EP2707330A1

EP2707330A1 - Method and device for separating crystals from a solution

Info

Publication number: EP2707330A1
Application number: EP12720850.2A
Authority: EP
Inventors: Martin Offermann; Sascha Dyschy; Magnus Rastelbauer; Georg VAN LINN; Recep ÖCALAN; Andreas STEINHOFF; Alexander Bekurdts; Reimund SCHULZ
Original assignee: Sachtleben Pigment GmbH
Current assignee: Sachtleben Pigment GmbH
Priority date: 2011-05-11
Filing date: 2012-05-04
Publication date: 2014-03-19
Also published as: DE102011050288A1; WO2012152682A1

Abstract

The invention relates to a method for the solid-liquid separation of crystals from a liquid phase that contains the crystals, said crystals being obtained from a solution by means of crystallization (19), wherein a liquid phase that contains the obtained crystals is fed to a solid-liquid separating device (21) comprising a pusher centrifuge (21c). The aim of the invention is to allow an improved separation of the solution or liquid phase that is still adhered to the obtained crystals after a first crystallization step in a production method that includes a crystallization process, in particular a solids content of crystals can be reliably provided in a liquid phase in a quantity that is sufficient for an economical operation of a pusher centrifuge. This is achieved in that the liquid phase containing crystallized crystals is first fed to a screen worm centrifuge (21b), a decanter centrifuge, or a screen decanter and separated from a portion of the liquid phase in said centrifuge before entering the pusher centrifuge (21c).

Description

Method and apparatus for separating crystals from a solution

The invention is directed to a process for solid-liquid separation of crystals obtained by crystallization from a solution from a liquid phase containing them, wherein a liquid phase containing the resulting crystals is supplied to a solid-liquid separation apparatus comprising a pusher centrifuge.

Furthermore, the invention is directed to a device comprising a crystallization stage, which has at least one first crystallizer with at least one downstream, comprising a pusher centrifuge solid-liquid separation device.

Finally, the invention is also directed to the use of such a device in at least two stages continuously operated crystallization of a titanium-containing Aufschlusslösung or digestion solution mixture.

Due to their crystalline form, crystals which precipitate out of a solution or suspension during crystallization often entail undesirably high amounts of the solution or suspension from which they pass through even after screening or passing through a solid-liquid separation Crystallization were excreted. In particular, in those cases in which the adhering solution or suspension represents the actually desirable product to be produced, it is desirable to liberate the crystals as completely as possible from the adhering liquid. However, this is often difficult and problematic because of the crystal form, since it makes it difficult to sufficiently separate the crystals from the adhering liquid. For example, in crystallization conditions where the crystals grow irregularly, there is a difference between the crystals forming are trapped in the liquid to be removed or separated. Such so-called "nests" or liquid-holding cavities are formed, for example, when crystals form, dissolve and reform again in a rapid and irregular sequence. "A platelet-shaped crystal structure of the crystals also causes the solution or suspension to be drained by capillary forces between the crystals individual crystals is kept and thus a "drainage", for example, by screening or other solid-liquid separation devices only leads to insufficient results.

This problem particularly occurs in the production of titanium dioxide when the production process comprises a crystallization process.

Titanium dioxide is known as a pigment with very good properties which make it suitable for use in e.g. Make paints, coating compounds and plastic materials suitable. Today, titanium dioxide is produced almost exclusively by two processes, the sulphate process and the chloride process, continuously or discontinuously.

In the sulphate process, a titanium-containing raw material, in particular titanium-containing ore such as ilmenite (FeTiO3) or titanium slag or another titanium-containing material, is digested with concentrated sulfuric acid (fresh acid, oleum and / or recycled sulfuric acid). This "digestion" reaction is very vigorous and strongly exothermic The digestion can be carried out both continuously and batchwise The resulting solid digestion cake is then matured for several hours at temperatures between 100 ° C and 200 ° C and then with Water or dilute sulfuric acid. Depending on the raw material used, the solution may contain various salts in dissolved form.

When using ilmenites or other titanium ores, the digestion solution essentially contains titanium oxide sulfate (titanyl sulfate, TiOS0 ₄ ) and iron (III) sulfate (Fe ₂ (SO ₄ ) 3) in dissolved form. When titanium slags are used, for example by reducing ilmenite with coke at 1200 ° C and containing about 80-87% titanium dioxide, the digestion solution contains titanyl sulfate, titanium (III) sulfate (Ti ₂ (S0 ₄ ) ₃ ) and Iron (II) sulfate (FeS0 ₄ ). The titanium slags can be digested with concentrated sulfuric acid at 100-180 ° C, the digestion must be done in acid, otherwise T1O ₂ would precipitate. Furthermore, the digestion solutions of titanium ores and / or titanium slags contain various other metal sulfates from the materials used for the digestion (ore, acid, etc.), such as aluminum sulfate, magnesium sulfate, chromium (III) sulfate, etc. in dissolved form and solid, not open-minded gait.

