European Patent Application No. 81110557.6, publication No. 0 054 314, discloses a method of producing a dry cement mixture with reduced or substantially eliminated content of water soluble ch romate, said method comprising grinding a starting material including cement binder clin¬ ker and having a content of water soluble chromate, and adding a chromate reducing and/or neutralizing agent in a non-dissolved condi¬ tion and in an amount of 0.01 -10% by weight of the starting material before, during or after the grinding process in order to eliminate or substantially reduce said water soluble chromate. The preferred reducing agent mentioned in the said patent specification is ferrous sulphate (FeSO^. nf- O) wherein n is normally 7, preferably provided with an oxidation-preventing coating. The product of this type, "MELSTAR M" or "FERROMEL 20™" , specifically disclosed in the patent application is a 96% FeSO ₄ .7H«O provided with an oxidation-preventing coating which also gives this particulate material suitable free-flowing properties which allow it to be stored and used in a practical manner in connection with the preparation of the cement product. Thus, for example, the product may be stored in silos, withdrawn and subjected to pneumatic transport, etc. All of these handling properties depend on the product's capability of being free-flowing and protected against undue oxidation . Also, the coating tends to reduce the cor- rosivity otherwise associated with ferrous sulphate heptahydrate.

Although the above-mentioned specific 96% FeSO _^ . Tf-LO product yields excellent results in the product described in the above-mentioned European Patent Application and yields a cement product in which the added ferrous sulphate is capable of completely reducing the chromate ions released, it would be preferable to be able to use a cheaper and more easily available reducing agent in the process disclosed in the above patent application in view of the fact that the process is per¬ formed on a large scale and consumes large amounts of the reducing agent. Also, it would be desirable to further reduce the corrosivity of the ferrous sulphate reducing agent.

OMPI sA- ipo _

The normal commercial or technical grade of ferrous sulphate is a heptahydrate without the above-mentioned coating . This heptahydrate contains a small amount of water and thereby has a tendency to be ^¬ come "snowy" (it closely resembles melting snow) and has a tendency to cake or agglomerate when it is handled and stored in silos and feed hoppers . These properties render the commercial or technical grade ferrous sulphate heptahydrate unsuitable for storage in silos and for pneumatic transport, etc. : therefore, the commercial or technical grade ferrous sulphate cannot in practice be used as a reducing agent in the above-mentioned chromate reducing process . As another disadvantage, technical or commercial grade ferrous sulphate heptahydrate has increased corrosiveness compared to the coated ferrous sulphate heptahydrate.

In the present context, the term "ferrous sulphate heptahydrate" is used to characterize the starting material . Some of the treatments dis¬ closed in the following may result in removal of part of the crystal water in accordance with what is well known in the art, but it is essential that the product substantially retains the water solubility properties which are characteristic to the heptahydrate, and, for convenience, the product is therefore characterized as heptahydrate or simply hydrate in the present specification and claims.

Applicants have tried to subject ferrous sulphate heptahydrate to drying processes in order to reduce its corrosiveness and to obtain free-flowing characteristics . It has been found, however, that normal drying processes will impair the chromate-reducing capability of the ferrous sulphate heptahydrate (a proportion of the ferrous ion is oxidized to the ferric ion during the drying) , and normal drying processes also have a tendency to cause agglomeration of the technical grade ferrous sulphate heptahydrate. However, when the drying is performed in a particular way, that is, at temperatures below 120°C, in particular at temperatures of at the most 80°C, and preferably temperatures of at the most 60°C, such as a temperature in the range of 20-60°C, it has been found possible to avoid oxidation of the ferrous sulphate heptahydrate and hence to obtain a substantial retainment of its chromate-reducing capabilities. Also, the moderate

drying has been found to have a beneficial effect on the free flowing properties of the product.

