WO2005068358A1

WO2005068358A1 - Production of 'useful material(s)' from waste acid issued from the production of titanium dioxyde

Info

Publication number: WO2005068358A1
Application number: PCT/IB2003/006065
Authority: WO
Inventors: Michael Robinson
Original assignee: Millennium Chemicals
Priority date: 2003-12-18
Filing date: 2003-12-18
Publication date: 2005-07-28

Abstract

A continuous multi-step precipitation process for working the 'strong filtrate' stream from the production of titanium dioxide by the sulphate method, the said process comprising of at least the following steps; Step 1: Partial neutralisation of the said stream to pH 1.5 to 2.5 with alkaline, basic, or metal values of at least one 'useful material'; Step 2: Further partial neutralisation to pH 3.5 to 4.5 with at least a fourth part of the required neutralant being calcium containing compound(s) and the remainder alkaline, basic or metal values of at least one 'useful material'; Step 3: Separation of the so-formed precipitate as precipitate 'A'; Step 4: Neutralisation of the remaining filtrate to an alkaline pH and the separation of the so-formed precipitate as precipitate 'C' and; Step 5: Acidification with sulphuric acid of the resulting liquor to a pH lower than 7 and the recovery of sulphate salts.

Description

Production of "useful materials)" from waste acid issued from the production of titanium dioxide The invention relates to a process for the production of a 'useful material', which has bulk commercial use, from waste acid issued from the production of titanium dioxide by the sulphate method. Such materials are the sulphates of ammonia, sodium, potassium, magnesium, ferrous iron, or any other metal the sulphate of which has bulk commercial use and optionally iron-oxide particulates In the manufacture of titanium dioxide from titaniferous ores and/or slags by the sulphuric acid attack process, known as the sulphate process, an acidic solution of titanyl sulphate is produced. This is then hydrolysed to form hydrous titania which is normally separated from the acidic liquor by filtration. The so-obtained filtrates, termed 'strong filtrates', contain sulphuric acid and metal sulphates, chiefly those of iron, together with aluminium, chromium, vanadium, manganese and magnesium. Filtrates also contain unrecovered titanium and traces of other materials from the raw materials, additives and materials of construction of the plant. An amount of these 'strong filtrates' can be recycled to earlier stages of the process to effect economies of acid usage but, by the nature of the process, disposal means are required for the remainder. Regarding the recovered hydrous titania, it is subjected to washing stages on further filtration equipment and this gives rise to a need to dispose of a larger quantity of filtrates containing low concentrations of the above materials, termed 'weak filtrates'. Both waste streams require treatment. Various means have already been tried to minimise the waste treatment requirements of the process, particularly with respect to the 'strong filtrates'. One such means, described in EP 133 505 involves concentration by evaporation along with partial purification of this stream so as to be able to recycle a greater amount of the acid to the earlier stages of the process. However, apart from being very costly this still leaves the impurities to be disposed of. Another approach, more commonly adopted is to render the waste stream(s) neutral or slightly alkaline by the addition of calcium containing neutralisation agents such as calcium carbonate and/or calcium oxide or hydroxide. This produces a precipitate of calcium sulphate, known as gypsum, accompanied by the various impurity metals as hydroxides, and also a clarified liquor for discharge. However, this method requires landfill of the large volumes of solid precipitate produced. Furthermore, if this process is carried out with a single stage of precipitate separation, the gypsum is coloured with the impurity hydroxides, chiefly iron, and is termed pink gypsum. Improvements have been made whereby this neutralisation and separation process is carried out sequentially in stages producing firstly a white gypsum from the bulk of the acid values and possibly some white-coloured impurity hydroxides, followed by secondly a red gypsum containing the bulk of the impurities. The red gypsum material has found some use as an agricultural soil conditioning agent. In locations where the white gypsum has some resale value this can be disposed of, lessening the costs of treatment and reducing somewhat the amount of landfill needed, but, because gypsum is a common material both from natural sources and from by-product sources such as flue gas desulphurisation processes, this is becoming more difficult. Another scheme which has been adopted for example by Ishihara in

