GB2285766A

GB2285766A - Ceramic casting compositions

Info

Publication number: GB2285766A
Application number: GB9401126A
Authority: GB
Inventors: James Alfred Fooks
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1994-01-21
Filing date: 1994-01-21
Publication date: 1995-07-26
Anticipated expiration: 2014-01-21
Also published as: GB2285766B; FR2715396A1; GB9401126D0; PT101638A; PT101638B; FR2715396B1; DE19501530A1

Description

2285766 CERAMIC CASTING COMPOSITION The invention concerns a process for

improving the properties of clay minerals for use in a composition for forming ceramic articles by a slip casting process.

The process of slip casting is widely used in the ceramics industry for forming articles of tableware and sanitaryware and also for fabricating articles of complex shape which would be difficult to make by any other method.

In the slip casting process a suspension or slip is prepared from a mixture of suitable raw materials, usually china clay, ball clay, quartz and feldspar.

The solid components are dispersed in water, usually at a solids concentration in the range of from about 70 to about 75% by weight of solids, and a dispersing agent is added to control the rheological and casting properties of the slip. The dispersing agent may be, for example, an alkali metal hydroxide or carbonate, a water soluble salt of a polysilicic acid, a water soluble salt of a poly(acrylic acid) or of a poly(methacrylic acid), a water soluble tannate, a water soluble humate or a mixture of any two or more of these reagents.

In the traditional casting process, the slip is poured into a mould made of plaster of Paris and allowed to remain in the mould for a period of time depending on the thickness of the cast required, the rheological properties of the slip and the nature of the raw materials. For example, in making tableware articles, the thickness of which is generally about 2 to 3mm, the casting time may be of the order of from 5 to 15 minutes, whereas in making sanitaryware articles, which are generally required to have a thickness of about 9mm, the casting time may be in the range of from 60 to 120 minutes.

During the casting period, water is drawn out of the slip by capillary action of the plaster mould, and a skin or cast of the clay-containing composition Is formed on the inner surface of the mould. After the required thickness of cast has been built up, the slip remaining in the mould is drained off, and the cast is allowed to dry in the air for a period of time before the mould is opened and the cast removed. For tableware articles this drying period may be up to one hour, but for sanitaryware articles it can be as long as from three to four hours.

Thus it can be seen that the casting of ceramic articles is a relatively slow process. A large number of moulds are required to achieve an acceptable production rate, and the process is quite labour intensive. It has long been an aim of the ceramic casting industry to speed up the process and thus improve productivity and reduce costs. An important parameter is that which is known In the industry as the casting rate, or the rate of increase of the thickness of the cast with time. Small improvements in the casting rate of a ceramic composition can be made by optimising the proportions of each ingredient in the composition and the properties, for example the particle size distribution, of each ingredient. It is also possible to effect improvements by optimising the nature and amount used of the dispersing agent. More substantial improvements in the casting rate can be achieved by raising the temperature of the casting composition, but this tends to lead to problems with the mould and to excessively rapid drying of the cast article with resultant cracking. The casting rate may also be improved by raising the differential pressure across the mould. In the traditional slip casting process this pressure differential is due solely to the capillary suction of the plaster mould and is of the order of 1 bar. The pressure differential may be Increased by applying pressure to the casting slip or vacuum to the outside of the mould or a combination of these two expedients, but the capital cost tends to be high and it is difficult to apply these techniques when 5 the mould is of complicated shape.

In addition to the casting rate, other parameters which are important in the slip casting process are the casting concentration, or the percentage by weight of solids in a slip which is sufficiently fluid to be pourable into the mould, and the green strength of the cast article. The cast articles must be sufficiently strong to be able to withstand the glazing operation and all necessary handling before the firing operation. In the past, attempts to improve the casting rate of a ceramic composition have often resulted in a decrease in the casting concentration or in the green strength of the cast article.

The properties which are desirable in china clays for use in the ceramics industry differ from those which are required in china clays for use in paper coating compositions. China clays for use as paper coating pigments are required to be very white in colour and to have a relatively fine particle size distribution so as to provide a coating of high light reflectivity and high gloss. On the other hand, china clays for use in the ceramics industry are generally of coarser particle size distribution and need not be so white in their unfired state. However, since they are required to give a ceramic article of good white colour after firing, it is necessary that they should have a low content of discolouring impurities such as iron compounds. Also, in order that their fluxing properties should be predictable, it Is necessary that they should have a low and consistent content of impurities containing alkali metal compounds. The amount of sodium and potassium present in the clay is found by chemical analysis, and is expressed as percentage by weight of Na20 and K20 respectively.

