GB1581539A

GB1581539A - Making plaster from ferrous sulphate

Info

Publication number: GB1581539A
Application number: GB4165576A
Authority: GB
Original assignee: Laporte Industries Ltd
Current assignee: Evonik LIL Ltd
Priority date: 1976-10-07
Filing date: 1976-10-07
Publication date: 1980-12-17
Also published as: ES462972A1; CA1084957A; DE2745023A1; JPS5346498A; AU2926377A; AU506984B2; FR2367030A1

Description

(54) MAKING PLASTER FROM FERROUS SULPHATE (71) We, LAPORTE INDUSTRIES LIMITED a British Company of Hanover House, 14 Hanover Square, London, W.1., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to making plaster from ferrous sulphate.

Ferrous sulphate, known as copperas in its heptahydrate crystalline form, is produced in considerable quantities as a byproduct in certain major industrial processes.

In the "sulphate" process for the production of titanium dioxide by-product ferrous sulphate is produced both as copperas and as waste liquors containing, for example, up to 300 g/l ferrous sulphate and up to 300 g/l free sulphuric acid. In sulphuric acid pickling process waste liquors are produced containing, for example, up to 460 g/l ferrous sulphate and up to 90 g/l free sulphuric acid. Liquors containing ferrous sulphate cannot legally be discharged as effluent in many countries and their disposal is a major problem.

The present invention relates to the alleviation of this problem by the utilisation of ferrous sulphate. More particularly the present invention relates to the production of a plaster utilising ferrous sulphate as raw material.

By plaster we mean a material comprising one or more at least partially dehydrated compounds which on contact with water recrystallise in a higher state of hydration thereby causing the material to set, said material being usable to form, for example, surface coatings on the interior walls and ceiings of buildings, moulds for use in the manufacture of articles or moulded artefacts. The most commonly used plaster consists essentially of calcium sulphate hemihydrate produced from gypsum and is a relatively cheap and abundant product.

The present invention provides a process for the manufacture of a plaster from ferrous sulphate comprising forming a reaction mixture consisting essentially of ferrous sulphate, any free acid therein having been neutralised, and one or more calcium compounds selected from calcium carbonate, calcium hydroxide and calcium oxide, in an aqueous medium, heating the reaction mixture in the presence of steam at a temperature greater than 100"C and under a pressure greater than atmospheric pressure to form a crystalline plaster product.

Preferably the ferrous sulphate is ferrous sulphate heptahydrate.

The ferrous sulphate may be in the form of a liquor produced as a by-product by an industrial process as above described and generally containing a proportion of free sulphuric acid. According to the invention any such free acid is neutralised with, for example, calcium compound, thereby forming calcium sulphate dihydrate or synthetic gypsum, when the free acid is sulphuric acid. The higher the proportion of the sulphuric acid in the ferrous sulphate the higher the proportion of synthetic gypsum formed.

Preferably any free acid present in the ferrous sulphate is neutralised by reaction with the one or more calcium compounds used to form the reaction mixture and in the course of the formation of the reaction mixture, sufficient additional calcium compound being added to achieve this, and the resulting acid salt remaining in the reaction mixture.

It is not necessary, however, in the present invention for any free sulphuric acid to be present, the neutralisation step merely being omitted if none is present.

In terms of the properties of the product we can obtain at least some benefit according to the present invention over a very wide range of ratios of ferrous sulphate to sulphuric acid. The preferred molar ratio of ferrous sulphate to free acid is from 1:0 to 1:6.

The present invention may involve a drying step and it may therefore be advantageous to ensure that as little water as is possible is present during the earlier stages of the process. Liquors resulting from industrial processes are generally dilute making necessary the use of a more extensive dewatering and drying plant than might otherwise be necessary. Preferably, therefore, the ferrous sulphate is in the form of copperas which has been dissolved or slurried in a relatively small amount of water or, preferably, dissolved in a ferrous sulphate containing liquor as above described.

The amount of water added may be relatively small in view of the release of water of crystallisation from the copperas. Preferably the amount of water used apart from the water of crystallisation present in the ferrous sulphate heptahydrate is from 4 to 100 and particularly preferably from 20 to 50 ml/ lOOg ferrous sulphate heptahydrate.