US 3 016286, is to carry out the neutralisation of the strong filtrates with an ammonia- containing stream so as to produce ammonium sulphate which can be recovered to be used in agriculture for example. After an initial seeded precipitation of the hydroxides of the impurity metals aluminium, titanium, vanadium, and chromium, an iron oxide precipitate was produced from the filtered solution by the addition of a seed material and ammonia, whilst aerating. It was also found possible to crystallize alums incorporating various impurity metals such as manganese and magnesium. However, implementation of this approach suffered due to the subsequent growth in the production of by-product ammonium sulphate from other chemical processes, for example caprolactam manufacture. This is itself now declining due to the introduction of improved processes. A further scheme is represented in DE 1 467 292 by Yawata. In this a white gypsum recovery step was initially carried out and then the filtered solution was seeded, neutralised, and aerated to produce iron oxide particles. In the above two processes cleaning of the initial solution of materials such as titanium is achieved by precipitation after the initial partial neutralisation and filtration step. Yet another scheme is shown in GB 1 530 740 by Montedison where additional iron values were added to the solution either as iron sulphate crystals or as scrap iron. The resulting iron sulphate was purified by crystallisation before being re- dissolved, seeded, neutralised and aerated to recover iron oxide particles. EP 638 515 uses adjustment of pH with ammonia to precipitate impurities such as titanium, followed by air/ammonia treatment to recover iron oxides, followed by the removal of manganese and magnesium using an organic solvent based reactive extraction, finally followed by ammonium sulphate recovery. Related patents EP 1 064 223 and EP 1 064 227 use addition of iron values, recovery of white gypsum using a calcium-containing material, followed by pH adjustment with ammonia to pH 3 to 5 to remove some impurities, followed by air/ammonia treatment to recover iron oxides. In an additional step the resulting ammonium sulphate solution was treated with lime to recover the ammonia for recycle and also white gypsum. In this last case the white gypsum produced from ammonium sulphate using lime is more expensive than that produced from calcium carbonate, and in any case white gypsum is a less desirable product under today's circumstances than ammonium sulphate. Furthermore, in the last two cases the final neutralisation using ammonia to precipitate the minor impurities as hydroxides leads to a very fine precipitate that is difficult to separate from the solution by filtration or other means. From the standpoint of long term economic stability, a purified stream of the sulphate of a 'useful material', and optionally iron oxide particulates, represent the most desirable products which can be potentially produced in large quantities from the 'strong filtrates' stream, whereas white gypsum is less desirable. When one considers the treatment of the 'weak filtrates' stream, a red gypsum material for agricultural use represents the least undesirable option due to the dilute nature of this stream. The problem addressed by the present invention was precisely to develop a process that makes it possible to recover from the 'strong filtrate' stream high-quality products, as extensively as possible for meaningful utilisation. More precisely, the instant invention relates to a continuous multi-step precipitation process for working the 'strong filtrate' stream from the production of titanium dioxide by the sulphate method, the said process comprising of at least the following steps: - Step 1: Partial neutralisation of the said stream to pH 1.5 to 2.5 with alkaline, basic, or metal values of at least one 'useful material', - Step 2: Partial neutralisation without aeration of the resulting liquor to pH 3.5 to 4.5 with at least a fourth of the required neutralant being a calcium-containing compound and the remainder an alkaline, basic or metal value of at least one 'useful material', - Step 3: Separation of the so-formed precipitate containing the sulphate or hydroxide or oxide of the added calcium and the impurity metal content as precipitate 'A' - Step 4: Neutralisation of the remaining filtrate to an alkaline pH, in particular to a pH of 7 to 9, and the separation of the so-formed precipitate as precipitate 'C' and - Step 5: Acidification with sulphuric acid of the resulting liquor to a pH lower than 7 and the recovery of sulphate salts. In particular, the present process may comprise of the following steps some of which are optional: - 'Step A' : Optional addition of a useful sulphate-containing material, which may be in an impure form, to the 'strong filtrate' stream, - Step 1: Partial neutralisation of the stream to pH 1.5 to 2.5 with alkaline, basic, or metal values of at least one 'useful material', followed by - Step 2: Partial neutralisation without aeration of the resulting liquor to pH 3.5 to 4.5 with at least a fourth of the required neutralant being a calcium-containing compound and the remainder being alkaline, basic or metal values of at least one 'useful material', - Step 3: Separation of the so-formed precipitate containing the sulphate or hydroxide or oxide of the added calcium and the impurity metal content as precipitate 'A', - 'Step B': Optionally, aerating and maintaining at least a portion of the liquor at pH 4 to 6 with a soluble alkali or basic neutralant in the presence of seed material, so as to precipitate iron oxide particles in a hydrated state under hot conditions, and their separation as precipitate 'B', - Step 4: Neutralisation of the remaining filtrate to an alkaline pH, in particular to a pH of 4 to 9, and separation of the so-formed precipitate as precipitate 'C, and - Step 5: Acidification with sulphuric acid of the resulting liquor to a pH lower than 7 and the recovery of sulphate salts. According to the instant invention, 'useful material' means ammonia, sodium, potassium, magnesium, ferrous iron sulphates, any other metal sulphate which has bulk commercial use and mixtures thereof. In accordance with a preferred embodiment of the instant process, sulphate of calcium is not considered as a 'useful material'. According to the instant invention, alkaline, basic or metal values covers alkaline materials such as ammonia, ammonium hydroxide, sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium hydroxide, iron scraps and mixtures thereof.