Hence, one aspect of the present invention comprises producing a dry cement mixture, said method comprising grinding a starting material including a cement binder clinker and having a content of water soluble chromate, and adding a chromate reducing and/or neutralizing agent in a non-dissolved condition and in an amount of 0.01 -10% by weight of the starting material before, during or after the grinding process in order to eliminate or substantially reduce said water so- luble chromate, the reducing and/or neutralizing agent being a fer¬ rous sulphate hydrate product with a high ratio between ferrous ion and total i ron and a low corrosiveness, the ferrous sulphate hydrate product being prepared from a technical or commercial grade ferrous sulphate heptahydrate product which tends to cake when handle^, and stored in silos and feed hoppers by rendering loosely bound water of the product unavailable for causing the caking by moderate dr iing . The drying is preferably performed at a temperature of at the ^v nost 120°C, preferably at the most 80°C, more preferably at the most *-0°C and especially in the range of about 20-60°C.

However, for most practical applications in connection with the above- mentioned process it is preferred that the loosely bound water of the ferrous sulphate heptahydrate product is rendered unavailable for causing caking of the product by absorption by physical or chemical means.

I n the present context, "absorption by physical or chemical means" covers any absorption of the water into a surface of a material or into interstices in the material or into interstices and the surface of a material, and the chemical absorption refers to any chemical reaction that will remove the water, typically by combining with the loosely bound water.

I n their broadest aspect, the measures for rendering loosely bound water unavailable for causing the caking process comprise one or more treatments selected from moderate drying, powder dilution, physical absorption, and chemical absorption .

The production of the dry cement mixture, including the grinding of the starting material including a cement binder clinker, and the addition of the chromate reducing and/or neutralizing agent in a non-dissolved condition before, during or after the grinding process may be performed as disclosed in the above-mentioned European Patent Application No. 81110557.6.

According to the invention, a preferred method of absorbing the loosely bound water is to mix the technical or commercial grade fer¬ rous sulphate heptahydrate with a water-absorbing material to provide the resulting ferrous sulphate hydrate with an oxidation-preventing and free flowing properties-imparting coating by powder dilution . The water-absorbing material may typically be a powder having a specific

2 surface in excess of about 2000 cm /g and is typically an inorganic material selected from materials occuring as by-products, raw mate- rials or waste products in the cement industry such as clay, fly ash, slag, chalk, gypsum, filter dust, and other products that are readily available in the cement industries (including cement) . Typically, the material will be a mixture comprising two or more of the above-men¬ tioned products .

The amount of the water-absorbing material may vary within wide limits, such as from 1 to 95%, calculated on the resulting product.

A particularly useful and cheap water-absorbing material is fly ash from coal-fired power stations. Such fly ash having a specific surface

2 (as measured by the Blaine method) of about 2000-5000 cm /g , in

2 particular of about 3000-4000 cm /g, has been found to be very well suited for absorbing the loosely bound water from the ferrous sul¬ phate heptahydrate with full retention of the reduction capability of the ferrous sulphate heptahydrate. Also, it has been found that fly ash increases the pH of the ferrous sulphate heptahydrate consider- ably, which means that the fly ash-treated ferrous sulphate hepta¬ hydrate has a considerably decreased corrosiveness, as compared to the starting ferrous sulphate heptahydrate.

Another effect of the addition of a relatively high amount (about 20% by weight or more) of powder is that the ferrous sulphate hepta¬ hydrate is diluted with the interposed particles of the powder, which further tends to counteract agglomeration . The fly ash is preferably added in an amount of 20-70%, in particular in an amount of 40-70%, preferably about 50%, calculated on the total mixtu re.

Another interesting material with which powder dilution of the ferrous hydrate heptahydrate may advantageously be performed is gypsum, either as such or in its less hydrated forms which are hemihydrate and anhydrite, or as a mixture of two or all of these. According to a particular embodiment of the invention, it has been found that the gypsum, hemihydrate or anhydrite should preferably have a pH of at the most 7, in particular a pH in the range of 5-7, preferably a pH of about 6, when admixed with the ferrous sulphate heptahydrate.

The powder dilution is usually performed by mixing in suitable mixing equipment such as a paddle mixer or a kneader or even high shear mixing or crushing equipment, but under conditions where no exces¬ sive heating of the material being mixed will take place. The material with which the ferrous sulphate heptahydrate is mixed may initially be in the form of a powder, or in the form of particles of agglomerated powder which, during the mixing, will be disintegrated into particles of powder size, or it may be in the form of granules, which, during the mixing, are reduced to powder size.