For the further production process, ie the hydrolysis of the titanyl sulfate, it is essential that the Fe (III) contained in the digestion solution is converted into Fe (II), since Fe (III) would also precipitate as Fe (OH) ₃ in the hydrolysis , which would lead to an undesirable contamination of the titanium dioxide and thus to a deterioration of the optical properties of the titanium dioxide pigment particles. Divalent iron, on the other hand, can be separated by crystallization, for example cooling crystallization, in the form of green salt (FeS0 ₄ -7H ₂ O) from titanium dioxide-containing sulfuric acid solutions.

The green salt can be used for wastewater treatment, ie for the precipitation of the phosphates contained in the wastewater in the form of sparingly soluble iron phosphate, for fertilizer production or for the Production of iron oxide pigments can be used. Alternatively, it may also be dehydrated and thermally decomposed to ferric oxide and sulfur dioxide. If the dissolution solution is obtained from a titanium slag, no further reduction measures are usually required. However, with regard to a solution of titanium ores, eg from ilmenites, it is necessary to reduce the Fe (III) to Fe (II). This reduction is generally carried out by adding metallic iron when dissolving the digestion cake in the digestion vessel or after dissolving the digestion cake by reducing the digestion solution in reduction towers with metallic iron. Alternatively, other reducing agents such as sulfide ores, sulfide iron compounds or Ti (III) -containing solutions may be added to digest the titanium ore.

Following the process steps of digestion and reduction, the resulting iron and titanium sulfate solution is freed by crystallization of iron (II) sulfate heptahydrate. The resulting titanyl sulfate solution is then hydrolyzed at elevated temperature to titania hydrate. This can then be calcined in an oven to separate the water of hydration and to obtain anhydrous titanium dioxide pigment.

Typically, to remove the green salt from titanium-containing digestion solution, titanium chloride tailings solutions containing about 5-6% by weight and etch-nitrate solutions from ilmenite contain about 16-20% by weight of iron (II) sulfate after reduction of Fe ³⁺ . a vacuum crystallization method used. This can be done as a batch or continuous process. Common processes are batch crystallization by evaporation crystallization or surface cooling crystallization in stirred / unstirred Crystallizers with or without a subsequent concentration stage or the continuous crystallization by evaporation or surface cooling crystallization in weighing crystallizers, forced-through horizontal crystallizers or stirred / unstirred stirred reactor cascades.

A two-stage vacuum crystallization process is known from DE 10 2007 032 418 AI. In this known method, titanium dioxide pigments are produced on the basis of the sulfate process, with firstly titania-containing ores (ilmenite) and other prepared ores, so-called titanium slags, separately digested with concentrated sulfuric acid and thus the metal oxides contained in the respective titanium-containing raw material (ore) to a large extent be brought into solution as sulfates. By targeted mixing of Fe (III) -containing ilmenite digestion solution with titanium (III) -containing

A nearly solid-free solution having This is precisely specified levels of T1O2, Fe ^2+, Ti ³⁺ is then generated and slag H2SO4 solution and by sedimentation or filtration methods. The T1O2 present in the solution is subsequently precipitated specifically by hydrolysis with water. This process involves a reduction and crystallization step, which can significantly increase the use of ilmenites for the production of T1O2. On the one hand, in the reduction stage, the Fe (III) present in the ilmenite solution is reduced and, on the other hand, the iron content of the digestion solution or digestion solution mixture is subsequently reduced by crystallization of the green salt (FeS <-V7 H ₂ O; ferrous sulfate (II) heptahydrate).

In such a, preferably at least two-stage crystallization plant, the highest possible production of green salt, ie a high precipitation of crystallized green salt from the original digestion solution or decomposition To achieve solution mixture, it is necessary after the first crystallization stage filtrate, ie the remaining digestion solution or digestion solution mixture, and green salt as far as possible and best possible to separate. This is achieved when, on the one hand, the least possible filter / centrifugate is passed on to the second crystallization stage and, on the other hand, a solution having the highest possible proportion of green salt solids is obtained. However, this can not be achieved to the desired extent by simple and customary green salt removal devices or green salt concentrations, for example lamellar clarifiers. If you want to prepare the enriched with Grünsalzfeststoffanteil part of the digestion solution or digestion solution mixture, for example, after a lamellar clarification using filtration centrifuges, this is causing problems. If, for example, as described in DE 10 2007 032 418 A1, pusher centrifuges are used, their throughput is greatly dependent on the green salt solids content and / or the crystal form (habit and arrangement of the surfaces) of the solution supplied. Furthermore, the use of pusher centrifuges involves the problem that, if crystallization takes place in a (first) crystallization stage, any crystals formed having a platelet-like structure of the green salt crystals can pass through the gaps of the slotted screen of a pusher centrifuge and produce an increased filter breakdown, which then reduces the solid-liquid separation efficiency of the pusher centrifuge and this solids content in the optionally subsequent second crystallization stage can pass, which then has to be removed there by means of the second crystallizer downstream Grünsalzabtrennvorrichtung from the resulting solution prior to further processing. A further problem is that a sufficient "dehydration" of the solution containing green salt is made more difficult by the fact that the usually platelet-shaped crystal structure of the green salt in such cases water solution (black solution) holds by capillary forces between them and thus a "drainage" leads, for example by thickening in clarifiers and / or screening only insufficient results.