According to a first aspect of the present invention, there Is provided a process for preparing a ceramic casting composition which comprises the following steps:

(a) treating a mixture of a kaolin clay and water at a pH in the range of from 2.8 to 7.5 with a minor proportion by weight of a water soluble organic compound having a plurality of basic groups and a weight average molecular weight not greater than 1000; (b) partially dewatering the treated mixture of kaolin clay and water, if necessary, to a plastic mass having a solids concentration in the range of from 50% to 78 by weight; (c) subjecting the resultant mass to mechanical working in the plastic state; and (d) forming the mechanically worked, treated kaolin clay into a ceramic casting composition.

In step (a) the kaolin clay preferably has a particle size distribution such that from 25 to 85 by weight of the particles have an equivalent spherical diameter smaller than 2pm. The mixture of the kaolin clay and water may be a fluid suspension containing from 5 to 30 by weight of dry kaolin clay, in which case the organic compound is mixed with the suspension which is then partially dewatered to a solids concentration in the range of from 50 to 78 by weight, preferably by filtration or by means of a centrifuge. Alternatively, the organic compound may be mixed into a plastic mixture of the kaolin clay and water having a solids concentration in the range of from 50 to 78 by weight. Most advantageously the plastic mass, after partial dewatering, if necessary, has a solids concentration in the range of from 72 to 77 by weight.

The water soluble organic compounds which are suitable for use in the process of the invention may be described as polyacidic organic bases. The water soluble organic compound is preferably chosen from the group consisting of polyethylene imine, hexamethylene tetramine, 1:12 dodecanediamine, ethylene diamine and polyethylene polyamines having the general formula: H2N(C2H4NH)nH, where n is from 2 to 10, which are generally prepared by a condensation reaction between ethylene diamine and an ethylene dihalide, and may have a linear, branched or cyclic structure. Examples of this last class of water soluble organic compounds include diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, and higher members of the series. A combination of two or more of these organic compounds may be used. The amount of the organic compound used is preferably in the range of from 0.01 to 0.1% by weight, based on the weight of dry kaolin clay. If more than about 0.1% by weight of the organic compound, based on the weight of dry kaolin clay, is used it is found that although the casting rate of the kaolin clay, when used in a ceramic casting composition, continues to increase with Increasing dose of the organic compound, the casting concentration becomes unacceptably low. Most preferably, the amount of the organic compound used is in the range from 0.015% to 0.06% by weight, based on the weight of dry kaolin clay.

In step (b) the mechanical work may be exerted upon the plastic mass by means of an auger-type pugmill, a Z-blade mixer, an edge runner mill or a similar device for working plastic materials. The amount of energy dissipated in the plastic mass is preferably in the range from 5 to 250kJ per kilogram of dry kaolin clay. If more than 250ki.kg-1 of energy are dissipated in the plastic mass, it is found that although the casting concentration of the kaolin clay, when used in a casting ceramic composition, continues to increase, the casting rate becomes unacceptably low. Most preferably the amount of energy dissipated in the plastic mass is in the range from 10 to 10OkJ per kilogram of dry kaolin clay.

Most preferably the amount of the organic compound used and the amount of energy dissipated in the plastic mass are adjusted so that the casting concentration of the treated kaolin clay, as measured as described below, is at least 3.5 percentage points higher than that of the untreated kaolin clay, and the casting rate, as measured as described below, is at least 1.Omm2.min-l.

in step (c) the ceramic casting composition may have a composition according to the following formulation:by weight Kaolin Clay Ball Clay Quartz Fluxing Material 15-60 0-35 20-30 15-26 The fluxing material is preferably feldspar or nepheline syenite.

The casting concentration of a kaolin clay for use in a ceramic casting composition is measured by adding to deionised water 250g of the kaolin which has been dried at 600C to a water content of less than lt by weight and crushed to pass through a sieve of aperture size 2mm. The amount of water used is based on an estimation of the expected casting concentration, and only gentle hand stirring is used to prepare the mixture. As the clay is added to the water, the mixture eventually become too viscous to stir and P84 grade sodium silicate solution, which has been diluted to 50% wt/vol, is run into the mixture from a burette as a dispersing agent. The dispersing agent is added in increments of 0.2m1 and the mixture is stirred by hand after each increment. Five minutes after each increment, the viscosity of the mixture is determined by means of a Brookfield Viscometer using Spindle No.3 at 20rpm. The procedure of adding an increment of the dispersing agent and measuring the viscosity of the mixture is repeated until a minimum value of the viscosity has been reached, which should be greater than 50OmPa.s If the initial quantity of water used was correctly estimated. Deionised water is then added until the measured viscosity has fallen to 50OmPa.s. A sample is then taken from the clay suspension, weighed, dried and reweighed to determine the percentage by weight of dry kaolin clay in the suspension. This percentage by weight is recorded as the casting concentration of the suspension.