Optimum results have been obtained using a combination of moderate drying and addition of an absorbing material . I n this case, it is preferred to mix the ferrous sulphate heptahydrate and the absorbing material and thereafter to perform the moderate drying on the re¬ sulting mixture. Thus, it has been found that addition of fly ash in an amount of 20-70%, such as 40-70%, in particular in an amount of about 50%, calculated on the total mixture, followed by moderate drying at no more than 80°C, in particular a drying at a temperature of about 20-60°C, results in a very suitable free-flowing powder with full retention of the reduction capability and reduced corrosivity. Gypsum, hemihydrate, anhydrite, or mixtures of two or all of these

may also advantageously be added in an amount of 20-70%, typically 40-70%, especially an amount of about 50%, calculated on the total mixture, followed by the same moderate drying procedu re.

It is an important recognition that by means of materials which are waste products or readily available cheap products in cement industry environments, the readily available and cheap ferrous sulphate hepta¬ hydrate can be modified into a form which is easy to handle and dose into the cement, and which still has retained almost the full reducing capability of the ferrous sulphate heptahydrate. The total amount of the ferrous sulphate hydrate-containing composition of the invention added to the cement is of the order of 0.01 -10% by weight, typically of the order of 1% by weight, and this means that the small amount of additive which is introduced into the cement together with the ferrous sulphate hydrate will not to any substantial extent incur any changes of the properties of the final cement product. When the powder with which the ferrous sulphate heptahydrate is treated is fly ash or gypsum, it is noteworthy that fly ash and gypsum are recognized desirable additives in cements.

As will appear from the working examples, calcium oxide in a re- latively low amount will be able to absorb the loosely bound water; this is due to chemical reaction with the water with formation of calcium hydroxide. While calcium oxide is, hence, capable of absor¬ bing the loosely bound water of the ferrous sulphate heptahydrate, the resulting product, when calcium oxide is used as the sole addi- tive, will normally not be optimal for handling and dosage, and it is often preferred not to use calcium oxide as the sole additive, but to combine it with other materials, such as fly ash, which impart a higher degree of volume or bulk to the product.

It is also possible to use a combination of fly ash with other materials which will absorb loosely bound water, such as filter dust, bentonite, slag, etc. It has been found that cement as such does not seem to be the optimum absorbing powder, and that fine grade chalk has a very high reactivity with the ferrous sulphate when used as the sole addi¬ tive, but this does not preclude the incorporation of cement and chalk as components of a multi-component material .

Based upon the teaching of the present application to the effect that ferrous sulphate heptahydrate may be effectively modified into a free-flowing powder with retention of its reducing capabilities by means of addition of an absorbing or reactive substance and/or by drying, it is within the capacity of the skilled art worker to combine the measures to obtain an optimum combination of free flowing pro¬ perties, high reduction capability, and low corrosiveness . According to the present invention, preferred measures at present seem to be a combination as mentioned above of addition of about an equal amount

2 of fly ash of a specific surface of about 3000-4000 cm /g, or gypsum, hemihydrate, and/or anhydrite, compulsory mixing of the components, e. g . in a paddle mixer, and moderate drying of the resulting granu¬ late product.

The granulate product resulting from this process can be charac- terized as a substantially free-flowing powder comprising ferrous sulphate hydrate in admixture with fly ash or gypsum, the ferrous sulphate hydrate of the mixtures being readily soluble in water and having a high ratio of ferrous ions, such as more than 80% and pre¬ ferably more than 90%, to total iron content in the ferrous sulphate heptahydrate. Normally, the ferrous sulphate hydrate in the compo¬ sition according to the invention will have a considerably increased pH as compared to the untreated ferrous sulphate heptahydrate, the pH increase typically being 1 pH unit or higher, with resulting re¬ duction of the corrosiveness of the product.

When used in the above-mentioned process for reducing the water soluble chromate content of cement, the composition of the present invention is employed in the same manner as described for the ferrous sulphate heptahydrate product used in the above-mentioned patent application , but normally in larger amounts corresponding to the lower content of ferrous ions due to content of the absorbing or reacting powders in the product.