With respect to ammonia, ammonia gas and aqueous ammonia are preferably used. The best suited is ammonia gas. According to a preferred embodiment, the alkaline, basic or metal values of 'useful material(s)' used in Steps 1 and 2 are the same. According to a first embodiment, it will be ammonia or ammonium hydroxide according to a second embodiment, sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium hydroxide or mixtures thereof and according to a third embodiment, iron scrap. According to a preferred embodiment, the Step 4 directed to the working up of the filtrate from Step 3 to a 'useful material' as precipitate 'C, may consist of a further neutralisation of the liquor to a pH of 7 to 9. This neutralisation is carried out with at least a fourth of the required neutralant for the impurities present being a calcium-containing material and the remainder being alkaline, basic or metal values of at least a 'useful material'. In a particularly preferred manner, partial neutralization of the free sulphuric acid in Step 1 is first effected until a pH value from 2 to 2.5 is attained. Optionally, at least a part of the so-generated heat is recovered for heating the subsequent partial neutralizations. The subsequent partial neutralization with at least a fourth of the required neutralant being calcium-containing compound(s), so as to form a separable precipitate containing the hydroxides and/or oxides of impurity metals and sulphate of the added calcium, is preferably carried out without aeration. The remaining free sulphuric acid is partly neutralized at a temperature between 50°C and the boiling point of solution. At the end of this partial neutralization, the precipitate 'A' is separated from the liquid phase. The neutralization from the pH value 1.5 to 2.5 until the pH value 4 to 4.5 could be achieved with a calcium containing compound alone. However, according to a specific embodiment, this neutralization is also effected with at least an alkaline, basic or metal value of at least a 'useful material'. According to a specific embodiment of the claimed process, the amount of calcium containing material may be adjusted to be at least equal to half of the amount required to achieve a pH value of 3.5 to 4.5 in step 2 and a pH value of 7 to 9 in step 4. Unexpectedly, the inventors have noticed that by carrying out the neutralisations to pH 3.5 to 4.5 in step 2 and to pH 7 to 9 in step 4 using a calcium containing material to replace a portion of the desired neutralent allows some gypsum to precipitate. This gypsum acts partly as a filter aid and partly as a solid with a large surface area for minority impurity compounds to deposit on or be absorbed on, so as to aid their removal. The instant process utilises this precipitation to efficiently achieve the removal of minority compounds such as Ti, N, Cr, Al, Fe, Νi, Pb, Cu, As, Hg, and also radioactive metals such as Radium. Notably, this is clearly achieved for Radium isotopes, the sulphate of which, while being very insoluble, is notably very slow to precipitate. Unexpectedly, the co-precipitation of calcium species assists this precipitation. According to one embodiment, the claimed process comprises of a preliminary step, termed 'Step A' involving the addition of at least one sulphate containing material to the 'strong filtrate' stream. This optional step allows for the introduction of an impure form of the 'useful material' to purify it of materials which precipitate as at least precipitate 'A'. According to another embodiment, the claimed process comprises of an additional step, termed 'Step B', this involves the addition of a seed material to at least a portion of the liquor from Step 3 followed by aeration whilst maintaining at pH 4 to 6 with a soluble alkali or basic neutralant. The iron oxide particles precipitated in a hydrated state under hot conditions are separated as precipitate B. Regarding the neutralization step leading to the precipitate B, it is realized under oxidising conditions with at least an alkaline, basic or metal values or mixture thereof, and preferably with at least ammonia. The oxidising agent used is preferably made of oxygen or a gas mixture containing oxygen, in particular air. It is preferably introduced into the reaction mixture. Compounds like hydrogen peroxide and sodium hypochlorite chlorate can also be used. According to a preferred embodiment, this partial neutralization is realized by the addition of a seed material, under hot conditions, to promote the precipitate of iron oxide in a hydrated state. This seed material could be made by any conventional way.

