Ceramic material

A porous granular ceramic material has a granule size of from 1 to 5 mm, a bulk density of from 200 to 700 g/l, preferably 350 to 550 g/l, a porosity of from 50 to 80% v/v, preferably from 65 to 80% v/v, and a liquid absorbency of from 20 to 200%, preferably from 40 to 150%, of water based on the weight of the granular material. The material may be made by calcining a mixture of a clay and a particulate combustible material. The material is suitable for use as animal litter or as an industrial absorbent for liquids.

The present invention relates to porous ceramic materials and to methods 
for their production. In particular, the present invention provides a 
porous ceramic material having absorbent properties which, at least when 
in a granular form, make it suitable for such end uses as animal litter or 
industrial absorbent for liquids. 
The present invention provides a porous granular ceramic material having a 
granule size predominantly within the range of from 1 to 5 mm, a bulk 
density within the range of from 200 to 700 g/l, a porosity within the 
range of from 55 to 80% v/v, and a liquid absorbency within the range of 
from 20 to 200% of water based on the weight of granular material. The 
material preferably has a bulk density within the range of from 350 to 700 
g/l, and a liquid absorbency within the range of from 40 to 150%, more 
preferably 70% to 150%, of water based on the weight of granular material. 
More preferably, the material has a bulk density of from 350 to 550 g/l, 
and a porosity of from 65 to 80% v/v. 
In one aspect of the invention, in such a porous granular ceramic material 
the pores are constituted by voids derived from the combustion of a 
combustible material previously contained therein. The said combustible 
material is advantageously lignite. The ceramic material is preferably a 
clay-based material. 
The present invention further provides a method for the preparation of a 
porous ceramic material comprising calcining a mixture of clay and a 
particulate combustible material, the temperature and time of calcination 
being such that a substantial amount of the clay minerals remain 
unvitrified and at least a part of the combustible material is burned off. 
Preferably, when the product is to be used as an animal litter, 
substantially all the combustible material is burnt off. 
The temperature of calcination is preferably in the range from 800.degree. 
C. to 1100.degree. C., more preferably from 1000.degree. C. to 
1100.degree. C. 
As examples of clays suitable for use in the invention there may be 
mentioned ball clay, especially carbonaceous clay. 
When the porous material of the invention is produced from a mixture of 
clay and lignite the ratio of clay:lignite is preferably within the range 
of from 1:9 to 9:1, more preferably from 1:2 to 2:1. 
The mixture of clay and a combustible material may be obtained by mixing 
particulate or powdered clay and combustible material with sufficient 
water to cause the particles to form agglomerates, and drying the 
agglomerated mixture. 
If desired or required, a surfactant or deflocculating agent may be 
employed to assist in forming a satisfactory mixture. 
The aqueous material is preferably dried to a moisture content of not more 
than 5%, wet basis. The mixture is preferably dried at a temperature of 
not greater than 200.degree. C. Advantageously, the conditions of the 
calcination step are such that the weight loss of the mixture is from 40 
to 65%, preferably 40 to 55%, based on the weight of the dried mixture. 
In a preferred process according to the invention the moist mixture is 
shaped prior to drying and, if necessary, the dried mixture is divided 
into granules. The mixture may, for example, be shaped by extrusion, for 
example using a pug mill or brick extruder, and the extrudate dried, 
broken and screened to recover granules within the desired size range. Any 
material screened out may be recycled for use so that the process produces 
very little waste. 
In a particularly advantageous manner of operation the mixture is extruded 
onto a flat surface so that the granules have at least one flat surface. 
This allows more efficient packing of the granules in a bed and, when used 
as animal litter, the improved packing reduces liquid penetration through 
the bed. 
Alternatively, the moist mixture may be formed directly into granules. 
Granulation may occur in the mixer or the mixture may be subjected to 
pelletisation for example on a drum or pan pelletiser. 
Preferably, the particle sizes of the clay and of the combustible material 
in the starting mixture are each not greater than 2 mm. 
Some carbonaceous clays are associated geologically with lignite and such 
clays may contain sufficient lignite to be used in their raw state in the 
method of the invention without the addition of further combustible 
matter. In that case such a clay may be used alone in place of the 
combination of clay and combustible material. 