According to a preferred feature of this invention the calcium compound is calcium carbonate in the form of ground limestone or ground chalk. Such forms of calcium carbonate do not react to completion with ferrous sulphate under normal pressure and at temperatures below 100 C, and what reaction there is is generally slower than the reaction between ferrous sulphate and calcium oxide (quicklime) or calcium hydroxide (slaked lime). Under conditions utilised in carrying out the present invention, as hereafter described, these forms of calcium carbonate may be made to react with ferrous sulphate sufficiently readily to represent an advantage in using such raw materials in comparison with using lime bearing in mind the relative raw material costs involved.Where the sole calcium compounds used are calcium oxide and/or calcium hydroxide they are preferably used in at least the amount required in theory to react with the sulphate ion present and in not more than 30% excess over that amount.

Where the sole calcium compound used is calcium carbonate it is preferably used in excess over the amount required in theory to react with the sulphate ion present and particularly preferably is used in an excess over that amount of at least 5% and, for example, up to 30% on a molar basis. When we refer to sulphate ion we include that derived from any free acid originally present.

The reaction mixture is suitably formed by adding the calcium compound to a mixture of the ferrous sulphate and water. The calcium compound will react fairly readily with any free sulphuric acid present and for this reason it may be desirable to control the rate of addition of the calcium compound so as to ensure that this initial reaction proceeds smoothly.

The reaction mixture is preferably maintained at the required temperature and pressure by heating it in an autoclave, e.g.

in the presence of water vapour at a temperature not greater than 175 C and particularly preferably at a temperature of from 115"C to 160 C. The autoclave need not be vented. Preferably, when a vented autoclave is used, it is controlled to give an internal pressure of from 10 to 50 pounds per square inch gauge (psig). The duration of treatment in the autoclave is that required for completion of the reaction between the calcium compound and the ferrous sulphate heptahydrate and the formation of a product which on drying will give the required properties in use and this duration will depend on the particular temperature and pressure used. Preferably the reaction in the autoclave is conducted for from 1 hour to 10 hours and particularly preferably from 2 hours to 6 hours in total.

To ensure a reasonable speed of reaction the particle size of the ground limestone or ground chalk is preferably such that at least 60% by weight and particularly preferably at least 95% by weight has a particle size below 53 microns.

Preferably as large a proportion as possible of the reaction between ferrous sulphate and the calcium compound occurs at a super-atmospheric pressure and a temperature greater than 100"C. Preferably, therefore, the temperature of the reaction mixture is maintained at below 100 C and preferably below 80"C during the formation thereof, thereby minimizing reaction until the required super-atmospheric pressure can be generated.

Even under the conditions used in carrying out the present invention the reaction between ferrous sulphate and calcium carbonate in the form of ground limestone or ground chalk will not normally go to completion. It is preferred to ensure reaction of all the ferrous sulphate since the presence of residual ferrous sulphate in the plaster product of the present invention may result in undesirable characteristics in said plaster caused, for example, by leaching of the ferrous sulphate from the plaster. Preferably, therefore, to ensure reaction of all of the ferrous sulphate, a quantity of calcium oxide or hydroxide is added to the reaction mixture. Preferably the quantity of added calcium hydroxide or oxide is sufficient in theory to react with between 5% and 40 and particularly preferably with from 5% to 25% of the sulphate ion derived from the ferrous sulphate originally incorporated in the reaction mixture. In this case it is possible, to use less than the quantity of calcium carbonate required in theory to react with the sulphate ion present although for economy it is still desirable to use more than 80%, preferably more than 95%, of that quantity. The reaction between the ferrous sulphate and the calcium hydroxide or calcium oxide is also preferably conducted under like conditions of temperature and pressure as the reaction with the calcium carbonate.The maintenance of a temperature of the reaction charge above 80"C is important, to prevent undue deterioration of the properties of the final product, while a reaction temperature above 100"C is necessary for the formation of that product.

Any addition of material to the reaction charge, therefore, is desirably conducted so as to maintain the temperature thereafter to at least 80"C. This may be achieved, for example, by preheating such material to at least 80"C preferably to at least 95"C.