The invention is therefore based on the object to provide a solution with which in a manufacturing process comprising a crystallization process, after a first crystallization stage, an improved separation of the solution or liquid phase still adhering to the crystals obtained can be achieved, in particular in a liquid phase Provide crystal solids content in a sufficient amount for an economical operation of a pusher centrifuge.

In a method of the type described in more detail, this object is achieved in that the crystallized crystals containing liquid phase before entering the pusher centrifuge initially a Siebschnecken- centrifuge, a decanter centrifuge or a Siebdekanter fed and separated therein from a part of the liquid phase.

In a device of the type described in more detail, this object is achieved in that between the first crystallizer and a pusher centrifuge at least one of the pusher centrifuge upstream Siebschneckenzentrifuge or at least one of the pusher centrifuge upstream decanter centrifuge or at least one of the pusher centrifuge upstream Siebdekanter is arranged.

Finally, this object is also achieved by the use of a device according to claim 27. By inventively provided combination of a Siebschneckenzentrifuge or a decanter centrifuge or Siebdekanters with a subsequent pusher centrifuge, it is possible that after a first crystallization stage in the resulting solution or in a liquid phase located crystals (in the embodiment, green salt) as far as possible to "dehydrate", ie from the In this case, the Siebschneckenzentrifuge or decanter centrifuge upstream of the pusher centrifuge or the upstream Siebdekanter causes by the built-in centrifuge each screw and the transport of the medium within the Centrifuge are produced many Umbruchkanten in the crystals, so that revolutions and breaks of the crystal or crystallization composite take place, which is the separation and drainage d it causes adherent or entrapped solution or liquid from the crystals. In addition, the process according to the invention makes it possible to operate a crystallization stage with subsequent solid-liquid separation continuously and economically, although one or both types of centrifuge may possibly only be operated suboptimally according to customary ideas, ie the achievable with regard to the achievable degree of concentration or separation Possibilities of the respective devices are not fully exhausted.

In particular, can be achieved by the inventive combination of upstream Siebschneckenzentrifuge, decanter centrifuge or Siebdekanter with a pusher centrifuge appropriate for the economic operation of a pusher centrifuge thickening of the liquid phase to be separated from the crystals to solids contents of more than 30 wt .-%. The invention is therefore further characterized in that the crystals holding liquid phase in the screen screw centrifuge, the decanter centrifuge or the Siebdekanter to a concentration of crystalline solids content of more than 30 wt .-%, preferably more than 45 wt .-%, is thickened.

In order to keep a liquid phase containing the crystals, for example a crystal pulp, sufficiently pumpable, the invention provides in a further development that the liquid phase containing the crystals in the screen screw centrifuge, the decanter centrifuge or the Siebdekanter to a concentration of crystalline solids content of 30-50 wt. % is thickened. 50% by weight solids content in the liquid phase represents approximately the limit above which no more liquid is present but already a solid which would then have to be transported or conveyed as a solid, ie could no longer be pumped.

For the liberation and separation of adhering solution, it is also often useful to wash the crystals obtained with a washing medium. The invention is therefore also distinguished by the fact that the crystals, preferably in already thickened liquid phase, are washed in the pusher centrifuge with a washing medium. It is of particular advantage to use the method according to the invention in the production of titanium dioxide if the production method comprises a crystallization step for the digestion solution or digestion solution mixture (black solution). The invention is therefore further distinguished by the fact that the method is part of a process for the production of titanium dioxide, which comprises the digestion of a titanium-containing raw material with sulfuric acid to a digestion solution (black solution) or digestion solution mixture and at least one downstream treatment stage ne at least two-stage crystallization of green salt (FeS0 * 7 H ₂ O) from the digestion solution or digestion solution mixture, wherein a solution containing the crystallized in a first crystallization step (FeSOfl H ₂ O) containing solution of a pusher centrifuge (first) Grünsalzabtrennvorrichtung is supplied and the crystallized green salt-containing liquid phase before entering the pusher centrifuge first a Siebschneckenzentrifuge or a decanter centrifuge or a Siebdekanter fed and therein by separation of a portion of the liquid phase to a concentration of Grünsalzfeststoffanteil of ≥ 30 wt .-%, preferably ≥ 45 wt.%, Thickened.

