AU2016295915A1

AU2016295915A1 - Method for reducing trivalent iron in the production of titanium dioxide according to the sulfate method

Info

Publication number: AU2016295915A1
Application number: AU2016295915A
Authority: AU
Inventors: Asborn DAHL; Jan Klauset; Mitja Medved; Thomas Pierau; Oystein RUUD; Per Thoen
Original assignee: Kronos International Inc
Current assignee: Kronos International Inc
Priority date: 2015-07-23
Filing date: 2016-07-18
Publication date: 2017-12-14
Also published as: JP2018523014A; EP3121295A1; BR112017025794A2; WO2017012710A8; KR20180031758A; RU2692544C1; EP3325679A1; MX2017015435A; WO2017012710A1; CN107849635A

Abstract

The invention relates to the method step of reducing trivalent iron in the production of titanium dioxide according to the sulfate method. According to the invention, after the removal of the poorly soluble decomposition residue, the reduction occurs by means of metallic iron in such a way that the trivalent iron is completely reduced to divalent iron and tetravalent titanium is partially reduced to trivalent titanium in a deliberate manner. A Ti(III) content of 1 wt% to 5 wt% with respect to the total titanium is preferably sought. The method step according to the invention preferably occurs in continuous operation at a temperature of > 50°C to < 85°C in a reduction reactor, wherein the progress of the reduction is monitored by means of the redox potential or the iron(III) content or titanium(III) content of the solution. The method according to the invention is characterized by the following advantages: said method can be largely automated, the reduction reaction proceeds steadily, different scrap qualities can be used, and improved TiO

Description

Process for the Reduction of Ferric Iron in the Production of Titanium Dioxide in a Sulfate

Process

Field of the invention

The invention relates to the reduction of ferric iron in the production of titanium dioxide in a sulfate process.

Technological background of the invention

The so-called sulfate process for the production of titanium dioxide is based on the digestion of iron-titanium raw materials in sulfuric acid, in which a digestion suspension containing dissolved ferrous sulfate and titanyl sulfate and a hardly soluble digestion residue is formed after the addition of diluted sulfuric acid to the solid digestion cake. After the digestion residue is separated from the digestion suspension, a digestion solution is formed, from which the dissolved ferrous sulfate is subsequently crystallized and also separated. Then, the dissolved titanyl sulfate is hydrolyzed, and the titanium oxide hydrate formed is calcined to the final product titanium dioxide.

The usually employed raw materials, such as ilmenite ore or iron-titanium slags, often contain major amounts of ferric iron, mostly in the form of the minerals hematite and magnetite. In contrast to ferrous iron in the form of FeS04, the dissolved ferric iron in the form of Fe2(S04)3 in the digestion solution has a negative impact on the intermediate products and the titanium dioxide end product. In addition, in contrast to ferrous sulfate, ferric sulfate has a very high solubility in the digestion suspension and in the digestion solution, so that separation is not possible, which raises problems in the subsequent process steps. For example, ferric iron is adsorbed on the high specific surface of the precipitated titanium oxide hydrate in the subsequent hydrolysis stage, leading to undesirable discoloration in the end product. In the known sulfate process according to the prior art, which is described in some detail in Ullmann's Encyclopedia of Industrial Chemistry, Chapter "Pigments, Inorganic, 2. White Pigments" (Wiley-VCH Verlag GmbH & Co. KGaA, 2011), a reduction of ferric to ferrous iron is therefore performed by adding metallic iron during and after the formation of the digestion suspension. The metallic iron is usually added to the digestion suspension as scrap iron in a loose form or in the form of pressed pellets with a defined density. The rate of the proceeding exothermic surface reaction depends on the surface quality, the thickness of the iron material employed, and the degree of mixing, and the temperature, which results from the reaction heat released and cooling effects in the reaction system. The mixing of the suspension is meanwhile effected by air blown into the digestion vessel from below. However, the air blown in causes the suspension to cool down, which leads to lower reaction rates, and in addition, it acts as an oxidant for ferrous iron and for hydrogen in statu nascendi, whereby ferric iron and water are produced. This reduces the recovery of the metallic iron employed, which has an effect on the economic efficiency of the process. The re-oxidation of the ferrous iron to ferric iron by air throughout the subsequent processing of the digestion solution is usually counteracted by a sufficient amount of trivalent titanium in the digestion suspension and in the digestion solution. The trivalent titanium is produced from the tetravalent titanium also by the addition of the metallic iron.