However, it is preferably a sample of sulphate filtrate to which caustic soda has been added to obtain a pH value around 6. The seed used influences the particle size of the so-produced FeOOH. Reprocessing of the iron pigment suspensions is effected by means of the known steps of filtration, drying and grinding. The FeOOH particles so obtained can be used directly, for example as a pigment, or it can also be converted to haematite (Fe₂O₃). Such a conversion could be achieved by calcination or hydrothermal conversion at pressure, both are known processes. The FeOOH can also be used directly in the ferrite forming process since it calcines to haematite at a lower temperature than that required for ferrite formation. Furthermore, the so-obtained precipitate 'B' may be worked up in an optional step, termed 'Step C, so as to produce saleable iron oxide product(s). In the case where the optional precipitate 'B' is worked up to a saleable state, less red gypsum is produced than would otherwise be the case therefore reducing the need for landfill. According to another embodiment, the precipitate 'A' from Step 3 and the precipitate 'C from Step 4 may be taken either separately or combined and supplemented with lime in an optional step, termed 'Step D', so as to form strongly alkaline red gypsum and to liberate ammonia. Treating precipitates 'A' and 'C with lime advantageously prevents escape of nitrogen values to the environment. Ammonia may be recovered by stripping with air and advantageously recycled for the heating of the first partial neutralization step. Accordingly to this specific embodiment, the so-obtained alkaline red gypsum may be mixed with the 'weak filtrate' stream from the washing steps of hydrous titania produced by the sulphate method, during the treatment of that stream with calcium containing material to form a red gypsum material and a clarified liquid effluent. In the case where precipitates 'A' and 'C are kept separately and only precipitate 'C is added to the 'weak filtrates' stream the impurity content of the final red gypsum stream produced is significantly lowered with benefit to its final use. As specified here above, the alkaline, basic or metal values of the 'useful material' used for partial neutralisation, is generally chosen with respect to the 'useful material'. In the embodiments of the invention where the 'useful material' is ammonium sulphate, in Steps 1, 2 and in optional 'Step C the alkaline or basic substance would be ammonia gas, ammonium hydroxide, alkaline carbonate or mixtures thereof. In the embodiments of the invention where the 'useful material' is sodium, potassium, or magnesium sulphate, the alkaline, basic or metal values in steps 1, 2 and optional 'Step C is sodium, potassium or magnesium hydroxides, carbonates, or the corresponding metal values for example metal scrap or mixtures thereof. If scrap metal is used provision must be made to deal with the hydrogen evolved. Also, in 'Step C, one can use scrap iron. In the embodiments of the invention where the 'useful material' is ferrous sulphate the most probable form of the added neutralant in Steps 1, 2 would be iron scrap with provision being made to deal with the hydrogen evolved. In optional 'Step C, the neutralant could be any alkaline, basic or metallic material not precipitating under the conditions of use and hence contaminating the FeOOH produced. Thus this could for example be ammonia, sodium carbonate, or iron scrap. In Step 4 for the embodiments of the invention where the 'useful material' is ammonium sulphate, sodium sulphate, potassium sulphate, or magnesium sulphate, or other like material the process for working up the remaining filtrate to a commercial 'useful material' comprises of at least a further neutralisation of the liquor to a pH of 7 to 9. The neutralisation, optionally carried out with aeration, is achieved with at least a fourth part of the required neutralant for the impurities present being calcium containing and the remainder being alkaline materials such as ammonia, ammonium hydroxide, sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium hydroxide, and so on. The separation of the precipitate 'C produced is followed by trimming the filtrate stream pH back to neutral or near neutral, and if desired evaporating the stream to produce the commercially 'useful material'. In Step 4 for the embodiment of the invention where the 'useful material' is ferrous sulphate, the process of working up the remaining filtrate comprises at least of the crystallisation of ferrous sulphate crystals and their recovery followed by further neutralisation of the liquor to pH 7 to 9 with at least a fourth part of the required neutralant for the impurities present being calcium containing and the remainder being an alkaline material. The alkaline material may be a scrap material the metal values of which are tolerable as effluent after the recovery of the precipitated materials in precipitate 'C. The main non-iron impurities considered in the instant step are manganese and magnesium. Since the pH target is particularly basic, calcium oxide or calcium hydroxide is preferably used. However, if so desired manganese can first be precipitated as the carbonate by the use of ammonia carbonate, which can be optionally made from the CO₂ evolved in the previous neutralisation step. Advantageously, the precipitation of the magnesium as the hydroxide co- precipitated with gypsum allows its removal more efficiently than as an 'alum' as it does not involve the loss of ammonium sulphate with the magnesium. In 'Step D' for the embodiment of the invention where the 'useful material' is ammonium sulphate, the ammonia which is produced may be recycled back to Steps 4 and 'Step B'. This invention is discussed below on the basis of examples without any limitation being constituted thereby. The parts and percentages quoted in the examples, relate to weight, unless otherwise stated.

