FI102891B - Method for treating gypsum - Google Patents

Description

109891

Method for treating gypsum I U L o 7

The invention relates to a process for the treatment of gypsum, wherein the process comprises converting the aqueous gypsum to a binder by dehydration.

Calcium sulfate CaSO 4 has several different forms. In nature it is eg. as anhydrite, anhydrous. Waste gypsum (so-called sulphox gypsum) from industrial processes and thermal power plants contains two moles of crystalline water per mole of calcium sulphate under normal conditions. In order to produce a binder from such calcium sulfate dihydrate, it is necessary to remove 1.5 water molecules by some method to obtain the hemihydrate CaSO · VMiO. This gypsum hemihydrate is widely used as an independent binder or component in compositions containing other binders in the building materials industry, such as in the manufacture of building elements such as gypsum boards.

CaSO 4 · ΛΑH 2 O obtained by conventional methods in conventional furnace dehydration consists mainly of the beta-modification of calcium hydrate. It is preferable to use alpha hemihydrate for the manufacture of structures requiring strength, such as building boards. It does not even arise in any waste gypsum production process, but must be prepared from the dihydrate form by a suitable drying process. Many patented methods are known in the art. Almost everyone uses an auto-25 spout, a pressure vessel where the temperature can be raised well above the normal boiling point of water.

·. German companies Sicowa GmbH u. C/o. KG and ProMineral GmbH

have developed a process whereby drying is done with saturated solutions of chloride-30, which is a kind of osmosis. The procedure involves washing and drying the product, which increases costs and produces waste water (source: Second International Conference on FGD and Chemical Gypsum, Toronto, May 12-15, 1991).

: 35 Various processes for the preparation of alpha-hemihydrate gypsum from different raw materials are disclosed in GB-2 011 363, DE-1 646 434 and DE-1 796 242. All known methods employ special treatment solutions and high temperatures.

2 102891

The object of the invention is to provide a process which does not require autoclave treatment or special solutions. To accomplish this purpose, the process of the invention is essentially characterized in that dehydration of the gypsum occurs first by dewatering it with a desiccant at low temperature, followed by drying the treated gypsum at a higher temperature.

The gypsum used as the raw material is, in particular, sulphoxic gypsum, i.e. gypsum formed by the removal of gaseous sulfur compounds.

In the first step of the process, particulate driers can be used, which are mixed with gypsum. Orthopilic acid is one such desiccant.

15

According to a preferred process variant, the water-absorbing desiccant used, such as ortho-silicic acid, is regenerated by drying and recycled in the process.

The invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a diagram illustrating a method for preparing alpha hemihydrate, Figs.

30 • · ·

In connection with the cleaning of flue gases, gypsum is formed in the reaction between the gaseous sulfur oxides and the calcium compounds, which is used as a raw material in the treatment process according to the invention. The reaction for the formation of sulphoxips is as follows: SO2 + CaCO3 + -> CaSO3 · I & H2O + CO2 2CaSO3 · y2H2O + 02 + 3H2O → 2CaSO4 · 2H2O

The sulfoxipsip treatment process comprises the following two steps: Step I, Dry Dehydration of Calcium Sulphate 5 Mix calcium sulphate with orthosilicic acid, mSiO 2 · nH 2 O in a weight ratio of 1: 1. Keep at 20 ° C in a closed container for 24 hours. In this "soft" step, ortho-silicic acid "absorbs" all free and physically bound water from the dihydrate. The temperature of the mixture rises to 40 ° C. The first crystals of the α-modification of the calcium sulfate he-10 hydrate are formed. Changes in bound water also occur at this stage.

Intermediate 15 Sulfox gypsum is sieved from mSiO 2 · nH 2 O through a 0.25 mm mesh sieve. The ortho-silicic acid is dried, and the ortho-silicic acid thus regenerated is treated with a new batch of sulpho-gypsum. Sulfox gypsum is sent for further dehydration.