The "external" mixing caused by the blown-in air has only a little influence on the "internal" mixing of the digestion suspension within the scrap bulk or the pressed scrap pellets. Therefore, local temperature peaks of above 85 °C may occur within the scrap bulk or the pressed scrap pellets, which trigger a premature and uncontrolled hydrolysis of the tetravalent titanium and thus result in an inferior Ti02 end product. The low internal mixing may further favor side reactions, such as the undesirable evolution of hydrogen. In addition, the long duration of the reduction reaction causes a continuous dissolution of further constituents, such as ferric iron, from the still undigested raw material particles from the digestion residue, which is also the reason why further metallic iron must be added for reduction.

In the sulfate process for the production of titanium dioxide according to EP 2 064 355 B1, the reduction step takes place, not in the digestion suspension, but after the separation of the insoluble digestion residue in the digestion solution, preferably before the crystallization of iron sulfate. The reduction is effected by introducing S02 gas, which reacts with Fe(lll) sulfate and water to form Fe(ll) sulfate and sulfuric acid. If required, residual iron(lll) may then be reduced by means of metallic iron (scrap). According to EP 2 064 355 B1, an iron(lll) content of from 0 to 1 g/l is stated as the target figure, avoiding the formation of trivalent titanium.

The reduction by means of scrap iron is effected by stirring the digestion solution batchwise and pumping it around a scrap pellet. The degree of reduction of the solution is determined by means of a redox potential measurement on the solution after cooling.

Thus, the mentioned methods for the reduction of ferric iron within the scope of the sulfate process for the production of titanium dioxide have various drawbacks, such as poor internal mixing, which leads to temperature peaks and concentration gradients, and too low a recovery (efficiency) of the amount of scrap employed, and undesirable re-oxidation reactions, which lead to increased scrap consumption and increased formation of iron sulfate.

In the sulfate process for the production of titanium dioxide according to DE 10 2007 032 417 A1, the reduction of ferric iron takes place in two steps. At first, the digestion solution is overreduced, for example, by adding metallic iron, and only in a second step, the desired degree of reduction is adjusted by mixing the overreduced digestion solution with unreduced digestion solution at a corresponding ratio. Thus, the process has the disadvantage of an additional process step.

Object and summary of the invention

It is the object of the invention to provide a process for the reduction of ferric iron within the scope of a sulfate process for the production of titanium dioxide that overcomes the drawbacks of the prior art.

The object is achieved by a sulfate process for the production of titanium dioxide from an iron-titanium raw material, comprising the process steps: a) digesting the iron-titanium raw material in sulfuric acid to form a digestion suspension; b) separating the hardly soluble digestion residue to form a digestion solution containing tetravalent titanium and ferric iron; c) reducing the digestion solution by using metallic iron in a reduction vessel while a desired degree of reduction is adjusted; d) crystallizing and separating iron(ll) sulfate from the reduced digestion solution; e) hydrolyzing and calcining the hydrolysis product to produce titanium dioxide; characterized in that the desired degree of reduction of the digestion solution in process step c) is adjusted in the reduction vessel, wherein the ferric iron is completely reduced to ferrous iron, and tetravalent titanium is selectively and partially reduced to trivalent titanium.

Further advantageous embodiments of the invention are stated in the depending claims.

Figures

Figure 1: Schematic process scheme of the process according to the invention for the reduction of ferric iron.

Description of the invention

All values disclosed in the following relating to the size in pm etc., the concentration in % by weight or % by volume, the temperature, volume or mass fluxes etc., are to be understood as including all the values within the range of measuring errors as known to the skilled person.

The invention proceeds from the prior art as described in Ullmann's Encyclopedia of Industrial Chemistry. However, according to the invention, the reduction step is performed in the digestion solution only after the separation of the digestion residue, the so-called clarification. Figure 1 schematically represents the process according to the invention.

In the digestion solution, after the clarification and before the reduction, the dissolved iron is usually present at from 3% by weight to 9% by weight as ferric iron in the form of Fe2(S04)3, and at from 5% by weight to 25% by weight as ferrous iron in the form of FeS04. The dissolved titanium is present as tetravalent titanium in the form of titanyl sulfate.

The process according to the invention enables a desired degree of reduction of the digestion solution to be adjusted, namely the complete reduction of ferric iron to ferrous iron, and the selected reduction of part of the tetravalent titanium to trivalent titanium.

According to the invention, after clarification, the digestion solution is passed, preferably through a supply tank, into a reduction vessel filled with metallic iron, preferably in the form of scrap iron. The scrap iron is present, for example, as a bulk material or in the form of pressed scrap pellets, hereinafter commonly referred to as "scrap". The introduction of the digestion solution is preferably effected from below, the bottom of the reduction vessel preferably including a suitable liquid distribution system, such as sieve bottoms with a suitable pressure loss, or pipe manifold systems, which are known from extraction and distillation technology and commercially available, through which the digestion solution is uniformly passed through the scrap.