Any water remaining in the product formed in the autoclave may be removed by drying which may be conducted in air, for example in a tunnel drier. If a substantial proportion of the water originally present in the reaction mixture has not been removed by, for example, using a vented autoclave, it may be necessary to precede the drying step by a dewatering step including, for example, filtration or centrifuge treatment. Preferably, the temperature of the product is maintained at, at least, 80"C to prevent deterioration thereof, e.g. at from 85"C to 125 C and particularly preferably at a temperature of from 100"C to 125"C until the conclusion of any dewatering or drying thereof. The duration of the drying step may be, for example, up to 24 hours.

The dried product may be pulverised and mixed with appropriate additives known in the art for the control of the properties of plaster of Paris to form a plaster powder.

This powder material may be mixed with water to form a settable plaster mixture which may be used as a wall surface covering or as a material for the construction of moulds or of moulded artefacts. Alternatively the plaster product formed by the said heating of the reaction mixture in the autoclave may be directly formed, with the addition or removal of the required quantity of water to obtain the correct consistency and of any appropriate additives as described above, into a plaster mix and shaped into desired artifacts, for example, blocks, tubes, rods or sheets.

The product of the present invention shows some properties similar to those of conventional plaster. However, despite the very high content of iron compounds in the said product, .particularly where ferrous sulphate has been the sole, or substantially the sole, source of sulphate ion we have found that the product of the present invention may be either non-magnetic or only slightly magnetic. In contrast, the product obtained by forming the same reaction mixture and heating it in air is relatively highly magnetic. We therefore believe that the restricted access of oxygen to the reaction mixture resulting from heating in an autoclave in the presence of steam prevents, or reduces, the formation of magnetic iron oxides. This is an important feature of the present invention since in many uses of a plaster the presence of magnetic materials would be undesirable.One way of limiting the contact between the autoclave charge and such oxygen as may be in the autoclave initially, or as may be admitted to it by opening the autoclave during processing is to arrange that the autoclave charge presents a relatively small surface area to the atmosphere in the autoclave. When the autoclave charge is spread on shallow trays a more magnetic product may be formed than when the autoclave charge is confined in a relatively deep layer.

The present invention will now be illustrated by means of the following Examples.

EXAMPLE I 278g of copperas (10 moles) containing less than 2% of free sulphuric acid was slurried in 1 1 water in a 5 1 beaker at a temperature of 60 C. 1000g of natural dry chalk (10 moles) was ground in a micropulveriser until at least 95% by weight of the particles had a greatest particle diameter of 53 microns. The ground chalk was added to the copperas slurry over 30 minutes the addition being conducted particularly carefully until the free acid had been neutralised.

The slurry was then stirred in the beaker for 15 minutes while maintaining the temperature at 60 C. The slurry was then placed in an unstirred autoclave for 3 hours at a temperature of 126 C and an internal pressure of 22 psig. After 3 hours the autoclave was vented off to atmospheric pressure and the contents stirred while the temperature remained at above 95"C.

137.5g calcium hydroxide was slurried in 400 ml boiling water and was added to the slurry over 15 seconds which slurry was then stirred vigorously for 4 minutes after which time it had a pH of 7.7. The slurry was then returned to the autoclave for a further 1 hour at a temperature of 126 C and a pressure of 22 psig. The autoclave was vented off to atmospheric pressure and the contents reslurried, and filtered at a temperature of 85"C over 4 minutes. The hot filter cake was discharged onto a steel tray, chopped into t" lumps and dried in an air oven for 17 hours at a temperature of 120t C. The dried product was discharged, cooled and ground. It was sub stantially non-magnetic.The filtrate showed a content of less than 1 ppm of Fe+2 showing that substantially all of the iron content of the copperas had remained in the product. The resulting product, tested as a plaster, showed a setting time of 6.5 minutes and a water demand of 44 mls/ 1 00g under standard conditions.

To 30g of the product was added 0.3g calcium hydroxide and 0.04g keratin and the resulting mixture having a pH of 10.0 was micropulverised. When tested as a plaster the product showed a setting time of 50 minutes under the same standard conditions.

A series of plasters manufactured by a process substantially as in the above Examples showed a water demand of from 30 to 48 ml/100g in comparison with a commercial plaster manufactured from natural gypsum which showed a water demand of from 36-40 ml/100g. A 100 mm cube made from plaster manufactured substantially according to the above Example showed a compressive strength of 41 kN/100 mm2 in contrast to a similar cube made from the commercial plaster which showed a com pressive strength of 37 kN/100 mum2. The density of the plaster manufactured from products manufactured substantially as in the above Example was from 1.2 to 1.8 g/cc.