It has been found that the residence time of the green salt in a Siebschneckenzentrifuge or a decanter centrifuge or Siebdekanter, for example, may be only about a maximum of five seconds, so that built in these devices usually at most a comparatively thin cake of about 1 cm thickness can be. These conditions prevent adequate washing of the green cake or mash, ie the black liquor / digestion solution / digestion solution mixture concentrated with the increased and sufficiently high green salt solids content. However, a concentration of green salt solids content in or part of the solution obtained in the crystallization of> 30% by weight, preferably ≥45% by weight, can be produced with a screen screw centrifuge or a decanter centrifuge or a screen decanter. With such a proportion of solids, however, a downstream pusher centrifuge can then be operated in an economical manner. If, for example, a pusher centrifuge is characterized in that a residence time of the green salt or green salt cake of approx. 25 seconds is achieved, a solid cake, ie green salt cake, of about 6 cm in thickness can be built up here. As a result of the higher residence time in the pusher centrifuge, time and larger solid cake starch can also be done the necessary washing of the green salt. The combination of a pusher centrifuge with upstream Siebschneckenzentrifuge or upstream decanter centrifuge or upstream Siebdekanter thus a two-stage crystallization plant for crystallization of green salt in the titanium dioxide production by the sulfate process can be operated continuously and economically, although one or both types of centrifuge, if necessary, after conventional technical and economic Notions and possibilities are operated only suboptimally. By this combination of centrifuges, it is possible to free the resulting in crystallization in a crystallizer in the first crystallization stage solution in increased flow rates of green salt to a large extent, so then in the second crystallization stage, the further crystallization or recrystallization preferably among others Crystallization is no longer influenced in an undesirably high degree crystallized in the first crystallization step green salt.

It is also advantageous here if the solution containing a crystallizer of the first crystallization stage and containing the crystallized green salt is first fed to an upstream lamellar clarifier before it is fed and fed into a Siebschneckenzentrifuge or a decanter centrifuge or Siebdekanter. In this case, it is then possible, by means of a more or less pure sedimentation process, to achieve already a concentration, ie thickening, of a part of the solution and a green salt solids content of about 26% by weight and to supply a clarified filtrate fraction for further processing or processing. The centrifuges downstream of the lamellar clarifier no longer have to be designed for the complete liquid solution fraction. The method according to the invention is therefore distinguished Furthermore, characterized in that the solution or liquid phase containing the crystals or the crystallized green salt is first supplied to one of the screen screw centrifuge or the decanter centrifuge or the plate decanter upstream lamella clarifier. The device according to the invention is further distinguished by the fact that at least one screen screw centrifuge or decanter centrifuge or Siebdekanter a lamellar clarifier is connected upstream. Since it is possible with a Siebschneckenzentrifuge or a decanter centrifuge or a Siebdekanter to produce a liquid phase or a Grünsalzbrei that is variable in its solids content between 30 wt .-% and almost 100 wt .-%, there is a further embodiment of the invention in that between an auger centrifuge or a decanter centrifuge or a Siebdekanter and the downstream associated pusher centrifuge an intermediate or collecting container, in particular a funnel container, is arranged, in which the solids-containing outflow of the associated Siebschneckenzentrifuge or decanter centrifuge or the associated Siebdekanter is introduced. This intermediate or collecting tank or hopper parallel untreated solution from the first crystallization stage or outlet from the lamellar clarifier or thickener, which may also be prepared before entering the intermediate or collecting container with admixed untreated solution from the first crystallization stage, fed and with the Green salt pulp containing green salt solids content, from the Siebschneckenzentrifuge or the decanter centrifuge or the associated Siebdekanter leaking to be mixed to a desired Grünsalzfeststoffanteil having solution. This mixed solution is then fed to the downstream pusher centrifuge. The process according to the invention is therefore further distinguished by the fact that the crystals or the crystallized out Green salt containing and preferably thickened solution or liquid phase is fed to an intermediate or collecting container before feeding to the pusher centrifuge and there with a partial stream of crystals containing the crystallized in the first crystallization stage green salt (FeS0 ₄ -7 H ₂ 0) containing solution or liquid phase on the desired concentration of solids or Grünsalzfeststoffanteil is mixed. In addition, in an embodiment of the method, the invention provides that the thickened liquid phase flowing out of the lamellar clarifier, preferably before it enters the screen screw centrifuge, is a partial stream of a green salt (FeSO ₄ .7H ₂ O ₎ crystallized out in the first crystallization stage ) is mixed. In order to be able to carry out in particular the above-mentioned mixing processes, the device according to the invention is characterized in an embodiment in that an intermediate or collecting container is connected between the screw conveyor centrifuge and the pusher centrifuge. The thickening or concentration of the liquid phase or solution containing the crystals or the crystallized green salt can be carried out continuously or discontinuously, which the invention also provides. It is advantageous according to the invention further, if in the downstream treatment stage, the crystallization of green salt (FeS0 ₄ -7 H ₂ 0) from the digestion solution or digestion solution mixture is carried out continuously in at least two successive crystallization stages, wherein the temperature in the first crystallization stage is adjusted so that there preferably more than 40% of the total crystallization occurring in the green salt (FeS0 ₄ -7 H ₂ 0) crystallized out as a coarse crystalline salt and the temperature in a subsequent second crystallization stage is adjusted so that in this crystallization step over the green salt obtained in the first crystallization step finely crystalline green salt (FeS0 ₄ -7 H ₂ 0) is obtained.

In such a crystallization process, it is furthermore expedient if the temperature of the first crystallization stage is adjusted to a value between 25 ° C and 35 ° C and the temperature of the second crystallization stage is set to a value between 10 ° C and 20 ° C. ,

Of very particular advantage is the inventive method in that it allows to use different titanium-containing raw materials as starting materials and to mix their digestion solutions. The invention is therefore further characterized in that the downstream treatment stage, a digestion solution (black solution) or digestion solution mixture is supplied, which is obtained by selective mixing of Fe (III) -containing ilmenite digestion solution with titanium (III) - containing slag digestion solution.