A suitable plant for preparing a ferrous sulphate heptahydrate com¬ position of the present invention is a paddle mixer with a dedusting unit in which the ferrous sulphate heptahydrate of technical grade

can be mixed with the material, typically fly ash, in connection with a drying drum to which the mixture is passed after thorough mixing . If necessary, the dried product can be subjected to disintegration, and thereafter, the resulting free-flowing, stable ferrous sulphate hepta- hydrate composition of reduced corrosivity can be stored and handled in the same manner as the coated ferrous sulphate heptahydrate product described in the above-mentioned patent application .

According to the embodiment of the invention in which the ferrous sulphate heptahydrate is to be admixed with the gypsum, hemihydrate or anhydrite (or any mixture thereof) which is to be used as a start¬ ing material in the production of cement (in which case it may, in certain cases, be preferred not to perform any additional drying) , the mixing ratio may, if desired, be adapted so that the resulting mixture is directly suitable for addition to the cement as the total gypsum addition to the cement in question . Depending on the type of the cement to be produced, the mixing ratio between ferrous sulphate heptahydrate and the gypsum, hemihydrate, anhydrite, or mixture of two or all of these may be between 1 :2 and 1 : 50, in particular be¬ tween 1 :5 and 1 : 20, such as about 1 : 10.

While the present specification discloses in detail how an absorbing material, in particular fly ash, may be used to solve the problems of agglomeration and acidity associated with technical or commercial grade ferrous sulphate heptahydrate used as a reducing agent for addition to cement before, during or after the milling thereof in order to eliminate water soluble chromate in the cement during the use of the cement, it is contemplated that also other reducing agents which are used for the same purpose in a similar manner and which show similar handling problems with respect to caking or agglomeration and/or acidity can be brought into a readily dispersable form showing similar advantages as described herein by admixture with an absorb¬ ing powder, in particular an absorbing powder comprising fly ash, in the manner described herein, suitably combined with drying as de¬ scribed herein and such improvement of such other reducing agents is within the scope of the present invention .

OMPI

The reserve potential chromate reducing capability of a cement (re¬ ferred to in Examples 4, 5 and 6) , i . e. the ability of the cement to reduce further amounts of chromate, can be determined by the follow¬ ing procedu re:

30 g of the cement is stirred for 15 minutes with 30 ml of a solution e+ _ + containing 100 mg Cr /I . After filtration, the amount of Cr in the filtrate is measured colorimetrically by the diphenylcarbazide method β + and expressed as mg Cr /kg cement (M100) .

The above-mentioned procedure is repeated using a new portion of cement and a soluting containing 50 mg Cr /I . The amount of Cr is again expressed as mg Cr /kg cement (M50) .

The reserve potential chromate reducing capability of the cement can then be calculated from the following formula

50 x 50

R = (50 - -) mg Cr ⁶⁺ /kg 100-M50

EXAMPLE 1

In a laboratory paddle mixer, 1 .5 kg of technical grade ferrous sulphate heptahydrate was mixed with an equal weight amount of fly ash from coal-fired power stations, the fly ash having a specific

2 ssuurrffaaccee aass mmeeaassuurreedd bbyy tthhee BBlaine method of 3300 cm /g . The mixing was continued for five minutes .

After the mixing, the resulting product was subjected to moderate drying at 40°C to a weight loss of 1 .3%. Thereafter, the product was a dry, free-flowing powder having a particle size of about 0.5 mm.

I n a similar manner, the ferrous sulphate heptahydrate was mixed with 3% by weight, calculated on the ferrous sulphate, of powdery calcium oxide. After the mixing, the mixture was subjected to mo¬ derate drying at 40°C. Again, the result was a free-flowing dry powder. The particle size was about 0.5 mm.

On these powders, the reduction capability, measured as the content

2+ of Fe in per cent of the total iron content, was determined by titra- tion with potassium dichromate. Also, the pH of a 1% slurry was determined.

The results appear from Table 1 below:

Table I

Amount, calculated on the ferrous sul- Reduction pH of 1% Powder additive phate heptahydrate capability slurry

Fly ash 100% 99% 5.1

Calcium oxide 3% 97% 3.5-4.0

It will be noted that the addition of fly ash results in a dry powder with practically full retainment of the original reduction capability of the ferrous sulphate heptahydrate and with a pH of 5.1 which is a considerable increase from the original pH (3.3) of the technical grade ferrous sulphate hydrate.