The casting rate of the kaolin clay is measured by casting samples of the suspension which has a measured viscosity of 50OmPa.s in standard plaster of Paris moulds for different lengths of time. The thickness of the cast obtained from each mould is measured and a graph is drawn of the square of the thickness in millimetres against the time in minutes. A straight line should be obtained, the slope of which is recorded as the casting rate in mm2.min-1 According to a second aspect of the present invention, there is provided a process for preparing a cast article comprising the steps of:

(a) introducing into a mould a casting composition prepared by a process in accordance with the first aspect of the present invention; (b) permitting a cast to develop; (c) removing residual casting composition from the mould and permitting the cast to dry; and -a- (d) removing the dried cast from the mould.

According to a third aspect of the present invention, there is provided the use, as an ingredient of a clay casting composition, of a kaolin clay 5 prepared by the following steps:

(a) treating a mixture of a kaolin clay and water at a pH in the range of from 2.8 to 7.5 with a minor proportion by weight of a water soluble organic compound having a plurality of basic groups and a weight average molecular weight not greater than 1000; (b) partially dewatering the treated mixture of kaolin clay and water, if necessary, to a plastic mass having a solids concentration in the range of from 50% to 78% by weight; and (c) subjecting the resultant mass to mechanical working in the plastic state.

Two alternative methods for carrying out the invention are illustrated in the accompanying drawings in which:

Figure 1 is a diagrammatic flow chart of a process in which the water soluble organic compound is mixed with a relatively dilute aqueous suspension of the kaolin clay; and Figure 2 is a diagrammatic flow chart of a process in which the water soluble organic compound is mixed with a plastic mass of the kaolin clay.

In the processes of both Figure 1 and Figure 2, an aqueous suspension containing from 5 to 30% by weight of a ceramic grade kaolin clay is introduced through a conduit 1 and is partially dewatered by means of a filter press 4 to form a cake having a plastic consistency and a water content in the range of from about 30 to about 35% by weight. The filter cake is conveyed to an auger-type pugmill 6 which extrudes the plastic material to form vermiform pieces which are thermally dried in a dryer 7. If desired, a proportion -g- of the dry product of the dryer 7 may be recycled by the route 8 to be mixed with the plastic clay in the pugmill 6 to reduce the water content of the plastic mass to within the range from about 22% to about 30% by 5 weight.

In the process of Figure 1, an additional mixing tank 3 is situated upstream of the filter press 4 to enable the aqueous suspension of the kaolin clay to be mixed with an aqueous solution of the organic compound, which is supplied through a conduit 2.

In the process of Figure 2, an aqueous solution of the organic compound is fed directly into the augertype pugmill through a conduit 5.

The invention will now be illustrated by reference to the following examples:

EXAMPLE 1

A kaolin clay from a French source having a particle size distribution such that 64% by weight of the particles had an equivalent spherical diameter smaller than 5pm and 43 by weight of the particles had an equivalent spherical diameter smaller than 2pm was mixed with water to form a suspension containing 20% by weight of dry kaolin clay and having a pH of 4.0. The suspension was divided into portions, some of which were treated with a polyethylene polyamine having a weight average molecular weight of 309 in amounts of 0.015%, 0.03 or 0.075% by weight, based on the weight of dry kaolin. Each portion was then dewatered by pressure filtration, and each batch of filter cake was further divided into sub-portions which were subjected to different amounts of mechanical work in a laboratory auger-type pugmill. In order to dissipate the required amount of work in the plastic filter cake, it was found to be necessary to reduce the water content of each sub-portion to within the range of from 25 to 28% by weight. After treatment each of the sub-portions of plastic material were dried at 600C to a water content of less than 1 by weight and the casting concentration and casting rate of the kaolin clay in each sub-portion were measured as described above.