At the other end of the reduction vessel, in this case at the upper end of the vessel, the digestion solution is withdrawn, for example, as overflow. In the course of the reduction process, the scrap iron dissolves in the digestion solution, and new scrap is supplied as needed.

In a particular embodiment of the invention, it is ensured that the amount of metallic iron available for reduction that is permanently in contact with the digestion solution to be reduced remains approximately constant during the proceeding scrap consumption. Preferably, this can be achieved by ensuring, for example, that the surface of the digestion solution in the vessel is always below that of the scrap packing by correspondingly supplying the scrap in such a way that it protrudes from the digestion solution. Alternatively, a defined level of the scrap packing may also be maintained below the surface of the digestion solution by supplying the scrap in accordance with a suitable level measurement.

The digestion solution is preferably transported through the reduction vessel at empty-tube velocity, which corresponds to a Reynolds number of at least 200, preferably at least 800, and more preferably at least 1600.

As shown below, the process according to the invention can be substantially automated and run in both "batch operation" and "continuous operation".

In one embodiment of the process according to the invention as a "batch operation", the digestion solution overflowing from the reduction vessel is passed through line A (see Figure 1) directly back into the reduction vessel and partially or completely through line B (see Figure 1) through the supply tank back into the reduction vessel. On its way, the digestion solution passes a heat exchanger (Figure 1), by means of which the temperature of the solution in the whole process step of reduction can be adjusted to below 85 °C. Preferably, the temperature of the solution in the whole process step of reduction, but especially in the reduction vessel, is adjusted to a value below 85 °C and above 50 °C, so that no premature hydrolysis occurs on the one hand and a sufficiently high reaction rate can be maintained on the other, in order to be able to achieve a desired plant capacity. For this purpose, an automated measurement of the temperature of the solution takes place both at the entry into the reduction vessel (T1) and at the exit from the reduction vessel (T2). More preferably, the temperature in the reduction reactor is to be from 60 °C to 70 °C.

Alternatively, the heat exchanger may be installed in line A and/or in line B. In addition, the temperature of the solution in the reduction vessel can be controlled through the amount supplied into the reduction reactor per unit time, and through the amount of circulation flow per unit time.

Further, the progress of the reduction reaction in the digestion solution overflowing from the reduction reactor can be determined continuously by means of a redox potential electrode. Alternatively, other methods, which may be suitable for automation, from the field of instrumental analytics may be employed for determining the concentration of ferric iron and/or trivalent titanium compounds, which are known to the skilled person, in order to monitor the progress of the reduction. Suitable methods include, for example, optical, spectroscopic and electro-analytical determination methods that are employed continuously in-line or discontinuously off-line.

In the following, reference to a redox potential measurement shall include any other measuring method by which the progress of the reduction or the concentration of ferric iron and trivalent titanium in the digestion solution can be determined.

In a preferred embodiment of the invention, the complete reduction of the ferric iron and the partial Ti(IV) reduction is performed in such a way that a content of trivalent titanium of from 1 to 5% by weight, more preferably from 1.2 to 2.3% by weight, based on the total titanium concentration, is achieved in the digestion solution.

In the "batch operation", a batch of the unreduced digestion solution is circulated through the reduction vessel (line A, see Figure 1) or passed through the supply tank and the reduction vessel (line B, see Figure 1) until the desired redox potential value of the digestion solution that corresponds to a complete reduction of the ferric iron and a partial reduction of the tetravalent titanium is reached.

In one embodiment of the process according to the invention as a "continuous operation", the supplied amount of unreduced digestion solution is controlled by the redox potential measurement. For this purpose, the amount of unreduced digestion solution supplied to the reduction vessel is regulated through the measurement of the redox potential and/or the temperature at the exit of the reduction vessel (T2) in such a way that the target value for the concentration of trivalent titanium in the solution and/or the sought temperature are reached at the exit of the reduction vessel, without a circulation of the digestion solution through line A and/or line B being required. After the target value for the concentration of trivalent titanium is reached at the exit of the reduction vessel, the thus reduced digestion solution can be passed on to the subsequent process steps.