In an embodiment of the device according to the invention, it is provided that the first crystal separation device comprises one or more pusher centrifuges.

In this case, furthermore, the crystallization stage can be formed as a continuously operable, at least two-stage crystallization stage and the first crystallizer and at least one second crystallizer serially connected in series with at least one downstream second solid-liquid separation device, in particular a downstream second crystal (green salt) separation device - Provide, what the invention also provides. Finally, in an expedient development, the device is characterized by the fact that the second solid-liquid separation device or the second crystal separation device comprises one or more peeler centrifuges.

Incidentally, the advantages and refinements associated with the use of the at least two-stage crystallization process are otherwise the same as those described and disclosed in DE 10 2007 032 418 A1, so that in this regard reference is expressly made to the disclosure content of this prior-published patent application.

The invention is explained below by way of example with reference to a drawing. This shows in

Fig. 1 in a schematic representation of the arrangement of

Facilities of a two-stage crystallization or one of the digestion stage for the digestion of a titanium-containing raw material with

Sulfuric acid to a digestion solution (black liquor) or digestion solution mixture downstream treatment stage with a two-stage crystallization of green salt from the digestion solution or digestion solution mixture and in

Fig. 2 performed in a plant according to FIG. 1

Method as a block diagram.

The illustrated in Figures 1 and 2 as an exemplary embodiment two-stage crystallization stage is part of a production plant for the production of titanium dioxide, in particular titanium dioxide pigment by the sulfate process. Via a transport line 16, digestion solution or digestion solution mixture of the downstream treatment stage or process stage shown in FIGS. 1 and 2 is fed from an upstream process stage for carrying out a crystallization. This treatment stage or stage comprises a feed tank 17 designed as a stirred tank, which is connected via a line 18 to a first crystallizer 19. This first crystallizer 19 forms the first crystallization stage of the overall two-stage crystallization process. On the discharge side, the first crystallizer 19 is connected via a line 20 to a first solid-liquid separation device or first crystal separation device, here a first green salt separation device, designated as 21 first green salt separation device, the crystals obtained in the crystallization, here Green salt, are supplied with a solution containing these crystals. This first crystal (green salt) separation device 21 comprises a lamellar keller 21a, a screw conveyor centrifuge 21b, and a reciprocating centrifuge 21c, and an intermediate or collection container 36 connected in conduit communication between the screw conveyor centrifuge 21b and the pusher centrifuge 21c.

The first crystallizer 19 is operated at a temperature between 25 ° C. and 35 ° C. as a vacuum crystallizer, so that from the digestion solution (mixture) the green salt crystallizes essentially and in the predominant amount as a coarsely crystalline salt. This crystallized green salt is then separated from the solution containing it in the first solid-liquid separation device or first crystal separation device 21. This first crystal separation device comprises a plurality of centrifuges 21b, 21c connected in series with an upstream static thickener 21a in the form of a lamella clarifier. On the output side, the first crystal separation device 21 via lines 22a, 22b, 22c with a further (second) storage tank 23, the filtrate or Zentrifugatbehälter, formed as a stirred tank, to which through the lines 22a, 22b, 22c respectively the clear flow or the centrifugate of the respectively associated solid-liquid separator 21a, 21b or 21c is supplied , The second storage tank 23 is connected via a further line 24 to the input side of a second crystallizer 25 in line connection, so that clarified by the line 24, "dehydrated" and cleared of green salt (digestion) solution or (digestion) solution mixture from the second storage tank 23 the On the output side, the second crystallizer 25 is connected via a further line 26 to a second solid-liquid separator or second crystal separator, here a second Grünsalzabtrennvorrichtung 27 in line connection, which in the Crystallization in the second crystallizer 25 obtained as a green salt crystals are supplied with a solution containing them.In the second crystallizer 25, a vacuum crystallization in the temperature range between 10 ° C and 20 ° C is carried out, so that the second crystallizer 5 on the outlet side via the further line 26 leaving (digestion) solution or (digestion) solution mixture usually only a small proportion of finely crystalline green salt, which is then separated in the second crystal (green salt) separation device 27 from the solution containing this. This second green salt removal device 27 comprises a thickener in the form of a lamella clarifier 27a and two peeler centrifuges 27b, in which a suspension reservoir tank 27c is connected between the lamellar clarifier 27a and the peeler centrifuges 27b. From the second crystal separation device 27, further lines 28a, 28b leading to the respective clear flow or the respective centrifugation lead to a transfer tank 29 designed as a stirred vessel, from which then a discharge line 30 leads, which connects the crystallization stage for transferring of the crystals, here the green salt, liberated (digestion) solution or (digestion) solution mixture with a hydrolysis step.

