in figure 1. Sulphate- containing titanium dioxide hydrate filter cake (7) , optionally mixed with doping chemicals, is fed into a drier (1) which is directly heated with flue gas (8) . The dust- containing gases (9) which leave the drier (1) with 90 to 400 β C, preferably 90 to 150°C, are de-dusted in a filter (2) and subsequently withdrawn into the atmosphere (10) . The dry material (11) taken out of the drier (1) and dust (12) taken out of the filter (2) are compacted together with dust carried out of the kiln (13) in a compactor (3) , for example a roller compactor. The compacted dry material (14) is fed into the rotary kiln (4) such that it is led in counter-current to the flue gas (16) created in a burner. The titanium dioxide (15) taken from the rotary kiln (4) is cooled in a customary manner, milled and optionally subjected to an additional treatment. The hot exhausts (17) containing dust, H 2 S0 4 and S0 2 which leave the rotary kiln at 250 to 450°C, preferably 300 to 350°C, are preferably de-dusted in an electrostatic precipitator (5) . The de-dusted exhausts (18) whose content on sulphur-containing compounds H 2 S0 4 and S0 2 is two- to threefold higher than conventionally accumulating exhausts are de- sulfurized in waste-gas purification plants (6) and subsequently withdrawn into the atmosphere (19) . A preferred embodiment relating to a rotary kiln according to the invention is represented in figure 2. The rotary kiln (20) is characterized in that its length for a given throughput capacity is less than 50 % of those conventional rotary kilns for Ti0 calcination. In addition to the ring shaped weirs at the upper solid feed end of the kiln (21) and on the lower product discharge end (22), four weirs (23) for example are installed in a distributed manner over the length of the kiln. Heating occurs by means of the burner (25) installed in the combustion chamber (24) . The possible introduction of secondary air into the combustion chamber is not shown. The product discharge(26) occurs at the burner side end. The dried solid containing titanium dioxide hydrate falls into the rotary kiln through the inlet pipe (27) . Alternatively, the feeding of the solid can occur by means of a conveying screw or belt. The dust-containing calcining exhausts are led from the gas outlet housing (28) over the line (29) to the dust precipitator (5) . Optionally, dust removed in the gas outlet housing (28) can be drawn off over the outlet (30) and added to the dried titanium dioxide hydrate (11, 12) . The advantages of the invention are illustrated by the examples without having the scope of the invention limited to them. The portions and percents given in the examples are with respect to weight in as far as nothing else is noted. The volumes given in m 3 correspond to normal conditions. Examples Comparative example: Sulphur-containing titanium dioxide hydrate mixed with doping chemicals and rutile nuclei was calcined in a conventional rotary kiln to titanium dioxide pigment clinker. The kiln had a length of 60 m, an inner diameter of 2.6 m and weirs at the inlet and outlet at a height of 0.3 m. The rotational speed was 0.15 min" 1 . This was charged with 6.8 t/h filter cake which contained 44% as Ti0 2 and 0.24 t/h S0 4 2" . The burner was operated with 925 m 3 /h natural gas. A total of approximately 18,800 m 3 /h air was introduced (primary air to the burner + secondary air + leakage air) . The temperature of the gases behind the solids outlet weir was 1,230°C corresponding to a gas velocity of 5.7 m/s (calculated for an empty kiln) . On the burner side, 3.0 t/h titanium dioxide pigment with a temperature of 980°C was discharged on average. The discharged amount, influenced by changing gas velocity due to the temperature control, varied between 2.85 and 3.1 t/h with acceptable quality fluctuations. Approximately 25,000 m 3 /h moist exhaust with a temperature of 395°C which contained 4.1 g S0 2 /m 3 , 6.7 g I^SO^m 3 and 4.8 g titanium-containing dust/m 3 were emitted from the upper end of the furnace. That corresponds to a gas flow rate of 61,000 m 3 /h under operating conditions. The gas which was cooled and scrubbed dust-free in a scrubber tower and was then freed from H 2 S0 4 mist in an ESP and catalytically freed from S0 2 by means of activated charcoal. 8 Example According to the invention, 17.4 t/h of the filter cake described in the comparative example were dried in a solid drier (type: stirred fluidized bed) at 125°C. The drier was operated with flue gas which was created by burning 890 m 3 /h natural gas with 26,500 m 3 /h air. Approximately 38,000 m 3 /h moist exhaust were emitted from the drier which was de-dusted in a filter. 