In an alternative embodiment of the "continuous operation", the digestion solution is circulated from the supply tank at a selected constant supply through the reduction vessel and through line A (see Figure 1). The volume flow of the solution circulated through line A is regulated in such a way that the desired redox potential value that corresponds to a complete reduction of the ferric iron and a partial reduction of the tetravalent titanium is reached. For this purpose, after a start-up phase, which is usually performed in a batch operation, the operation is gradually shifted to continuous mode. This means that the supplied amount of unreduced digestion solution is slowly adjusted from zero supplied amount to the desired supplied amount. Meanwhile, the volume flow of the solution circulated through line A is controlled through the redox potential measurement in such a way that reduced digestion solution in the overflow of the reduction vessel, which exhibits complete reduction of the ferric iron and a partial reduction of the tetravalent titanium, is continuously withdrawn and can be passed on to the subsequent process steps.

According to the invention, the quality of the overflow can be controlled selectively through regulation of the amount circulated through line A, even with varying concentrations of the fractions to be reduced in the clarified digestion solution. This control is exerted through the measurement of the redox potential at the exit of the reduction vessel, like in the embodiment variants described above.

In contrast to DE 10 2007 032 417 A1, the selected reduction of the digestion solution, i.e., the adjustment of the desired degree of reduction, takes place in the reduction vessel according to the invention, while DE 10 2007 032 417 A1 teaches that the digestion solution is at first overreduced in the reduction vessel, and only in a subsequent step, the desired degree of reduction is adjusted in a mixing vessel by admixing unreduced digestion solution. This saves a process step according to the invention.

In addition, the process according to the invention offers further advantages over the processes from the prior art: - The process according to the invention can be substantially automated and can be run in batch operation and, in particular, in continuous operation. - Manual supervision of the scrap addition or scrap afterdosing is not required because an about constant amount of metallic iron is always available for the reduction process. - The reduction reaction can proceed uniformly and in a controlled way, so that a defined degree of reduction can be reached in the digestion solution in terms of a redox potential measured against a standardized reference potential. - Problems due to air stirring or internal mixing as performed according to the prior art, such as re-oxidation, premature hydrolysis, hydrogen formation from side reactions and uncontrolled cooling of the solution, can be avoided by the process. An improved titanium dioxide product quality is achieved thereby. - The temperature control allows for the use of different scrap qualities, for example, very reactive thin can body stock, which cannot be employed in the process according to the prior art because of a high reaction rate and a high accompanying heat development. - All in all, the need for scrap is lower, which thus results in a higher efficiency of the process. In addition, a smaller amount of crystallized iron sulfate is obtained in the subsequent process step of iron vitriol crystallization. - Other minor components contained in the digestion residue are not leached out during reduction in this process, but remain in the residue because of a previous separation. This has the effect that the digestion solution contains less accompanying elements, which must be separated off, for example, in the subsequent crystallization, and adversely affect the quality of the titanium dioxide end product.

Examples

The invention will be described in more detail by means of the following Examples without intending to limit the scope of the invention thereby.

Example 1: Continuous operation without circulation A 20 m3 reduction reactor (reduction vessel) with a height of 5 m was filled with about 9 tons of metallic iron in the form of a mixture of pressed scrap pellets and scrap bulk in such a way that a packing exceeding the overflow level by 0.5 to 1 m was formed above the overflow at a height of 4 m. In a start-up procedure, the reduction reactor was first filled completely with the clarified and about 55 °C warm unreduced digestion solution from a 200 m3 large supply tank. Subsequently, the digestion solution present in the reduction reactor was circulated through the redox potential measuring site and the heat exchanger until an exit temperature of 60 °C was reached at the overflow, and a concentration of 1.2 g/l trivalent titanium (corresponding to about 1.5% by weight trivalent titanium, based on the overall titanium content) in the solution was reached at the redox potential electrode. The volume flow circulated through a heat exchanger and through line A during this start-up procedure was 25 m3/h, a temperature difference between the reactor bottom and the reactor exit of about 3 °C occurring. The power of the heat exchanger that could be given off to the circulated solution was up to 500 kW and was automatically retrieved for both cooling and heating depending on the desired temperature at the reactor exit.

Subsequently, the operation mode was changed to continuous by regulating and adapting the amount supplied from the supply tank stepwise through measurement of the redox potential in the overflow at the reactor exit in such a way that circulation of the volume flow was no longer required for achieving the sought temperature range and the sought titanium(lll) content. In a continuous operation mode, a stable operation point was obtained at a supply volume flow of 35 m3/h. The dissolving rate of metallic iron was at 700 kg/h under the stated conditions. This consumption resulted in a complete reduction of ferric iron and in the sought concentration of trivalent titanium of 1.2 g/l in the digestion solution at the reactor exit. The reduced digestion solution was then passed on to the subsequent process steps.