The results obtained are set forth in Table 1 below:- Table 1 by weight of polyethylene Energy polyamine dissipa!rd (kJ.kg- 0 0.015 0.03 0.075 0 cc 62.1 62.5 61.6 59.5 CR 2.5 3.0 5.4 5.6 is cc 65.1 65.1 CR 1.7 1.9 36 cc 65.0 66.1 CR 0.92 -.1.4 54 cc 65.9 66.2 CR 0.58 1.3 68 cc 66.8 67.5 CR 0.65 1.1 cc 67.5 68.0 66.7 67.0 CR 0.45 0.56 0.61 0.80 151 cc 68.1 68.8 CR.59 0.58 Note: CC is the casting concentration ( by weight of dry kaolin) CR is the casting rate (mm2.min-1).

It will be observed that the three sub-portions which satisfy the criteria that the casting concentration should be at least 3.5 percentage points greater than that of the untreated kaolin and that the casting rate should be at least lmm2.min-1 are those which have been treated with 0.03 by weight, based on the weight of dry kaolin, of the polyethylene polyamine and have been subjected to mechanical working sufficient to dissipate in the plastic mass from 36 to 68kJ.kg-1.

EXAMPLE 2

A kaolin clay from a Portuguese source having a particle size distribution such that 15% by weight of the particles had an equivalent spherical diameter larger than lOpm and 50% by weight of the particles had an equivalent spherical diameter smaller than 2pm was mixed with water to form a suspension containing 20t by weight of dry kaolin clay and having a pH of 4.2. The suspension was divided into portions, some of which were treated with the same polyethylene polyamine as was used in Example 1 in amounts of 0.03%, 0.06t or 0.1t by weight, based on the weight of dry kaolin. Each portion was then dematered by pressure filtration, and each batch of filter cake was further divided into sub- portions which were subjected to different amounts of mechanical work In a laboratory auger-type pugmill as described in Example 1 above. After treatment each of the sub-portions of plastic material were dried at 600C to a water content of less than 1% by weight and the casting concentration and casting rate of the kaolin clay in each sub-portion were measured as described above.

The results obtained are set forth in Table 2 below:- Table 2

Energy by weight of polyethylene dissipated polyamine (kJ.kg 1) 0 0.03 0.06 0.1 0 cc 58.7 60.0 59.2 57.9 CR 1.5 1.9 2.8 3.9 29 cc 63.0 62.4 61.1 CR 1.4 1.5 1.9 61 cc 62.2 62.5 63.4 62.3 CR 0.6 1.0 1.2 1.7 108 cc 62.3 62.9 CR 0.3 0.8 cc 63.2 64.5 64.1 62.9 CR 0.2 0.5 0.6 1.0 Note: CC is the casting concentration (% by weight of dry kaolin) CR is the casting rate (mm2.min-1).

It will be observed that the sub-portions which satisfy the criteria that the casting concentration should be at least 3. 5 percentage points greater than that of the untreated kaolin and that the casting rate should be at least lmm2.min-1 are those which have been treated with 0.03% or 0.06t by weight of the polyethylene polyamine and subjected to mechanical working such that 29 or 61kJ of energy per kilogram of dry kaolin clay have been dissipated in the plastic mass. It is also possible to make an acceptable product if the clay suspension is treated with 0.1% by weight of the polyethylene polyamine, but at the expense of an unnecessarily high expenditure of both reagent and energy.

EXAMPLE 3 A kaolin clay from a Cornish source having a particle size distribution such that 22% by weight of Z the particles had an equivalent spherical diameter larger than lOpm and 35 by weight of the particles had an equivalent spherical diameter smaller than 2pm was mixed with water to form a suspension containing 27. 5 5 by weight of dry kaolin clay and having a pH of 3.8. The suspension was divided into portions, some of which were treated with the same polyethylene polyamine as was used in Example 1 in amounts of 0.03, 0.06% or 0.1 by weight, based on the weight of dry kaolin.

Each portion was then dewatered by pressure filtration, and each batch of filter cake was further divided into sub- portions which were subjected to different amounts of mechanical work in a laboratory auger-type pugmill as described in Example I above. After treatment each of the sub-portions of plastic material were dried at 600C to a water content of less than lt by weight and the casting concentration and casting rate of the kaolin clay in each sub-portion were measured as described above.