EXAMPLE 2 122.5g copperas containing less than 2% free sulphuric acid was slurried in 8.8 galls water in a stirred tank and the slurry agitated whilst live steam was sparged into the tank to raise the temperature of the slurry to 65"C. 44.lg natural dry chalk of which at least 95% by weight had a greatest particle diameter of 53 microns was added to the tank from a vibrating hopper over + hour. This reduced the slurry temperature to 45 C. The slurry was then pumped into a warmed 20 gall autoclave filled with a stirrer, a pressure control valve, an inlet port which could operate under pressure and a bottom drain exit port. The slurry was sparged with steam to raise its temperature to 70 C after which the autoclave was sealed.The autoclave interior was raised to a temperature of 126"C and a pressure of 40 psig over a time of 1 hour 10 minutes while stirring. These conditions were maintained for 3+ hours maintaining stirring.

The pressure was then vented down to a pressure of 22 psig stirring being continued.

6.2 lbs lime in 2 galls of near boiling water were pumped into the autoclave under a pressure of 25 psig over a time of 5 minutes 5 seconds. The conditions in the autoclave were then maintained under stirring at a temperature of 126"C and a pressure of 22 psig for a further time of 1 hour. The slurry was then pumped over a time of 5 minutes from the autoclave into a filter press, which had been prewarmed to 90 C.

The resulting hot filter cakes containing 59% solids were transferred to an air oven sufficiently quickly to prevent them cooling to 80 C and were dried at a temperature of 120 C for 17 hours after which they were cooled and pulverised.

To 30g of the resulting plaster product was added 0.3g calcium hydroxide and 0.04g keratin and the resulting mixture was micropulverised. Under the same standard conditions as used in Example 1 this plaster gave a setting time of 80 minutes. Tested in a similar manner to that used in Example 1 the plaster showed a water demand of 40 mls /1 00g and a similar compressive strength and density to that found in Example 1.

WHAT WE CLAIM IS: 1. A process for the manufacture of a plaster from ferrous sulphate comprising forming a reaction mixture consisting essentially of ferrous sulphate, any free acid therein having been neutralised, and one or more calcium compounds selected from calcium carbonate, calcium hydroxide and calcium oxide, in an aqueous medium, heating the reaction mixture in the presence of steam at a temperature greater than 100 C and under a pressure greater than atmospheric pressure to form a crystalline plaster product.

2. A process as claimed in claim 1 wherein the ferrous sulphate is ferrous sulphate heptahydrate.

3. A process as claimed in claim 1 or 2 wherein the free acid is neutralised by reaction with the one or more calcium compounds in the course of the formation of the reaction mixture the resulting acid salt remaining in the reaction mixture.

4. A process as claimed in claim 3 wherein the molar ratio of ferrous sulphate to free acid is from 1:0 to 1:6.

5. A process as claimed in any preceding claim wherein the amount of water used to form the reaction mixture is from 4 to 100 ml per lOOg ferrous sulphate excluding any water of crystallisation present.

6. A process as claimed in any preceding claim wherein the calcium compound used to form the reaction mixture is calcium carbonate in the form of ground limestone or ground chalk.

7. A process as claimed in claim 6 wherein at least 60 % by weight of the ground limestone or ground chalk has a particle size below 53 microns.

8. A process as claimed in claim 6 or 7 wherein the quantity of ground limestone or ground chalk is in excess over that required in theory to react with the quantity of sulphate ion present said excess being up to 30%.

