Production of commercially useful materials for waste gypsum boards

A commercially useful material is produced from waste gypsum boards containing calcium sulfate dihydrate, by a method comprising the steps of (a) grinding the waste gypsum boards to obtain particles having a substantially uniform size; (b) drying the particles obtained in step (a) to obtain moisture-free particles; (c) heating the moisture-free particles obtained in step (b) at a temperature of 128 to 162.degree. C. to convert the calcium sulfate dihydrate to calcium sulfate hemihydrate; (d) adding water to the particles obtained in step (c) to form a paste; (e) forming the paste obtained in step (d) into a shaped product of a predetermined size; and (f) drying the shaped product obtained in step (e) at a temperature of 105 to 120.degree. C. to obtain a commercially useful material. The materials produced by the method of the invention can be used for a variety of industrial and household purposes. Typical uses include the absorption of oil, grease and chemicals on floors and elsewhere, in animal toilet applications as a cat box absorbent, as carriers for chemicals, such as pesticides and herbicides, in various agricultural and horticultural applications. They can also be used as fillers for agricultural and horticultural applications. When admixed with an adhesive agent such as Portland cement, epoxy or polyester adhesives, they can be applied onto walls and/or floors to provide a decorative coating.

BACKGROUND OF THE INVENTION 
The present invention pertains to improvements in the field of waste 
recycling. More particularly, the invention is directed to a method of 
producing commercially useful materials from waste gypsum boards, such as 
waste GYPROC (trademark) boards. 
Waste gypsum boards coming from demolition and gypsum board factories 
represent a serious threat to the environment as they accumulate by 
thousands to hundred thousands of tons in many dumping sites. They retain 
humidity and thus promote the formation of undesirable mildew. Degradation 
of the calcium sulfate contained in the waste gypsum boards under the 
influence of infra-red and ultraviolet radiations causes the generation of 
H.sub.2 S and SO.sub.2 which contaminate the environment. 
Gypsum boards contain calcium sulfate dihydrate, silicon dioxide, 
cellulosic fibers as well as various metal oxides such as aluminum oxide, 
calcium oxide, ferric oxide and magnesium oxide. Calcium sulfate dihydrate 
is known to have liquid absorbing properties, as disclosed for example in 
U.S. Pat. Nos. 4,163,674 and 4,264,543. However, waste gypsum boards are 
not suitable for use as absorbents or any other useful products such as 
fillers for agricultural and horticultural applications. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to overcome the above 
drawbacks and to provide a method of producing a commercially useful 
material from waste gypsum boards. 
According to one aspect of the invention, there is thus provided a method 
of producing a commercially useful material from waste gypsum boards 
containing calcium sulfate dihydrate. The method of the invention 
comprises the steps of: 
a) grinding the waste gypsum boards to obtain particles having a 
substantially uniform size; 
b) drying the particles obtained in step (a) to obtain moisture-free 
particles; 
c) heating the moisture-free particles obtained in step (b) at a 
temperature of 128 to 162.degree. C. to convert the calcium sulfate 
dihydrate to calcium sulfate hemihydrate and thereby obtain particles 
containing calcium sulfate hemihydrate; 
d) adding water to the particles obtained in step (c) to form a paste; 
e) forming the paste obtained in step (d) into a shaped product of a 
predetermined size; and 
f) drying the shaped product obtained in step (e) at a temperature of 105 
to 120.degree. C. to obtain a commercially useful material. 
The present invention also provides, in another aspect thereof, a 
commercially useful material derived from waste gypsum boards and produced 
by the above method. 
Applicant has found quite unexpectedly that commercially useful products, 
particularly liquid absorbing materials, filler materials for agricultural 
and horticultural applications and decorative coating materials for walls 
and floors, can be produced at a low cost from waste gypsum boards by 
converting the calcium sulfate dihydrate contained in the waste gypsum 
boards to calcium sulfate hemihydrate having binding properties and 
serving to bind the other components of gypsum boards. The waste gypsum 
boards must first be ground to obtain particles having a substantially 
uniform size, which are then dried to obtain moisture-free particles, for 
example, by heating the particles at a temperature of 105 to 120.degree. 