Example 1 A large sample of acidic 'strong filtrates' from a TiO₂ sulphate plant was taken. It was found to contain 273 g/1 SO₄ ^~ and 19.6 g/1 Fe⁺⁺ the specific gravity being 1.21. 1. Steps 1. 2 and 3 To this was added sufficient ammonia, which in this instance was in aqueous solution, to adjust the pH to 1.5. A further small amount of dilution water was found to be needed to prevent crystals forming on standing. The solution analysed as follows. % SO₄ ^{" :} 17.1 % by weight

And therefore; ppm Fe/ % SO₄ ^~ 850 ppm Mn % SO₄ ^~ 24 ppm Mg/ % SO₄ ^~ 170 ppm Ca/ % SO₄ ^~ 9 ppm Al/ % SO₄ ^~ 62 ppm Ti/ % SO₄ ^~ 6 ppm Cr/ % SO₄ ^~ 8 ppm V/ % SO₄ ^" 19 Also ppmΝb/ % SO₄ ^" 0.25 ppm Zn/ % SO₄ ^~ 0.2 a) Reference embodiment A 500 ml sample of the pH 1.5 stock solution was treated with ammonia, which in this instance was in aqueous solution, to adjust the slurry pH to 4.5. The amount used was recorded. The precipitate which formed was filtered off with difficulty as it was extremely fine and slow to filter even when an organic flocculent was added. The precipitate was washed with 100 ml of water and the washings added to the filtrate, which was then analysed. b) Embodiments according to the instant invention Test 1 : A 500 ml sample of the pH 1.5 stock solution was treated with finely ground calcium carbonate which in this instance was in aqueous suspension to adjust the slurry pH to 4.5. The amount used was recorded. The precipitate which formed was filtered off with ease as it was granular and filtered freely. The precipitate was washed on the filter with 100 ml of water and the washings added to the filtrate, which was then analysed. Tests 2. 3. 4 : Further 500 ml samples of the pH 1.5 stock solution were taken and treated initially with the calcium carbonate suspension and then with the ammonia solution in proportions calculated from the above to give 25 % (test 2), 50 % (test 3), and 75 % (test 4) of the amount of total neutralant required as calcium carbonate, the slurry pH was then adjusted to 4.5 with the ammonia solution. In each case the precipitate which formed was filtered off with ease because it was granular and filtered freely. The precipitates were washed on the filter with 100 ml of water and the washings added to the filtrates, which were then analysed.