Calcium oxide (3%) also results in a dry powder with retainment of the reduction capability, but not with the same pH increase.

Similar experiments were performed with other materials, including 10% bentonite which resulted in retainment of the complete reduction capability, but was apparently not a sufficient amount to secure free flowing properties of the resulting powder. Based upon this, it is justified to assume that a somewhat higher amount of bentonite would yield a satisfactory result.

EXAMPLE 2

In a granulating disc mixer, technical grade ferrous sulphate hydrate was mixed with an equal weight amount of fly ash with a specific

2 surface (Blaine method) of 4250 cm /g, and the mixtu re was passed through a drying drum. The results of these experiments, which were performed on a pilot scale, appear from Table I I below:

Table I I

Expeπ iment No. Experimental conditions

1 I nlet air Outlet air Nm air Production °C °C per min . kg/h

1 40.7 29.3 34 11 .4

2 39.9 28.5 34 13.7

3 No drying

The properties of the products appear from Table I I I :

Table I I I I -

Experiment No. Powder properties Reduction pH of capability 1% slurry

1 Dry, free flowing 89% 4.4 2 Dry, free flowing 92% 4.0 3 Slightly moist 88% 4.5

EXAMPLE 3

In a production cement mill plant comprising two serially connected cement mills, ferrous sulphate heptahydrate admixed with fly ash and prepared according to Example 2, Experiment 2, was added to the

OMPI

second mill . The product was dispensed by means of a feed hopper and a weighfeeder. From the weighfeeder, the product was conveyed by pneumatic transport over a distance of about 150 m to the inlet into the cement mill . The dosing was performed for about 1 hour 20 minutes, and the total consumption of the product was about 800 kg.

During the experiment, samples of the ferrous sulphate/fly ash pro¬ duct were withdrawn, and every tenth minute, samples of the cement after the primary cement mill (without addition of the ferrous sul¬ phate/fly ash product) and after the secondary mill (with addition of the ferrous sulphate/fly ash product) were withdrawn .

The addition per hour of the product was 583 kg to a cement/fly ash product comprising about 84 tons per hour of cement and about 4 tons per hour of added fly ash . This corresponds to a dosage of the ferrous sulphate/fly ash product of about 0.7%.

It was noted that the product flowed freely in the feed hopper in the same manner as the coated ferrous sulphate product which is normally used as reducing agent and which is dosed by means of the same dis¬ pensing equipment. On the whole, the ferrous sulphate/fly ash pro¬ duct was less dusty and more agreeable to handle than "FERROMEL 20™" .

Table IV indicates the samples withdrawn during the experiment and the content of water soluble chromate in the cement samples, mea¬ sured after 1 day, after 8 days and after 14 days, respectively.

From the results stated in the table, it appears that the reference sample (after the primary mill) contains an average of 6.2 mg Cr /kg cement, and that addition of about 0.7% of the ferrous sulphate/fly ash product, corresponding to about 0.35% FeSO ₄ .7H ₂ O, a complete reduction of the content of water soluble chromate compounds in the cement after the secondary cement mill is obtained .

, _OMH

Table IV

Content of water soluble

6-

Sample, kg chromate (mg Cr per kg) after

1 day 8 days 14 days

Cement after primary cement mill

Time:

11.20 a.m 2 6.0 6.0 6.0

11.30 2 5.8 5.8 5.8

11.40 2 6.0 6.0 6.0

11.50 2 6.2 6.2 6.2

12.00 p.m. 2 6.2 6.2 6.2

12.10 - - - -

12.20 2 6.4 6.4 6.4

12.30 2 6.5 6.5 6.5

Cement after secondary cement mill

Time:

11.20 a.m. 2 0.4 0.4 0.4

11.30 2 <0.1 <0.1 <0.1

11.40 10 <0.1 <0.1 <0.1

11.44 10 <0.1 <0.1 <0.1

11.55 10 <0.1 <0.1 <0.1

12.05 p.m. 10 <0.1 <0.1 <0.1

12.10 3 2 <0.1 <0.1 <0.1

12.15 10 <0.1 <0.1 <0.1

12.20 10 <0.1 <0.1 <0.1

12.25 10 <0.1 <0.1 <0.1

15.00 10 <0.1 <0.1 <0.1

EXAMPLE 4

In continuation of the experiments described in Examples 2 and 3, production testing was carried out in industrial scale. In a paddle- mixer/kneader unit, technical ferrous sulphate heptahydrate was mixed with an equal amount by weight of fly ash with a specific

2 surface (Blaine method) of 3700 cm /g, and the mixture was dried by

- r

OMPI

^• A. WIPO

passing it th rough a drying drum equipped with lifters to promote the contact between the drying air and the flow of material . The drying air was heated by means of a gas fired heater unit mounted on end hood of the drying drum. Throughout the test period, the outlet air temperature was kept between 65°C and 70°C, and the rate of air flow was maintained at about 11 ,000 Nm /hour (dry basis) .

The mixture of ferrous sulphate heptahydrate and fly ash was fed to the drying drum by means of a weighfeeder. By adjusting the feed rate upwards and downwards under continuous sampling and testing of the product discharged from the drying drum installation, a pro¬ duction rate of about 7 tons/hour was found to give an optimum pro¬ duct quality as regards free flowing properties and retained chromate reduction capability measured in per cent of the chromate reduction capability of the ferrous sulphate heptahydrate contained in the feed. At this optimum production rate, a total amount of about 95 tons of ferrous sulphate/fly ash mixture was produced and loaded into road bulk tankers for transport to the cement mill plant. An average sample from the 95 tons test batch had a retained chromate reduction capability of 95%, and the pH of a 1% slurry was 4.2.

The 95 tons test batch of the dried mixture of the ferrous sulphate heptahydrate and fly ash was used as chromate reduction additive in the production of cement on the same two serially connected cement mills as used for the experiment described in Example 3. The additive

2+ feed rate was set to maintain the same addition of Fe to the second stage of the two serially connected cement mills as when using the

"FERROMEL 20™" as chromate reducing additive in the normal course of production .

It was observed that the product from the test batch 'lowed freely in the storage silo as well as in the feed hopper, and normal production control sampling and testing for determination of reserve potential of chromate reducing capability in the cement according to the method described above confirmed that the said reserve potential chromate reducing capability remained unaltered at between about 30 and 40 mg Cr /kg cement when changing type of chromate reduction additive

from the type of coated ferrous sulphate heptahydrate (" FERROMEL

20™") used in the normal course of production to the product from the test batch provided that the cement mill output and the amount of

2+ Fe contained in the feed of chromate reduction additive were kept constant.

The said production control samples were also tested for content of

6+ Cr after 8 and 14 days, and all such tests showed that the content of Cr was below 0.1 mg/kg cement.

EXAMPLE 5

In the same industrial mixing and drying plant as was used for the carrying out of the experiment described in Example 4, technical grade ferrous sulphate heptahydrate was mixed with an equal amount, by weight, of gypsum and the mixture was dried to a powder with optimum product quality as regards free flowing properties and re- tainment of chromate reduction capability measured .

The gypsum used for the test production was synthetically produced industrial gypsum form Boliden Kemi, Sweden, of the type normally used in the manufacturing process for all cement made in Denmark. Technical data for the gypsum are set out in Table V below:

^{' •} OMPI

Table V

Chemical analysis % (dry basis)

CaSO ₄ , 2H ₂ O 97.50

P ₂ O ₅ 0.40

AI ₂ O ₃ 0.09

Fe ₂ O ₃ 0.06

Na ₂ O ₃ 0.25 K ₂ O 0.03

F 0.30

98.63

pH of 25% slurry = 6.0.

Moisture (dried at 40°C) = 5.6%.

The air flow through the drying drum was kept at about 13,000

3 Nm /hour (dry basis) , and the outlet air temperature was kept be¬ tween 65°C and 70°C throughout the test period. Under these ope- rating conditions for the drying drum, the optimum production rate was found to be about 8 tons/hour. At this optimum rate, a total amount of about 25 tons of ferrous sulphate/gypsum mixture was produced and loaded into a road bulk tanker for transport to the cement mill plant. An average sample from the 25 tons test batch had a retained chromate reduction capability of 98%, and the pH of 1% slurry made with the product was 4.5.