In carrying out the process, digestion solution or digestion solution mixture is introduced into the stirred feed tank 17 after a sedimentation and filtration step, from which it is pumped into the first crystallizer 19. After the first crystallization stage, the crystallized green salt in the first crystal separation device 21 is separated from the solution containing the obtained crystals (arrow 31), and the (digestion) solution is passed as filtrate or clear flow via line 22a into the second storage tank 23. From this, it is then pumped into the second crystallizer 25, in which further green salt is crystallized out. The crystals which form in the second crystallizer 25 in the form of finely crystalline green salt are separated from the solution containing them (arrows 32a / 32b) in the second crystal separation device 27 and the (digestion) solution or (digestion) solution mixture which only has a low level of finely crystalline green salt, is pumped through line 28a into transfer tank 29. From this tank 29, the titanium-containing (digestion) solution then passes into a downstream hydrolysis stage, advantageously recycling the cold, effluent (digestion) solution or (digestion) solution mixture by heat exchange with the warm digestion solution or digestion solution mixture flowing into the crystallization process he follows.

For crystallization, about 60 ° C warm, reduced Aufschlusslösung or digestion solution mixture via the storage tank or storage tank 17 via the line 18 in the first crystallization stage or the first crystallizer 19th directed. Here, the solution is cooled to about 30 ° C. The solution which proceeds in the form of a salt slurry from the first crystallizer 19 through the line 20 is separated in the first crystal (green salt) separation device 21 by means of the thickener 21a into a solution or clearing 22a and into a crystallized crystals in the form of a thickened salt slurry (33 ) containing liquid phase separated. By centrifuging the thickened salt slurry (33) first in the screw conveyor centrifuge 21b and then in the downstream pusher centrifuge 21c, green salt is separated from the respective liquid phase, which is discharged via a line or conveyor 31 and fed to a further use. The part of the solution and the liquid phase which has been freed from the crystals in the first solid-liquid separation device 21, is led into the storage tank 23 via lines 22a and as a respective liquid phase centrifugate via lines 22b and 22c. From there, this solution then passes through the conduit 24 into the second crystallization stage, ie, the second crystallizer 25. The temperature of the solution is lowered in the second crystallizer 25 to about 15 ° C and again green salt is crystallized out. The solution, which is in the form of a salt slurry from the second crystallization stage and contains the crystallized (green salt) crystals, is fed through the conduit 26 to a second thickener 27a of the second crystal (green salt) separation apparatus 27. The resulting from liquid phase, thickened and the crystallized crystals containing crystal slurry (34) is separated from the liquid phase via the peeler centrifuges 27b (32a / 32b) and fed to a further use. The clear water of the sludge clarifier / thickener 27a and the liquid phase centrifugate are supplied to the transfer tank 29 via the conduits 28a and 28b, and then the solution is passed through the discharge line 30 for subsequent hydrolysis. In the static thickener or lamellar clarifier 21a of the first crystal (green salt) separation device 21 on the one hand a filtrate or clear water and on the other hand a thickened portion of the crystals, a Grünsalzbrei, produced as a liquid phase, which a Kristallfeststoffanteiel (here: Grünsalzfeststoffanteil) of about 26 wt. -% and which is derived via a line 33 from the static thickener or lamella clarifier 21a and the Siebschneckenzentrifuge 21b is fed. The Siebschneckenzentrifuge rotates at about 1000 revolutions / min and thereby separates from the liquid phase in the crystals enclosed or adhering to (digestion) solution or (digestion) solution mixture, so-called black solution, as a centrifugate from the salt slurry. This centrifugate is supplied to the storage tank 23 via the line 22b. The discharge screw of the Siebschneckenzentrifuge 21b rotates at about 15 revolutions / min faster than the drum rotating at 1000 revolutions / min and carries the thickened salt slurry as a liquid phase via a line 35. The salt slurry or liquid phase then has a green salt solids content (generally a crystal solids content) of about 45% by weight and is then fed to the pusher centrifuge 21c. In this case, an intermediate or collecting container 36 or hopper container is interposed in the embodiment. Since it is possible with the aid of the Siebschneckenzentrifuge 21b, the crystal solids content, here the green salt solids content of the Siebschneckenzentrifuge 21b leaving liquid phase (here salt slurry) in a range of about 30 wt .-% to approximately 100 wt .-%, it is within In the embodiment it is possible to pump a partial flow of the underflow leaving the lamellar clarifier or thickener 21a via the line 33 as a liquid phase via a branch line or parallel line 37 parallel to the screen worm centrifuge 21b into the intermediate or collecting container 36 or funnel container and here with the much higher concentrated to mix the Siebschneckenzentrifuge 21b on the line 35 leaving salt slurry and thus in the intermediate or collecting container 36 a salt slurry with the desired total concentration of Grünsalzfeststoffanteil (generally crystalline solids content) of for example about 45 wt .-% set. In addition, the possibility is provided to supply the line 33 and thus the partial stream of the liquid phase leaving the lamellar clarifier 21a via a first branch line 38 to a solution containing saline salt, as it exits from the first crystallizer 15, and to mix it. Alternatively or additionally, via a second branch line 38a solution containing saline, as it exits from the first crystallizer 15, the parallel line 37 are supplied and / or can via a third branch line 38b saline solution, as it exits the first crystallizer 15, in the intermediate or collecting container 36 or funnel container are introduced. From the intermediate or collecting container 36, the salt slurry produced there is then supplied by means of a line 39 as liquid phase of the scoop centrifuge 21c.