9.1 t/h dried titanium dioxide hydrate dust accumulated with drying. This was compacted with a roller compactor and then fed into a rotary kiln according to the invention. The rotary kiln had a length of 28 m and an inner diameter of 3.2 m. Aside from the 0.3 m high weirs at the kiln ends, four equally high weirs are installed evenly distributed over the length of the kiln. The rotational speed is 0.5 min "1 . The burner was operated with 960 m 3 /h natural gas. A total of approximately 17,700 m 3 /h of air was introduced. The temperature of the gases behind the discharge weir was 1,320°C; the gas velocity 3.3 m/s. 7.65 t/h titanium dioxide pigment with a temperature of 975°C was discharged. The product discharge and the pigment quality vary within narrower limits as with the comparative example. A moist exhaust current of approximately 21,100 m 3 /h with a temperature of 310°C which contained 13.4 g S0 2 , 14.8 g H 2 S0 4 and 3.6 g dust per m 3 /h as dry gas accumulated from the rotary kiln. It was cleaned as in the comparative example. The substantial advantages of the invention are represented by the comparison with the art: Example Comparative according to the example invention according to the art rotatin kiln: length m 28 60 external surface area m 2 335 600 specif, surface surface m 2 /t Ti0 2 43,8 200 Exhaust of the rotary kiln (S0 2 /H 2 S0 4 -containing) : temperature °C 310 395 spec, volume* m 3 /t Ti0 2 5,880 20,390 dust discharge 0.83 4.18 % of Ti0 2 prod filter cake drying: Exhaust (S-free) : temperature °C 125 spec, volume* m 3 /t Ti0 2 6,590 — Total consumption of 242 308 natural gas m 3 /t Ti0 2 *m 3 here, volume under operating conditions.

Claims: 10 What is claimed is: 1. A process for the production of titanium dioxide by dissolving titanium raw materials with sulfuric acid, producing a metal sulfate-containing titanyl sulfate solution, hydrolyzing the titanyl sulfate, separating the titanium dioxide hydrate from the metal sulfate- containing sulfuric acid by filtration, optionally bleaching the titanium dioxide hydrate by reducing treatment in acidic solution and calcining the titanium dioxide hydrate optionally mixed with doping chemicals and rutile nuclei to titanium dioxide, characterized in that the sulfate-containing titanium dioxide hydrate is dried in a dryer at temperatures from 90 to 400°C, preferably 90 to 200°C, especially 90 to 150°C, and the titanium dioxide hydrate dust from the dryer exhaust is separated by filtration, the dried titanium dioxide hydrate is calcined in a directly heated rotary kiln with counter-current flow of solids and gasses at temperatures in the range from 800 to 1100°C (solids temperature) and the exhaust is de-dusted and de- sulfurized. 2. Process according to claim 1, characterized in that the dried titanium dioxide hydrate is compacted and subsequently calcined. 3. Process according to claim 1, characterized in that the gases containing dust, H 2 S0 4 and S0 2 which are emitted from the rotary kiln with 250 to 450°C, preferably 300 to 350 β C, are de-dusted and de-sulphurized independently from the exhausts of the titanium dioxide hydrate dryer. 4. Process according to claim 1, characterized in that the exhausts from the calcining furnace are de-dusted in an electrostatic precipitator or a dust filter and the separated dust is introduced into the calcining furnace together with the dried titanium dioxide hydrate. 11 optionally after a compacting step. 5. Calcining furnace for the production of titanium dioxide from dried sulphate-containing titanium dioxide hydrate in the form of a rotary kiln with a length-diameter ratio in the range 5 to 10. 6. Calcining furnace according to claim 5, characterized in that aside from the ring-shaped weirs at the kiln ends, additional weirs are installed in the rotary kiln distributed over the length of the furnace. 7. Calcining furnace according to claim 5, characterized in that 1 to 5 additional ring-shaped weirs are installed in the rotary kiln. 8. Calcining furnace according to claim 5, characterized in that the rotary kiln is equipped with a device for altering the speed of rotation.

Inventors: HOLLE BERND, LAILACH GUENTER, HOLLE, BERND, LAILACH, GUENTER
Applicants: HOLLE BERND, KERR MCGEE PIGMENTS GMBH & CO, LAILACH GUENTER, HOLLE, BERND, KERR-MCGEE PIGMENTS GMBH & CO. KG, LAILACH, GUENTER
Application year: 1999
Main classification: C09C1
IPC codes: C01G23, C09C1, C22B34