C. in a circulating air oven for a period of time sufficient to evaporate 
all traces of water. The moisture-free particles are heated at a 
temperature of 128 to 162.degree. C., preferably about 145.degree. C., to 
partially dehydrate the calcium sulfate dihydrate and to thereby convert 
same to calcium sulfate hemihydrate. Care should be taken to avoid heating 
the moisture-free particles at a temperature above 162.degree. C., since 
at a temperature of 163.degree. C. calcium sulfate hemihydrate loses all 
its water of crystallization and is converted to anhydrous calcium 
sulfate. Water is added to the resulting product to form a paste. The 
paste is formed into a shaped product of a predetermined size, for 
example, granules, pellets or balls, and the shaped product is then dried 
at a temperature of 105 to 120.degree. C. to obtain the desired absorbent 
or filler material. If the shaped product is dried at a temperature less 
than 105.degree. C., the evaporation of water is insufficient so that the 
absorption characteristics of the final product are adversely affected. At 
a temperature above 120.degree. C., on the other hand, there is a partial 
conversion of the hydrated calcium sulfate to calcium sulfate hemihydrate, 
which weakens the final product and leads to the formation of cracks 
therein. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Various additives can be dry-mixed with the product obtained in step (c) 
with a view to increasing the absorption characteristics, density, 
hardness and/or water-resistance of the material obtained in step (f); 
fungicidal agents and/or coloring agents can also be added. For example, 
when it is desired to increase the absorption characteristics of the final 
product, wood cellulose fibers, textile fibers, paper pulp, peat fines, 
mineral fines, charcoal, agricultural waste materials such as corn stalks 
and peanut shells, or porous plastic materials can be added. Use is 
preferably made of cellulosic fibers in an amount of 0.5 to 30 wt. %. When 
it is desired to increase the density of the final product, powdered slag 
can be added; it is preferably used in an amount of 5 to 80 wt. %. Since 
powdered slag exhibits binding properties, it can also be used to increase 
the hardness of the final product. In such a case, the powdered slag is 
preferably added in an amount of 1 to 80 wt. %. When using powdered slag, 
acidified water is preferably added in step (d) to reduce the pH so that 
the final product has a pH of about 7. On the other hand, when it is 
desired to increase the water-resistance of the final product, gypsum 
plaster which is also called "Plaster of Paris" can be added. The gypsum 
plaster is preferably used in an amount of 10 to 80 wt. %. A clumping 
agent such as sodium bicarbonate can also be added. 
A particularly preferred absorbent material produced in accordance with the 
invention is derived from about 50 wt. % waste gypsum boards and contains 
about 25 wt. % powdered slag, about 20 wt. % gypsum plaster and about 5 
wt. % cellulosic fibers. Such an absorbent material not only exhibits 
increased hardness and absorption characteristics but is also highly 
resistant to degradation by water. 
A particularly preferred filler material produced by the method of the 
invention is derived from about 50 wt. % waste gypsum boards and contains 
about 25 wt. % gypsum plaster and about 25 wt. % powdered slag. Such a 
filler material exhibits not only increased density and hardness but is 
also resistant to degradation by water. 
The materials produced in accordance with the invention can be used for a 
variety of industrial and household purposes. Typical uses include the 
absorption of oil, grease and chemicals on floors and elsewhere, in animal 
toilet applications as a cat box absorbent, as carriers for chemicals, 
such as pesticides and herbicides, in various agricultural and 
horticultural applications. They can also be used as fillers for 
agricultural and horticultural applications. When admixed with an adhesive 
agent such as Portland cement, epoxy or polyester adhesives, they can be 
applied onto walls and/or floors to provide a decorative coating.

The following non-limiting examples illustrate the invention. 