Preferably, when the product is to be used as animal litter, substantially 
all the combustible material is burned off during calcination. With many 
clays, e.g., carbonaceous clays, this enables a white product to be 
obtained. 
Thus, in accordance with a preferred embodiment of the invention the 
porous, granular ceramic product is made by forming a moist mixture 
comprising clay and lignite, shaping, drying and subdividing the mixture 
to form granules, and calcining the granules to burn out the lignite. 
The product usually has a substantially neutral pH and is usually strong 
enough to resist attrition and dust formation in use. The material has an 
open-pore structure, the matrix of which is permeable. That is, the 
material contains a network of relatively large pores that are 
interconnected by relatively small pores. 
The properties of the product depend upon a variety of factors, which may 
be adjusted in order to obtain a product of particular desired 
characteristics. These factors include, for example: the type of clay 
employed, which affects the colour and strength of the product; the 
particle sizes of the clay and combustible material, which determine the 
pore size and the absorption properties; the ratio of clay to combustible 
material, which controls the density and hence the porosity and strength; 
the drying time and temperature, which may affect the particle shape and 
also the strength; and the kiln environment (that is, the temperature, 
firing profile and atmosphere), which can affect the colour, density, 
strength and absorption properties. For example, an increased strength may 
be obtained in a material according to the invention by using a higher 
drying temperature or a higher kiln temperature. 
Where an extrusion procedure is employed, the type of extruder will affect 
the strength and particle shape of the final product. The type of 
comminuting or crushing device used to break the material after shaping 
and drying will also affect the particle shape and size distribution of 
the product. 
The following examples illustrate the invention:

EXAMPLE 1 
50 kg of milled clay (moisture content 28% wet basis), 90 kg milled lignite 
(moisture content 31.8% wet basis) and 14.7 liters of water are placed in 
an Eirich mixer and mixed for 5 minutes. The resulting moist granular 
mixture is fed into a pug mill or brick extruder, which compresses the 
mixture and extrudes it in the form of a continuous rod or rods. The rods 
are passed through a drier at 100.degree. C. until the moisture content is 
reduced to not more than 5% wet basis. This hardens the rods so that they 
may be crushed and sieved to recover granules of a size range of 1-5 mm. 
The selected granules are finally calcined by passage through a rotary 
kiln at 1050.degree. C. until substantially all the lignite is burned off. 
The product of this process is a porous granular ceramic material of 
irregular shape, with a granule size within the range of from 1 to 5 mm. A 
bulk product density of 460 g/l is achieved by this process. The product 
is of substantially neutral pH and is capable of absorbing 77% of its own 
weight of water or urine. The porosity of the product is within the range 
of from 55 to 80% v/v. The product has high intrinsic strength and is very 
resistant to attrition and dust formation, both dry and in use. The 
properties of the material reduce the amount of scattering of material by 
animals when used as litter. Discoloration of the product in use is 
minimal. 
(The bulk density of the product is calculated by determining the weight of 
a given volume (not less than 500 cm.sup.3) of material treated in 
accordance with B.S.1460 but using manual tapping. The liquid absorption 
is determined by covering the material with liquid, mixing, and leaving to 
stand for 5 minutes, the residual liquid then being removed. The 
absorption in % is calculated from the material weight (W.sub.m) and 
liquid weight absorbed (W.sub.l) according to the equation: (W.sub.l 
.times.100)/W.sub.m. The porosity is determined using the same method, 
except that the porosity is calculated from the material volume V.sub.m 
and the volume of liquid absorbed V.sub.l according to the equation: 
(V.sub.l .times.100)/V.sub.m. The volume V.sub.m is the volume of the 
material per se, that is, excluding the volume between particles. The pH 
of the material is determined by dispersing particles of the material of 
size less than 190! in deionised water and after mixing allowing to stand 
for at least 12 hours, the sample then being stirred for one minute prior 
to measuring the pH of the solution.) 
EXAMPLE 2 
50 kg of carbonaceous clay (moisture content 28% wet basis) and 90 kg of 
lignite (moisture content 31.8% wet basis) are milled separately to a 
particle size of below 2 mm and then placed in an Eirich mixer with 14.7 
liters of water and mixed for 2 minutes. A mixture containing discrete 
granules is obtained. The granules are dried directly at 150.degree. C. 
for 20 minutes. Fines and oversize material are recycled to the mixing 
stage. The remainder is fired at 1070.degree. C. for 20 minutes in an 
excess of oxygen. 