9. A process as claimed in any one of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. stantially non-magnetic. The filtrate showed a content of less than 1 ppm of Fe+2 showing that substantially all of the iron content of the copperas had remained in the product. The resulting product, tested as a plaster, showed a setting time of 6.5 minutes and a water demand of 44 mls/ 1 00g under standard conditions. To 30g of the product was added 0.3g calcium hydroxide and 0.04g keratin and the resulting mixture having a pH of 10.0 was micropulverised. When tested as a plaster the product showed a setting time of 50 minutes under the same standard conditions. A series of plasters manufactured by a process substantially as in the above Examples showed a water demand of from 30 to 48 ml/100g in comparison with a commercial plaster manufactured from natural gypsum which showed a water demand of from 36-40 ml/100g. A 100 mm cube made from plaster manufactured substantially according to the above Example showed a compressive strength of 41 kN/100 mm2 in contrast to a similar cube made from the commercial plaster which showed a com pressive strength of 37 kN/100 mum2. The density of the plaster manufactured from products manufactured substantially as in the above Example was from 1.2 to 1.8 g/cc. EXAMPLE 2 122.5g copperas containing less than 2% free sulphuric acid was slurried in 8.8 galls water in a stirred tank and the slurry agitated whilst live steam was sparged into the tank to raise the temperature of the slurry to 65"C. 44.lg natural dry chalk of which at least 95% by weight had a greatest particle diameter of 53 microns was added to the tank from a vibrating hopper over + hour. This reduced the slurry temperature to 45 C. The slurry was then pumped into a warmed 20 gall autoclave filled with a stirrer, a pressure control valve, an inlet port which could operate under pressure and a bottom drain exit port. The slurry was sparged with steam to raise its temperature to 70 C after which the autoclave was sealed.The autoclave interior was raised to a temperature of 126"C and a pressure of 40 psig over a time of 1 hour 10 minutes while stirring. These conditions were maintained for 3+ hours maintaining stirring. The pressure was then vented down to a pressure of 22 psig stirring being continued. 6.2 lbs lime in 2 galls of near boiling water were pumped into the autoclave under a pressure of 25 psig over a time of 5 minutes 5 seconds. The conditions in the autoclave were then maintained under stirring at a temperature of 126"C and a pressure of 22 psig for a further time of 1 hour. The slurry was then pumped over a time of 5 minutes from the autoclave into a filter press, which had been prewarmed to 90 C. The resulting hot filter cakes containing 59% solids were transferred to an air oven sufficiently quickly to prevent them cooling to 80 C and were dried at a temperature of 120 C for 17 hours after which they were cooled and pulverised. To 30g of the resulting plaster product was added 0.3g calcium hydroxide and 0.04g keratin and the resulting mixture was micropulverised. Under the same standard conditions as used in Example 1 this plaster gave a setting time of 80 minutes. Tested in a similar manner to that used in Example 1 the plaster showed a water demand of 40 mls /1 00g and a similar compressive strength and density to that found in Example 1. WHAT WE CLAIM IS:

1. A process for the manufacture of a plaster from ferrous sulphate comprising forming a reaction mixture consisting essentially of ferrous sulphate, any free acid therein having been neutralised, and one or more calcium compounds selected from calcium carbonate, calcium hydroxide and calcium oxide, in an aqueous medium, heating the reaction mixture in the presence of steam at a temperature greater than 100 C and under a pressure greater than atmospheric pressure to form a crystalline plaster product.

9. A process as claimed in any one of

claims 6 to 8 wherein calcium oxide or calcium hydroxide is added to the reaction mixture in a quantity sufficient in theory to react with from 5% to 40% of the sulphate ion derived from the ferrous sulphate originally incorporated in the reaction mixture.

10. A process as claimed in any preceding claim wherein the said heating of the reaction mixture is conducted in an autoclave at a temperature not greater than 175"C.

11. A process as claimed in claim 10 wherein the said heating of the reaction mixture is conducted under a pressure of from 10 p.s.i.g. to 50 p.s.i.g. in a vented autoclave.

12. A process as claimed in any preceding claim wherein the said heating of the reaction mixture is conducted for from 1 hour to 10 hours.

13. A process as claimed in any preceding claim wherein the temperature of the reaction mixture is maintained at below 80"C during the formation thereof.

14. A process as claimed in any preceding claim wherein the temperature of the product is maintained at at least 80"C until the conclusion of any dewatering or drying thereof.

15. A process as claimed in any preceding claim wherein the plaster product is recovered from any residual aqueous medium and is dried and pulverised, with the addition of any appropriate additives, to produce a plaster powder.

16. A process as claimed in any one of claims 1 to 14 wherein the plaster product formed by the said heating of the reaction mixture is directly formed, with the addition of any appropriate extra quantity of water and of any appropriate additives, into a suitable plaster mix in the shape of a desired artefact.

17. A process as claimed in claim 1 and substantially as described herein.

18. A process substantially as described in Example 1 herein.

19. A process substantially as described in Example 2 herein.

20. Plaster which is the product of a process as claimed in any one of claims 1 to 19.