20 Phase II Heat Treatment

The furnace, t = 175 ° C, is subjected to further dehydration of the sulfokips over a period of three hours. This removes the chemically bound water, giving rise to a calcium sulphate hemihydrate consisting mainly of the α-modification. In step I, the mixture is not heated, but the mixing of the substances: · .. can be done at relatively low temperatures (below 30 ° C), such as 18 to 25 ° C at room temperature. The temperature rise of the mixture is due only to the dehydration reaction.

30 • · * The temperature of the Stage II heat treatment step may also be lower or the residence time shorter than the above. If the temperature is lowered, the residence time is correspondingly longer, at 100 ° C for about 24 hours and at 140 ° C for about 12-14 hours. At 175 ° C, V2 h drying is also sufficient, but the best properties are obtained, if processing time is longer, approx. 3 h. A fast drying at high temperature (above 170 ° C) is preferred.

4, 102891

The resulting product, which has a higher concentration of α-modification of the calcium sulfate hemihydrate than that obtained by the conventional dehydration process, is suitable as a binder for many building materials in the construction industry and can be used as an additive or as a base material for building materials such as gypsum board.

The invention is illustrated by the following non-limiting example.

10 Example of Sulfokips Plaster Technology:

The raw material was sulphox gypsum from three different flue gas treatment plants, which according to the certificates contained about 10% moisture.

The following treatment was performed on each batch of sulfo gypsum.

5 kg of sulfoxic gypsum and 5 kg of industrially produced ortho-silicic acid were taken. These components were mixed together in a vessel for 3-5 minutes. The vessel was then sealed and left at room temperature for 20 to 24 hours. At the end of this time, the sulpho-gypsum was screened with orifice acid with 0.25 mm apertures. The sulfox gypsum thus obtained was subjected to heat treatment in an oven at 175 ° C for 3 hours. At the same time, ortho-silicic acid was regenerated for its new use.

• I «\ '·! In a comparative experiment, a batch of sulfoxypypsum was heat treated at 175 ° C without pretreatment with ortho-silicic acid.

<· • · • ··

The results are shown in Figures 2-4, which illustrate the properties of the gypsum obtained in three batches. Curve A depicts the particle size distribution of the plaster of -30 dun in the comparative experiment and curve B the particle size distribution of the gypsum obtained by the treatment method of the invention. The gypsum obtained by the process according to the invention - * \ 'non is characterized by a larger number of α-hemihydrate particles, which is reflected in larger particles. The specific surfaces calculated from Figures 2 to 4 are as follows:

Figure 2: A: 4370cm2 / g, B: 2092cm2 / g

Figure 3: A: 6259cm2 / g, B: 1815cm2 / g

Figure 4: A: 5111 cm 2 / g, B: 1911 cm 2 / g.

102891 5

Small specific surfaces are typical of the α-hemihydrate form.

The microscopic images of the particles in the batch of Figure 2 are shown in Figures 5-8. Figures 5 and 6 show pictures taken from a sample of curve A at 5000 magnification, and figure 7 shows a picture taken from sample B of curve at the same magnification, and figure 8 shows a picture taken at 3500 magnified from the same sample. images.

10

The invention is not limited to the use of orthopilic acid in the first step, but may also include other desiccants which may be mixed with orthopedic acid with powdered gypsum.

15 • '· • · ·

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Claims (5)

102891 A process for treating gypsum, wherein the aqueous gypsum is converted into a binder by dehydration, characterized in that dehydration of the gypsum occurs first by dewatering it with a desiccant at a low temperature, followed by drying the treated gypsum at a higher temperature. Method according to Claim 1, characterized in that the gypsum and the desiccant are mixed together at a temperature below 30 ° C, preferably at room temperature (18 to 25 ° C). Method according to claim 1 or 2, characterized in that the desiccant is separated from the gypsum after treatment. 15 Method according to claim 3, characterized in that the separated desiccant is regenerated and used for the treatment of a new batch of gypsum. 5. A process according to any one of the preceding claims, characterized in that the drying agent used is ortho-silicic acid. I 4 4 I · II 4 «4« «• · • · · Ml • · · · ·« · • • tr • <·· <• 1 »• 4» · • s • · {4 4 14 I 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Patentkrav: 102891