The screen screw centrifuge 21a is one which consists of a drive part arranged in a bearing housing, a screen holding drum, a sieve insert, a transport screw and a product housing enclosing the rotating parts. Usually, the solid / liquid mixture (here salt slurry as a liquid phase) is fed into a trained within the screw body feed space via a central inlet pipe and distributed from there through passages on the inserted into the Siebhaltetrommel screen member. The transport screw turns in the same direction of rotation as the screen holding drum, but at a slightly different speed and ensures even product acceleration and product distribution on the screen section. In a non-illustrated embodiment may be used instead of a Siebschneckenzentrifuge 21b but also a decanter centrifuge or a Siebdekanter use. Depending on the size of the plant and production plant, several of these filtration centrifuges can be connected in parallel and / or in series. Part of a first solid-liquid separator or first crystal separator or first green salt separator 21 and / or a second solid-liquid separator or second crystal separator or second green salt separator 27 be.

In decanter centrifuges, the liquid or suspension to be separated is applied approximately in the middle of a drum of the decanter centrifuge, so that the solid is conveyed through the differential speed to the drum rotating screw towards the smaller diameter of the drum, while the clarified liquid at the opposite end the drum overflows.

A Siebdekanter is a combination of Vollmantel- decanter centrifuge and Siebschneckenzentrifuge. In the decanter part, the feed suspension or solution is initially pre-thickened, that is to say largely dehydrated on the conical part of the full-width drum. The liquid is clarified in the cylindrical part and thrown off at the end. The pre-thickened solid is transported by the transport screw into the cylindrical sieve part where it is freed from the adhering residual liquid.

Even if, in the exemplary embodiment, the first solid-liquid separation device 21 has only one screen screw centrifuge 21b and a pusher centrifuge 21c connected in series therewith in fluid line fashion and a plate clarifier 21a connected in series with both centrifuges in fluid-line connection. It is, of course, also possible to form any combination of one or more screen screw centrifuges 21b with one or more pusher centrifuges 21c. This with and without upstream thickener or lamellar clarifier 21a. These devices or devices, in particular the Siebschneckenzentrifugen and / or the pusher centrifuges, depending on the application in each case (centrifuges of different types or different types) or with each other (centrifuges of the same type or the same type) fluid line parallel and / or in series. The minimum requirement is that a pusher centrifuge is always preceded by at least one screen screw centrifuge, a decanter centrifuge or a screen decanter.

Claims

patent claims

A process for solid-liquid separation of crystals obtained by crystallization (19) from a solution from a liquid phase containing them, wherein a liquid phase containing the obtained crystals is supplied to a solid-liquid separation device (21) comprising a pusher centrifuge (21c) .

characterized,

the liquid phase containing crystallized crystals, before entering the pusher centrifuge (21c), is first fed to and separated from a part of the liquid phase in a screen screw centrifuge (21b), a decanter centrifuge or a screen decanter.

2. The method according to claim 1, characterized in that the liquid phase containing the crystals in the Siebschneckenzentrifuge (21b), the decanter centrifuge or the Siebdekanter to a concentration of crystalline solids content of more than 30 wt .-% is thickened.

3. The method according to claim 1 or 2, characterized in that the crystals containing liquid phase in the Siebschneckenzentrifuge (21b), the decanter centrifuge or the Siebdekanter to a concentration of crystalline solids content of more than 45 wt .-% is thickened.

4. The method according to claim 1 or 2, characterized in that the crystals containing liquid phase in the Siebschneckenzentrifuge (21b), the decanter centrifuge or the Siebdekanter to a concentration of crystalline solids content of 30-50 wt .-% is thickened.

5. The method according to any one of the preceding claims, characterized in that the crystals are washed in the pusher centrifuge (21c) with a washing medium.

6. The method according to any one of the preceding claims, characterized in that the crystals are washed in already thickened liquid phase in the pusher centrifuge (21c) with a washing medium.

7. The method according to any one of the preceding claims, characterized in that it is part of a process for the production of titanium dioxide, which comprises the digestion of a titanium-containing raw material with sulfuric acid to a digestion solution (black solution) or digestion solution mixture and in at least one downstream treatment stage at least two-stage (19 includes, 25) crystallization of green salt (FeS04-7 H20) from the digestion solution or decomposition solution mixture, wherein a crystallized the (in a first crystallization step 19) green salt (FeS04 ^'7 H20) solution containing a thrust centrifuge (21c) comprising Grünsalzabtrennvorrichtung (21) is fed to the crystallized green salt containing liquid phase before entering the pusher centrifuge (21c) first of a Siebschneckenzentrifuge (21b) or a decanter centrifuge or a Siebdekanter and therein by separation of a part of the liquid gphase to a concentration of green salt solids content of ≥ 30 wt .-% is thickened.