Example 2: Continuous operation with circulation

The reduction reactor from Example 1 was filled with a clarified digestion solution according to the same start-up procedure as described in Example 1 and subsequently fed continuously with a constant volume flow of 50 m3/h from the supply tank. In order to reach a sought concentration of trivalent titanium of 1.2 g/l in the digestion solution at the reactor exit, the volume flow circulated through a heat exchanger and through line A was adjusted to 20 m3/h. Under these conditions, an exit temperature of 67 °C occurred at the overflow, the temperature difference between the reactor bottom and the reactor exit being about 6 °C. The reduced digestion solution was passed on to the subsequent process steps.

Example 3: Continuous operation with circulation

The reduction reactor from Example 1 was filled with a clarified digestion solution according to the same start-up procedure as described in Example 1 and subsequently fed continuously with a volume flow of 70 m3/h from the supply tank. The volume flow circulated through a heat exchanger and through line A was adjusted to 45 m3/h for an exit temperature of 63 °C to occur at the overflow. Under these conditions, the temperature difference between the reactor bottom and the reactor exit was about 5 °C. The dissolving rate of metallic iron was about 1400 kg/h, which resulted in a complete reduction of ferric iron and in the sought concentration of trivalent titanium of 1.2 g/l in the digestion solution at the reactor exit. The reduced digestion solution was passed on to the subsequent process steps.

In Examples 1 to 3, the measurement of the hydrogen concentration in the air extracted above the reactor was performed as an important safety measure in order to monitor the lower explosion limit. The extraction was effected through a line arranged centrally at the reactor lid with a volume flow of 6000 m3/h of air, which was sucked from the environment through lateral openings at the reactor lid. In all Examples, no hydrogen evolution could be detected. As compared to conventional processes, where concentrations of up to 3% by volume hydrogen are observed, this suggests a very good yield of the reduction reaction without substantial losses through a side reaction of hydrogen evolution.

Claims

CLAIMS:

1. A process for the production of titanium dioxide from an iron-titanium raw material, comprising the process steps: a) digesting the iron-titanium raw material in sulfuric acid to form a digestion suspension; b) separating the hardly soluble digestion residue to form a digestion solution containing tetravalent titanium and ferric iron; c) reducing the digestion solution by using metallic iron in a reduction vessel while a desired degree of reduction is adjusted; d) crystallizing and separating iron(ll) sulfate from the reduced digestion solution; e) hydrolyzing and calcining the hydrolysis product to produce titanium dioxide; characterized in that the desired degree of reduction of the digestion solution in process step c) is adjusted in the reduction vessel, wherein the ferric iron is completely reduced to ferrous iron, and tetravalent titanium is selectively and partially reduced to trivalent titanium.
2. The process according to claim 1, characterized in that the reduction in process step c) takes place in a reduction vessel filled with metallic iron, wherein the digestion solution is transported through the reduction vessel.
3. The process according to claim 1 or 2, characterized in that the temperature of the digestion solution in process step c) is within a range of from > 50 °C to < 85 °C, preferably within a range of from 60 °C to 70 °C.
4. The process according to one or more of claims 1 to 3, characterized in that the digestion solution is transported through the reduction vessel at empty-tube velocity, which corresponds to a Reynolds number of at least 200, preferably at least 800, and more preferably at least 1600.
5. The process according to one or more of claims 1 to 4, characterized in that the digestion solution is introduced at the bottom of the reduction vessel and withdrawn from the top of the reduction vessel.
6. The process according to one or more of claims 2 to 5, characterized in that the amount of metallic iron available for reduction that is permanently in contact with the digestion solution to be reduced remains approximately constant during the proceeding iron consumption.
7. The process according to one or more of claims 2 to 6, characterized in that the iron packing in the reduction vessel protrudes from the digestion solution.
8. The process according to one or more of claims 1 to 7, characterized in that process step c) can be run both in batch operation and in continuous operation.
9. The process according to claim 8, characterized in that the digestion solution is transported through the reduction vessel in one pass in continuous operation.
10. The process according to claim 8, characterized in that the digestion solution is circulated through the reduction vessel in continuous operation.
11. The process according to one or more of claims 1 to 10, characterized in that the progress of reduction in the digestion solution is determined through the redox potential of the digestion solution, or through the concentration of ferric iron and/or of trivalent titanium in the digestion solution.
12. The process according to one or more of claims 1 to 11, characterized in that the reduction is performed up to a content of trivalent titanium, based on the total titanium, of from 1 to 5% by weight, preferably from 1.2 to 2.3% by weight, in the digestion solution.