The data collected in the preceding experiments were:

The Fe, Mn, Mg, and Ca were in the same ratio to SO as in the pH 1.5 stock. These results show that the Step 2 conducted according to the instant invention leads to filtrates having a lower amount of SO₄ ^" and having only residual amount of the Chromium and Vanadium impurities.

2. Steps 'B'. 4 and 5 A goethite seed material was prepared following a standard method. A batch of filtrates generated as in test 3 above was used to produce goethite using a seed material, aeration, heating, and control of pH with ammonia to pH 4.5 by a known method. X ray diffraction identified the material precipitated as goethite. The Mn, Mg, and Ca were in the same ratio to SO₄ ^~ as in the pH 1.5 stock, but of course the Fe had been reduced. A further batch of filtrates was generated using 50/50 calcium carbonate and ammonia to produce a slurry with a pH of 4.5 and then filtering. The filtrates were treated with calcium carbonate suspension followed by ammonia, again using a 50/50 split, to produce a slurry with a pH of at least 9. The precipitate was again filtered off with ease and washed as before. The filtrates were analysed:

% SO₄ ^" 11.97 ppm Fe/ % SO₄ ^" <0.1 ppm Mn/ % SO₄ ^" 13 ppm Mg/ % SO₄ ^" 200 ppm Ca/ % SO₄ ^~ - ppm Al/ % SO₄ ^~ <0.1 ppm Ti/ % SO₄ ^~ <0.1 ppm Cr/ % SO₄ ^~ <0.1 ppm V/ % SO₄ ^" <0.1 ppmNb/ % SO₄ ^" <0.1 ppm Zn/ % SO₄ ^~ 0.1 All other metal impurity metals were below the limits of detection As shown by these results, the instant process efficiently achieves the removal of minority compounds like Ti, V, Zn and Nb.

Claims

1. A continuous multi-step precipitation process for working the 'strong filtrate' stream from the production of titanium dioxide by the sulphate method, the said process comprising of at least the following steps: Step 1: Partial neutralisation of the said stream to pH 1.5 to 2.5 with alkaline, basic, or metal values of at least one 'useful material', Step 2: Partial neutralisation without aeration of the resulting liquor to pH 3.5 to 4.5 with at least a fourth part of the required neutralant being calcium containing compound(s) and the remainder alkaline, basic or metal values of at least one 'useful material', Step 3: Separation of the so-formed precipitate, containing the sulphate or hydroxide or oxide of the added calcium and impurity metal content, as precipitate 'A' - Step 4: Neutralisation of the remaining filtrate to an alkaline pH and the separation of the so-formed precipitate as precipitate 'C and Step 5: Acidification with sulphuric acid of the resulting liquor to a pH lower than 7 and the recovery of sulphate salts.