The results obtained are set forth in Table 3 below:- Table 3

Energy by weight Of polyethylene dissipated polyamine 1 (kJ.kg 0 0.03 0.06 0.1 0 cc 64.6 64.4 63.8 63.1 CR 2.2 4.3 8.7 10.2 72 cc 68.3 69.5 69.1 68.9 CR 0.9 1.7 3.1 4.6 cc 69.0 CR 0.6 248 cc 69.8 CR 0.5 Note: CC is the casting concentration (% by weight is of dry kaolin) CR is the casting rate (mm2.min-1).

It will be observed that the sub-portions which satisfy the criteria that the casting concentration should be at least 3. 5 percentage points greater than that of the untreated kaolin and that the casting rate should be at least Imm2.min-1 are those which were treated with from 0.03% to 0.1% by weight of the polyethylene amine and which were subjected to mechanical working such that there was dissipated in the plastic mass 72kJ of energy per kilogram of dry kaolin.

11

Claims

CLAIMS:

1. A process for preparing a ceramic casting composition which comprises the following steps:

(a) treating a mixture of a kaolin clay and water at a pH in the range of from 2.8 to 7.5 with a minor proportion by weight of a water soluble organic compound having a plurality of basic groups and a weight average molecular weight not greater than 1000; (b) partially dewatering the treated mixture of kaolin clay and water, if necessary, to a plastic mass having a solids concentration in the range of from 50% to 78% by weight; (c) subjecting the resultant mass to mechanical working in the plastic state; and (d) forming the mechanically worked, treated kaolin clay into a ceramic casting composition.

2. A process according to claim 1, wherein, in step (a), the kaolin clay has a particle size distribution such that from 25% to 85% by weight of the particles have an equivalent spherical diameter smaller than 2pm.

3. A process according to claim 1 or 2, wherein the mixture of the kaolin clay and water is a fluid suspension containing from 5 to 30% by weight of dry kaolin clay and wherein the organic compound is mixed with the suspension which is then partially dewatered to a solids concentration in the range of from 50% to 78 by weight.

4. A process according to claim 1 or 2, wherein the organic compound is mixed into a plastic mixture of the kaolin clay and water having a solids concentration in the range of from 50% to 78% by weight.

5. A process according to any one or more of the preceding claims, wherein the plastic mass, after partial dewatering, if necessary, has a solids concentration in the range of from 72% to 77% by weight.

6. A process according to any one or more of the pFeceding claims, wherein the water soluble organic compound is chosen from the group consisting of polyethylene imine, hexamethylene tetramine, 1:12 dodecanediamine, ethylene diamine and polyethylene polyamines having the general formula: H2N(C2H4NH)nH, where n is from 2 to 10.

7. A process according to any one or more of the preceding claims, wherein, in step (b), the mechanical work is exerted upon the plastic mass by means of an auger-type pugmill, a Z-blade mixer, an edge runner mill or a similar device for working plastic materials.

8. A process according to any one or more of the preceding claims, wherein the amount of energy dissipated in the plastic mass is In the range from 5 to 250ki per kilogram of dry kaolin clay.

9. A process according to any one or more of the preceding claims, wherein the amount of the organic compound used and the amount of energy dissipated in the plastic mass are adjusted so that the casting concentration (as hereinbefore defined) of the treated kaolin clay is at least 3.5 percentage points higher than that of the untreated kaolin clay.

10. A process according to any one or more of the preceding claims, wherein the amount of the organic compound used and the amount of energy dissipated in the plastic mass are adjusted so that the casting rate (as hereinbefore defined) is at least l.omm2.min-1.

11. A process according to any one or more of the preceding claims, wherein the ceramic casting composition has the following formulation, in % by weight of the total composition:

treated kaolin clay 15-60 ball clay 0-35 quartz 20-30 fluxing material 15-20.

jz

12. A process according to claim 11, wherein the fluxing material is feldspar or nepheline syenite.

13. A process for preparing a cast article comprising the steps of:

(a) introducing into a mould a casting composition prepared by a process in accordance with the first aspect of the present invention; (b) permitting a cast to develop; (0) removing residual casting composition from the mould and permitting the cast to dry; and (d) removing the dried cast from the mould.

14. The use, as an ingredient of a clay casting composition, of a kaolin clay prepared by the following steps:

(a) treating a mixture of a kaolin clay and water at a pH in the range of from 2.8 to 7.5 with a minor proportion by weight of a water soluble organic compound having a plurality of basic groups and a weight average molecular weight not greater than 1000; (b) partially dewatering the treated mixture of kaolin clay and water, if necessary, to a plastic mass having a solids concentration in the range of from 50% to M by weight; and (c) subjecting the resultant mass to mechanical working in the plastic state.