The object of the present invention is a method for trea¬ ting ferrous sulphate for the preparation of ferrous sul- phate hydrate which can be used in cement manufacture for the reduction of chromium.

As is known, cement is manufactured from limestone, sili¬ cate, iron and aluminium minerals together with optional fluorine and sulphur correction materials, which are ground and mixed homogenously and thereafter burned to form so called cement clinker. Cement also contains chro¬ mium compounds, which on contact with water, for example in concrete production, is converted to the Cr ⁶⁺ -form. This soluble chromate is undesirable as it can cause al¬ lergies in persons who come into contact with the cement- water mixture. It is known (e.g. EP 0054314) , in order to reduce the Cr ⁶⁺ -content, to add ferrous sulphate to the dry cement clinker to reduce Cr ⁶⁺ to Cr ³⁺ , which reduced form is practically insoluble in the cement-water mixtu¬ re. The added amount ferrous sulphate is generally in the range of 0.5 to 1 % by weight of the cement clinker.

Ferrous sulphate has a tendency to absorb water wherefore it is difficult to handle as such, for example to pump or to blow. Ferrous sulphate is, however, commercially avai¬ lable e.g. in the form of its heptahydrate provided with an oxidation preventing coating, which coating imparts to the product also desirable flow characteristics. The use of such a coated preparation is, however, expensive ta¬ king into account the large amounts needed in cement ma¬ nufacture. It has also been suggested (FI 842873) to use technical or commercial ferrous sulphate heptahydrate without said coating, whereby the ferrous sulphate hepta- hydrate first is subjected to drying by heating or ab¬ sorption using physical or chemical methods. Mixing of gypsum or fly ash in the ferrous sulphate is mentioned as a particularly suitable chemical method, whereas calcium

oxide or cement alone are not said to give optimal re¬ sults.

The problem underlying the present invention was to find a material for chromium reduction which, on the one hand, contains only such components that are "familiar" in ce¬ ment manufacture and, on the other hand, are available at low cost and in large quantities, and the use of which in addition satisfies the environmental requirements one today places on the industry and its waste products. The problem according to the invention was solved by using, as chromium reduction agent, ferrous sulphate originating from the titanium pigment manufacture. The ferrous con¬ tent in such a ferrous sulphate side product, which pri- marily in its monohydrate form, lies typically in the range of from 13 to 18 % by weight and it has a high sul¬ phuric acid content, up to 30 % by weight, typically from about 10 to 30 % by weight, and is therefore extremely corroding and useless as such. This product contains in addition a considerable amount of moisture, up to 10 % by weight, and smaller amounts of trace elements. It is a product which is extremely difficult to handle and a bur¬ den on the environment, and has to be neutralized in or¬ der to be deposited. Such methods according to which the neutralization takes place in an aqueous solution to an alkaline pH, have been described, for example, in DE OS 4103311 and 3724677.

According to the invention, this moist and sulphuric acid rich ferrous sulphate obtained as a side product is neut¬ ralized and dried with a CaO-containing material, prefe¬ rably cement, optionally together with limestone, to a pH-value of 1.5 to 5, thereby allowing the reaction tem¬ perature to rise to at the most 120 °C. In the reaction, a large part of both the free water and the water bound in crystal water form leaves, and as major products fer¬ rous sulphate hydrate, which is water soluble, primarily

in the form of monohydrate, and gypsum, calcium sulphate, which is one of the starting materials in cement produc¬ tion, are obtained.

It is essential that the temperature does not exceed the said 120 °C in order to ensure that not all hydrate water leaves the product and that the iron is not oxidized to trivalent form which is useless for the chromium reduc¬ tion. The product obtained has excellent handling, espe- cially flow characteristics, and it can be pumped and blown in silos, and it does not dust, which is of essen¬ tial importance in industry in outdoor handling.

Thus, according to the invention, a method has been pro- vided according to which a waste product from the ti¬ tanium dioxide pigment industry in a simple and economi¬ cal manner kan be converted to a product which is easy to handle and which without any problems can be further uti¬ lized in full in cement manufacture. The product contains only such components which are already included among the cement manufacture materials and thus does not contribute with any side products whatsoever which are detrimental to the cement manufacture.

As CaO-containing material, cement or a cement containing material, e.g. slag or fly ash cement, optionally toge¬ ther with limestone, are used. The CaO-containing mate¬ rial is added to the starting ferrous sulphate in subs¬ tantially dry form. It is in principle possible to use any type of CaO-containing material, for example steel slag, blast furnace slag, precipitator dust etc. provided they exhibit sufficient hydraulic characteristics. Pro¬ ducts having a lower reactivity, e.g. filter dust from the cement and lime industry, limestone, and even fly ash, are usable in an initial stage of the process, but in such case a post-treatment with more reactive mate¬ rial, e.g. cement, has to be carried out. It is also con-

ceivable to use hydrated and unhydrated lime, but the use of these extremely reactive materials require a strict temperature control.