The 25 tons test batch of the dried mixture of ferrous sulphate heptahydrate and gypsum was used as chromate reduction additive in the production of cement on the same two serially connected cement mills as used for the experiment described in Examples 3 and 4. The

2+ additive feed rate was set to maintain the same addition of Fe as when using "FERROMEL 20™" as chromate reduction additive.

It was observed that the product from the test batch flowed freely in the storage silo as well as in the feed hopper, and normal production control sampling and testing for reserve potential ch romate reduction capability in the cement according to the method described above confirmed that the said reserve potential ch romate reduction capability remained unaltered between about 30 and 40 mg/kg when changing type of ch romate reduction additive from the type of coated ferrous sulphate heptahydrate (" FERROMEL 20™" ) used in the normal cou rse of production to the product from the test batch provided that the

2 2++ cceemmeenntt mmiillll oouuttppuutt aanndd tthhee aammoouunntt ooff FFee contained in the feed of chromate reducing additive were kept constant.

The said production control samples were also tested for content of g . Cr after 8 and 14 days, and all such tests showed that the content g ₊ of Cr was below 0. 1 mg/kg cement.

EXAMPLE 6

I n the same industrial paddle-mixer/kneader u nit as was used for the carrying out of the experiments described in Examples 4 and 5, one part by weight of technical grade ferrous sulphate heptahydrate was mixed with 10 parts by weight of gypsum of the same quality as used for the experiment described in Example 5. A total amount of 22 tons of ferrous sulphate/gypsum mixture was produced and loaded into open trucks for transport to the cement mill plant. Analysis of an average sample of the test batch showed that 99% of the chromate reducing capability of the ferrous sulphate used for the experiment was retained in the product. The pH of the product in 1% slu rry was 5.0.

At the cement mill plant, the 22 tons test batch was received and handled by the installations normally used for taking gypsum to the cement mill feed silos . This installation comprises a feed hopper with extraction belt and a system of rubber belt conveyors for transport and distribution to the gypsum feed silos for the cement mills . The test batch was loaded into the gypsum feed silo for the same two

serially connected cement mills as used for the experiments described in Examples 3, 4 and 5, and over a period of 5 hours, all of the 22 tons were fed to the inlet of the first stage mill as combined addition of gypsum and chromate reduction additive.

It was observed that the ferrous sulphate/gypsum mixture had the same handling properties as gypsum without addition of ferrous sul¬ phate heptahydrate. No difficulties were encountered with extraction and transport during the experiment.

During the experiment, samples of the cement produced were drawn at 30 minutes' interval and tested for determination of reserve poten¬ tial chromate reducing capability according to the method described above. The results of the testing are set out in Table VI below:

OMPI

Table VI

Reserve potential capability in mg Cr 6^ ⁺ /kg

Time cement

10.30 20

11 .00 16

11 .30 18

12.00 15 Ferrous sulphate and

2+

12.30 20 gypsum (1.12 kg Fe

13.00 16 per tonne of cement

13.30 17 produced)

14.00 19

14.30 17

15.00 20

15.30 15

16.30 30

17.00 36

17.30 40 "FERROMEL 20™"

18.00 35 added to second stage

18.30 37 mill (1.09 kg Fe ²⁺ per

19.00 39 tonne of cement pro¬

19.30 31 duced)

20.00 36

Immediately after all of the test batch had been used up, normal gyp- sum feed to the first stage mill and addition of "FERROMEL 20™" as chromate reduction agent in the second stage mill were restored. It

2+ was then observed that addition of Fe in the form of "FERROMEL

20™" in the second stage mill at the same rate of addition as the ferrous sulphate/gypsum mixture in the first stage mill resulted in an increase in the reserve potential chromate reducing capability of about

OMPI

g+ 2 ⁺

20 mg Cr /kg cement. Most likely, this higher demand for Fe when feeding the chromate reducing agent to the first stage mill is due to prolonged exposure of the ferrous sulphate to the cement grinding process in the first stage mill.

OMH_