The product obtained has all the properties of that of Example 1, except 
that the strength is marginally less. This process avoids the use of 
extruder machinery and is therefore cheaper to operate. Furthermore, water 
or urine absorption of the product may in some cases be greater than that 
of an extruded material made from a mixture of the same composition. 
EXAMPLE 3 
50 kg of milled carbonaceous clay (moisture content 28% wet basis), 90 kg 
of milled lignite (moisture content 31.8% wet basis), and 14.7 liters of 
water are placed in an Eirich mixer and mixed for 5 minutes. The resulting 
moist granular mixture is extruded in the form of a continuous sheet by a 
brick extruder or a twin roller extruder onto the belt of a continuous 
dryer. The sheet is passed through the dryer at 150.degree. C. until the 
moisture content is not more than 5% wet basis. The embrittled sheet is 
further broken by a roller crusher and after sieving granules of a size 
range of 1-5 mm are recovered. Fines are recycled to the mixing stage and 
oversize material is recycled to the crusher. The remainder is calcined by 
passing through a rotary kiln at 1050.degree. C. until substantially all 
the lignite is burned off. 
The product has similar properties to that of Example 1 except that the 
granules now have at least one flat surface. This allows the formation of 
an improved packed bed and, when used as animal litter, reduces liquid 
penetration through the bed. 
EXAMPLE 4 
50 kg of clay, 30 kg of lignite and 10 liters of water are placed in a 
mixer and mixed for 15 minutes. The resulting moist granular mass is fed 
into a brick extruder, which compresses the granules and extrudes them in 
the form of a continuous rod or rods. The rods are passed through a drier 
at 100.degree. C. This hardens the rods so that they may be crushed and 
sieved to recover a particles in the size range 1 to 5 mm. The selected 
particles are finally calcined by passage through a rotary kiln at 
1050.degree. C. 
The product of this process is a granular ceramic material of irregular 
shape, porous appearance, and a particle size within the range 1 to 5 mm. 
Bulk product densities of 250 to 650 g/l may be achieved by this process, 
but are preferably between 350 and 500 g/l. The product is of neutral pH 
and may absorb 70 to 200%, preferably 100 to 150%, of its own weight of 
water or urine. The product has high intrinsic strength and is very 
resistant towards attrition and dust formation, both dry and in use. The 
material is therefore not scattered or trodden about by animals when used 
as litter. Discoloration of the product in use is minimal. 
EXAMPLE 5 
50 kg of carbonaceous clay and 30 kg lignite are milled to below 2 mm and 
then placed in a mixer with 10 liters of water. After 15 minutes mixing, 
discrete granules are formed which can be dried directly at 150.degree. C. 
for 10 minutes. Fines and oversize material are recycled to the mixing 
stage. The remainder is fired at 1070.degree. C. for 10 minutes in an 
excess of oxygen. 
The product obtained has all the qualities of that of Example 4, except 
that the strength is marginally reduced. This process avoids the use of 
extruder machinery and is therefore cheaper to operate. Furthermore, the 
liquid absorption capability of the product is greater than that of an 
extruded material, being between 100 and 300% weight for weight, and more 
usually 150 to 250%. The use of carbonaceous clay gives a white fired 
product. 
EXAMPLE 6 
50 kg of carbonaceous clay is dispersed in 20 liters of water with the aid 
of 100 g sodium silicate as deflocculant. 35 kg lignite is separately 
dispersed in 20 liters of water with mixing. The two components are then 
mixed together for 15 minutes and the resultant mass extruded in sheet 
form onto the belt of a continuous drier. Drying at 105.degree. C. for 15 
minutes embrittles the sheet, causing it to fracture under its own weight. 
The broken pieces are conveyed onto a vibratory screen which collects 
material 1-5 mm in size, rejects fines, and recycles oversize material. 
The 1-5 mm fraction is fired at 1100.degree. C. for 2 minutes in a 
toroidal bed kiln. 
The product has similar properties to that of Example 4 except that the 
granules now have one flat surface. This enables them to form a better 
packed bed and, when in use as animal litter, reduces liquid penetration 
through the bed.