The amount of CaO-containing material to be added depends naturally on the sulphuric acid content in the starting material, but an amount of from 5 to 20 % by weight, cal¬ culated from the starting ferrous sulphate, is in most cases sufficient to neutralize to the desired pH and to dry the product. The starting ferrous sulphate is used in the form it is obtained from the pigment production, that is in crystallized, filter dry pressed form, which gene¬ rally means a moisture content of less than appr. 10 % by weight in the starting product. It is also possible to carry out a careful pre-drying of the starting material. The addition of CaO-containing material leads to an exo¬ thermic reaction, which aids in the removal of free mois¬ ture from the material. Care has to be taken, however, that the temperature does not rise too much, for the abo- ve reasons, which conveniently can take place by regula¬ ting the amount and rate of addition of neutralizing agent, but also by suitably chosing the degree of fi¬ neness of the material.

The flow characteristics of the obtained product can be further improved by treating the product with fly ash, either already together with the neutralizing material or after neutralization, using an amount up to 20 % by weight of the ferrous sulphate. Fly ash as such does not possess sufficient hydraulic properties to function alone as both a neutralization and drying medium, but can very well be used as a post-treatment agent, or filler.

According to an advantageous embodiment of the invention, the neutralizing agent, preferably cement, optionally together with limestone, is added in an amount which gi¬ ves a pH of appr. 3 to 4. It is also preferable to keep

the temperature below appr. 105 °C, optimally at appr. 80 to 105 °C, whereby a soluble product suitable for passi¬ vation, is obtained. According to a preferred embodiment, the treatment can take place so that in a first stage limestone is added, which, as mentioned above, gives a slower reaction and a lesser temperature increase, for example to a pH of appr. 1.4, and thereafter the more active material, e.g. cement, is added to raise the pH to the desired pH end value. This procedure provides an ex- cellent temperature control. If such a pre-treatment is carried out, the amount of cement or a material with com¬ parative reactivity can be reduced, for example to 2 to 15 % by weight of the starting material. The pH value is measured by dissolving one part by weight of ferrous sul- phate in 9 parts by weight of water.

The composition of the product obtained after the treat¬ ment varies to some degree depending on the composition of the starting material and the amount and type of neut- ralizing agents used. From the point of view of use, na¬ turally the ferrous content, i.e. the Fe ²⁺ -content, is of importance, and this is generally within the range of 10 to 18 % by weight, which corresponds to a ferrous sulpha¬ te content in pure hydrate form of appr. 30 to 55 % by weight. In addition, the product typically contains appr. 15 to 30 % by weight of gypsum, CaS0 ₄ , and moisture, typi¬ cally 2 to 5 % by weight, and other products, e.g. cement and limestone. The product is well preserved during ce¬ ment manufacture, i.e. there is no premature oxidation to Fe ³⁺ in the mill, the product does not irritate and it is compatible with the cement manufacture. The gypsum in the product can substitute part of the amount of gypsum nor¬ mally used in cement manufacture.

The following examples illustrate the invention, without restricting the same.

EXAMPLE 1

A test batch of appr. 1000 tons of raw ferrous sulphate hydrate from titanium dioxide pigment production which contained appr. 14 % by weight of Fe ²⁺ and appr. 24 % by weight of sulphuric acid, was neutralized with cement (Partek Cement AB, Rapid) using an amount of 8 % by weight calculated from the ferrous sulphate hydrate. The mixing took place in a continuous concrete mixing plant. The mixing time was short, only a few seconds, whereafter the mixture was deposited for end reaction. Mixing took place without any disturbances from the materials used. An analysis of the end material showed that the Fe ²⁺ -con¬ tent was substantially the same and that essentially no oxidation to Fe ³⁺ had taken place. The pH of the end pro¬ duct was 1.5.

EXAMPLE 2

Further test series have been carried out wherein raw ferrous sulphate has been neutralized with different ma¬ terials and also using different combinations. In the series 1 - 10, the ferrous sulphate according to the Example 1 was treated first with 4 and 8 % by weight of limestone, respectively, and therafter with 6 and 2 % by weight of Rapid cement, respectively, calculated from the ferrous sulphate. In the series 11 to 22, raw ferrous sulphate was mixed with various types of CaO-containing material in an amount of either 10 or 20 % by weight, calculated from the ferrous sulphate. The pH was measured (1 part by weight powder and 9 parts by weight water) after 1 day. The heat generation in the powder mixture was measured after a mixing time of 5 minutes. The re¬ sults are given in the Tables 1 and 2.