8. The method according to any one of the preceding claims, characterized in that the crystallized green salt containing liquid phase before entering the Schub- Centrifuge (21c) is first fed to a Siebschneckenzentrifuge (21b) or a decanter centrifuge or a Siebdekanter and thickened therein by separation of a portion of the liquid phase to a concentration of green salt solids content of ≥ 45 wt .-%.

9. The method according to any one of the preceding claims, characterized in that the solution or liquid phase containing the crystals or the crystallized green salt is first supplied to one of the Siebschneckenzentrifuge (21b) or the decanter centrifuge or the Siebdekanter upstream lamellar clarifier (21a).

10. The method according to claim 9, characterized in that from the lamella clarifier (21a) effluent, thickened liquid phase, a partial stream (38) of a crystals or crystallized in the first crystallization step green salt (FeS0 ₄ ^> 7 H ₂ 0) containing solution containing becomes.

11. The method according to claim 9 or 10, characterized in that the effluent from the lamellar clarifier (21a), thickened liquid phase before entering the Siebschneckenzentrifuge (21b) of the partial stream (38) of a crystals or crystallized in the first crystallization step green salt (FeSO ₄ '7 H ₂ O) containing solution is mixed.

12. The method according to any one of the preceding claims, characterized in that the crystals or the crystallized green salt containing and preferably thickened solution or liquid phase before being fed to the pusher centrifuge (21c) to an intermediate or collecting container (36) and fed there with a partial flow (37 , 38a; 38b) of the crystals and / or crystallized in the first crystallization stage green salt (FeS0 ₄ -7 H ₂ 0) containing solution (38b) or liquid phase (37, 38a) is mixed to the desired concentration of solid content or green salt solids content.

13. The method according to any one of the preceding claims, characterized in that the thickening of the crystals or the crystallized green salt-containing liquid phase or solution is carried out continuously or discontinuously.

14. The method according to any one of claims 5 to 13, characterized in that in the downstream treatment stage, the crystallization of green salt (FeS0 ₄ -7 H ₂ 0) from the digestion solution or digestion mixture continuously in at least two successive crystallization stages (19, 25) is set, wherein the temperature in the first crystallization stage (19) is set so that there crystallized green salt (FeS0 ₄ * 7 H ₂ 0) as a coarse crystalline salt and the temperature in a subsequent second crystallization stage (25) is adjusted so that in this Crystallization step (25) is obtained compared to the green salt obtained in the first crystallization step (19) finely crystalline green salt (FeS0 ₄ * 7 H ₂ 0).

15. The method according to claim 14, characterized in that in the first crystallization stage (19) more than 40% of the total crystallization occurring in the green salt (FeS0 ₄ * 7 H ₂ 0) is crystallized out as a coarse crystalline salt.

16. The method according to claim 14 or 15, characterized in that the temperature of the first crystallization Fe (19) is set to a value between 25 ° C and 35 ° C and the temperature of the second crystallization stage (25) is set to a value between 10 ° C and 20 ° C.

17. The method according to any one of claims 5 to 16, characterized in that the downstream treatment stage, a digestion solution (black solution) or digestion solution mixture is fed, obtained by targeted mixing of Fe (III) -containing ilmenite digestion solution with titanium (III) -containing slag digestion solution becomes.

18. An apparatus comprising a crystallization stage comprising at least one first crystallizer (19) with at least one downstream, a pusher centrifuge (21c) comprising solid-liquid separation device (21), characterized

in that at least one screen centrifuge (21b) upstream of the first crystallizer (19) and the pusher centrifuge (21c) or at least one decanter centrifuge upstream of the pusher centrifuge (21c) or at least one screen decanter upstream of the pusher centrifuge (21c) is arranged.

19. The apparatus according to claim 18, characterized in that it is part of a plant for the production of titanium dioxide by the sulfate process.

20. The device according to claim 18 or 19, characterized in that the solid-liquid separation device (21) forms a Kristallabtrennvorrichtung.

21. Device according to one of claims 18 to 20, characterized in that the at least one screen screw centrifuge (21c) or the at least one decanter centrifuge or the at least one Siebdekanter a lamellar clarifier (21a) is connected upstream.

22. Device according to one of claims 18 to 21, characterized in that between the Siebschneckenzentrifuge (21 b) and the pusher centrifuge (21 c) an intermediate or collecting container (36) is connected.

23. Device according to one of claims 18 to 22, characterized in that the crystal separation device (21) comprises one or more pusher centrifuges (21c).

24. The device according to one of claims 18 to 23, characterized in that the crystallization stage is designed as a continuously operable, at least two-stage crystallization stage and the first crystallizer (19) and at least one line connected thereto in series second crystallizer (25) with at least one downstream second solid-liquid separation device (27).

25. The device according to claim 24, characterized in that the second solid-liquid separation device forms a second crystal separation device (27).

26. Device according to claim 24 or 25, characterized in that the second solid-liquid separation device or second crystal separation device (27) comprises one or more peeler centrifuges (27b).

27. Use of a device according to any one of claims 18 to 26 in carrying out at least two stages continuously operated crystallization of a titanium-containing Aufschlusslösung or digestion solution mixture.

28. Use according to claim 27 for carrying out a method according to one of claims 7 to 17.