EXAMPLE 1 
120 kg of the waste humid gypsum boards were broken down into small 
particles and dried at 115.degree. C. in an air dryer until the particles 
became moisture-free. The temperature of the dryer was gradually increased 
to heat the moisture-free particles at a temperature of 150.degree. C. for 
a duration of 20 minutes. The particles thus obtained which contained 
hemihydrated calcium sulfate are hereinafter referred to "H.H.C.S. 
particles". 
10 kg of the H.H.C.S. particles were mixed with a suitable amount of water 
to form a paste suitable for shaping into granules. The granules were 
dried at 115.degree. C. then broken down into different mesh sizes from 
mesh 5 to 40. The broken granules were then redried in the same dryer at 
115.degree. C. 
EXAMPLE 2 
5 kg of the H.H.C.S. particles obtained in Example 1 were well mixed with 2 
kg of gypsum plaster. Water was added to the resulting mixture to form a 
paste suitable for shaping into granules. The granules were dried at 
115.degree. C. and then broken down into different mesh sizes from mesh 5 
to 40. The broken granules were then redried in the same dryer at 
115.degree. C. 
EXAMPLE 3 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
3 kg of gypsum plaster. 
EXAMPLE 4 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
4 kg of gypsum plaster. 
EXAMPLE 5 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
5 kg of gypsum plaster. 
EXAMPLE 6 
Example 4 was repeated, except that 0.5 kg of defibrated waste hygienic 
paper was added to the mixture containing the H.H.C.S. particles and 
gypsum plaster, prior to adding the water. 
EXAMPLE 7 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
1 kg of powdered slag. 
EXAMPLE 8 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
2 kg of powdered slag. 
EXAMPLE 9 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
3 kg of powdered slag. 
EXAMPLE 10 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
4 kg of powdered slag. 
EXAMPLE 11 
Example 2 was repeated, except that the H.H.C.S. particles were mixed with 
5 kg of powdered slag. 
EXAMPLE 12 
Example 8 was repeated, except that 1 kg of gypsum plaster was added to the 
mixture containing the H.H.C.S. particles and powdered slag, prior to 
adding the water. 
EXAMPLE 13 
Example 12 was repeated, except that the amount of powdered slag added was 
2 kg. 
EXAMPLE 14 
Example 12 was repeated, except that the amount of powdered slag added was 
3 kg. 
EXAMPLE 15 
Example 12 was repeated, except that the amount of powdered slag added was 
4 kg. 
EXAMPLE 16 
Example 12 was repeated, except that the amount of powdered slag added was 
5 kg. 
The bulk density, hardness, water absorption and resistance to water of the 
products obtained in Examples 1 through 16 were tested. 
The bulk density was determined by filing a graduated cylinder without 
tamping. The hardness was determined according to the standard test ASTM # 
E728-91, and compared with the hardness of granules made from 100 wt. % 
gypsum plaster. The water absorption was determined according to the 
procedure described in U.S. Pat. No. 4,183,763 (col. 11). The resistance 
to water was determined by soaking the granules (2 g) in water for 2-4 
hours and compressing with a spatula. 
The results are reported in the following Table: 
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Bulk 
density Water 
Ex. pound/ absorp- Resistance Suitable 
No. feet.sup.3 Hardness tion % to water application 
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1 46 56% 0.8 Week Animal litter 
2 48 71% 0.65 Moderate Animal litter 
3 49 80% 0.6 Moderate Animal litter 
4 51 86% 0.55 Moderate Animal litter 
5 50.5 99.5% 0.5 Good Animal litter 
6 45.2 83% 0.75 Moderate Animal litter 
7 50 80% 0.6 Good Animal litter 
8 55 92% 0.5 Good Animal litter 
9 58 99% 0.6 Good Animal litter 
10 62 102% 0.5 Very good Animal litter & 
absorbent 
11 69 106% 0.4 Very good Absorbent 
12 56 125% 0.38 Very good Absorbent 
13 58 142% 0.3 Very good Absorbent 
14 63 167% 0.3 Very good Absorbent 
15 68 189% 0.31 Excellent Absorbent 
16 72 207% 0.29 Excellent Absorbent 
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