TABLE 1

Types of limestone A: Moist limestone

B: Parfil 1, Partek Cement AB

C: Parfil 6, -"-

D: Bypass-dust

E: Precipitator dust

No. Type of lime % PH Cement PH stone %

1 A 4 1.51 6 1.80 2 A 8 1.58 2 1.70

3 B 4 1.58 6 1.81

4 B 8 1.64 2 1.72

5 C 4 1.62 6 1.77 6 C 8 1.67 2 1.73

7 D 4 1.67 6 1.93 8 D 8 1.73 2 1.86

9 E 4 1.65 6 1.90 10 E 8 1.75 2 1.84

The results show that limestone in the amounts used, was not reactive enough to sufficiently raise the pH, but that a sufficient pH raise is obtained with a further addition of cement. When mixing the limestone, a very small heat generation was obtained for the samples 1 to 6, for the samples 7 and 9 the maximum temperature was 32 °C and for the samples 8 and 10, it was 40 °C. When mi¬ xing the cement, the maximum temperature for the samples 1, 3, 5, 7 and 9 was appr. 60 to 80 °C, and for the re¬ maining samples appr. 40 °C.

TABLE 2

No. Additive % Temp . pH °C

11 Rapid-cement 20 ^" 80 3.3 12 Salmisaari desulphuration product 20 ^" 5 1.9

13 Copper-slag sand 20 ^" 30 1.8 14 Special steel slag (602 m ² /kg) 0 ^" 50 1.7 15 Ground steel slag (511 m ² /kg) 20 ^" 60 2.3 16 Steel slag (unground) 20 ^" 35 1.7 17 Blast furnace slag (404 m ² /kg) 10 69 18 Mason's cement 10 74 19 Standard cement 10 78 2.1(3d) 20 Quick cement 10 93 21 Standard cement + 10 fly ash 5 79 2.3(3d)

22 Standard cement 15 81 2.7(3d)

In the samples 19, 21 and 22, the ferrous sulphate had been pre-treated with 8 % by weight of limestone. From the results it can be seen i.a. that the temperature can be regulated by means of a suitable choice of cement ty¬ pe.

In the following Table 3, the pH and temperature increase (pH after one day and the maximum temperature of the mix¬ ture, respectively) in tests carried out by adding 10 % by weight of standard cement to three differently treated ferrous sulphate products, namely

1: ferrous sulphate treated with 8 % by weight of limestone

2: ferrous sulphate treated with 8 % by weight of

Rapid cement

3: ferrous sulphate treated with 8 % by weight of limestone and thereafter with 8 % by weight of stan dard cement

No. Max. temp. Start.prod. End prod. pH pH

1 90 1.58 1.98 2 33 1.86 2.80

3 22 2.95 4.04

The results show that for the products 2 and 3, a small or negligent temperature increase is obtained by means of the further addition of 10 % by weight cement, but that on the other hand the product 1 contains more free sulphuric acid.

EXAMPLE 3

In this example, the capacity of the ferrous sulphate hydrate products made according to the process, namely products 2 and 3 from the Table 3 was measured. One part by weight of products 2 and 3, respectively, was mixed with 92 parts by weight of Rapid cement and 7 parts by weight of gypsum. The content of sixvalent chromium (Cr ⁶⁺ ) was measured in the obtained product. Also the reduction capacity of the product was measured using the standard SFS 5183 (24.3.86) by adding to the mixture an excess of 50 mg/kg Cr ⁶⁺ and measuring the amount of re¬ duced chromium. Without the addition of extra chromium no measurable chromium content is obtained. The higher the value, the better reduction capacity of the product

Sample Cr ⁶⁺ Reduction capacity

2 0 39 mg/kg

3 0 37 mg/kg

By adding amounts between 0.5 to 1 % by weight of the product obtained according to the invention to various commercial cement types, reduction capacities up to 49 mg/kg could be measured using the above mentioned stan¬ dard method. The ferrous content (Fe ²⁺ ) in the treated

cement varied typically between 0.01 to 0.2 mg/kg. In the following table, a compilation of analysis results is presented from tests where ferrous sulphate hydrate has been added to cement with varying starting chromium con- tent.

TABLE 4

_Cr 6+ Addition Red. capacity Fe ²⁺ in cement mg/kg % mg/kg %

24 0.7 6 0.07

14 0.5 4 n.a.

25 0.5 13 n.a.

(Luja cement)

25 0.5 17 n.a.

(Rapid)

n.a. = not analyzed.