2. The continuous process according to claim 1, the said process comprising of at least the following steps: 'Step A': Optional addition of useful sulphate-containing material to said 'strong filtrate' stream, Step 1: Partial neutralisation of the stream to pH 1.5 to 2.5 with the alkaline, basic, or metal values of at least one 'useful material', - Step 2: Partial neutralisation, without aeration, of the resulting liquor to pH 3.5 to 4.5 with at least a fourth part of the required neutralant being calcium- containing compound(s) and the remainder alkaline, basic or metal values of the 'useful material', Step 3: Separation of the so-formed precipitate contaimng the sulphate or hydroxide or oxide of the added calcium and the impurity metal content as precipitate 'A', 'Step B': aerating and maintaining at least a portion of the liquor at pH 4 to 6 with a soluble alkali or basic neutralant in the presence of seed material, so as to precipitate iron oxide particles in a hydrated state under hot conditions, and their separation as precipitate 'B', - Step 4: Neutralisation of the remaining filtrate to an alkaline pH and the separation of the so-formed precipitate as precipitate 'C, and - Step 5: Acidification with sulphuric acid of the resulting liquor to a pH lower than 7 and the recovery of sulphate salts.

3. The continuous process according to claims 1 and 2, wherein 'useful material' means ammonia, sodium, potassium, magnesium and/or ferrous iron sulphates.

4. The continuous process according to any one of claims 1, 2 and 3, wherein alkaline, basic or metal values comprise of at least a compound selected from the group consisting of ammonia, ammonium hydroxide, sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium hydroxide, iron scraps and mixtures thereof.

5. The continuous process according to any one of claims 1 to 4, wherein alkaline, basic or metal values of 'useful material' (s) used in Steps 1 and 2 are the same.

6. The continuous process according to any one of claims 1 to 5, wherein the Step 4 comprises the neutralisation of the liquor to a pH of 7 to 9 with at least a fourth part of the required neutralant for the impurities present being a calcium containing material and the remainder being alkaline, basic or metal values of at least a 'useful material'.

7. The continuous process according to any one of claims 1 to 6, wherein the amount of calcium containing material is adjusted in Steps 2 and 4 to be at least equal to a half of the amount required for achieving the target pH value.

8. The continuous process according to any one of claims 1 to 7, wherein the precipitate 'A' from Step 3 and the precipitate 'C from Step 4 are taken either separately or combined and supplemented with lime so as to form a strongly alkaline red gypsum and to liberate ammonia.

9. The continuous process according to claim 8, wherein the so-obtained alkaline red gypsum is mixed with the 'weak filtrate' stream from the washing steps of hydrous titania produced by the sulphate method.

10. The continuous process according to any one of claims 1 to 9, wherein the 'useful material' is ammonium sulphate and the alkaline or basic substance used in Steps 1, 2 and in optional 'Step C is ammonia gas, ammonium hydroxide, alkaline carbonate or mixtures thereof.

11. The continuous process according to any one of claims 1 to 9, wherein the 'useful material' is sodium, potassium, or magnesium sulphate and the alkaline, basic or metal values used in steps 1, 2 and optional 'step C is sodium, potassium, or magnesium hydroxides, carbonates, metal scrap or mixtures thereof.

12. The continuous process according to any one of claims 1 to 9, wherein the 'useful material' is ferrous sulphate and the added neutralant in Steps 1, 2 is iron scrap.

13. The continuous process according to any one of claims 1 to 12, wherein in Step 4, the 'useful material' is ammonium sulphate, sodium sulphate, potassium sulphate, or magnesium sulphate, or other like material and the process of working up the remaining filtrate to a commercial 'useful material' comprises of at least a further neutralisation of the liquor to a pH of 7 to 9 with at least a fourth part of the required neutralant for the impurities present being calcium containing and the remainder being alkaline materials selected from ammonia, ammonium hydroxide, sodium hydroxide, sodium carbonate, potassium hydroxide, magnesium hydroxide and mixtures thereof.

14. The continuous process according to any one of claims 1 to 12, wherein in Step 4 the 'useful material' is ferrous sulphate and the process of working up the remaining filtrate comprises of at least the crystallisation of ferrous sulphate crystals and their recovery followed by a further neutralisation of the liquor to pH 7 to 9 with at least a fourth part of the required neutralant for the impurities present being calcium containing and the remainder being an alkaline material which is a scrap material the metal values of which are tolerable as effluent.