EP2398751A2

EP2398751A2 - Procedure to improve ball clays to be used in the manufacture of ceramic products

Info

Publication number: EP2398751A2
Application number: EP10711024A
Authority: EP
Inventors: José Javier MENENDEZ MEDINA; David VARELLA SÁNCHEZ; María Fernanda GAZULLA BARREDA; Francisco Javier GARCÍA TEN
Original assignee: Nanobiomatters Industries SL
Current assignee: Nanobiomatters Industries SL
Priority date: 2009-02-20
Filing date: 2010-02-19
Publication date: 2011-12-28
Also published as: WO2010094771A3; MX2011008841A; WO2010094771A2; BRPI1005986A2; EP2226307A1

Abstract

The present invention describes a procedure for enhancing kaolinitic clays characterized in that it comprises the steps of obtaining an aqueous suspension of clay; removing of the fraction greater than 60 μm using physical separation procedures; classifying of the clay in suspension on the basis of the particle size, using mechanical means; treating of the fraction with the smallest particle size; removing the water in the suspension in order to obtain the cake; and washing the cake with water in order to reduce the content of soluble salts, such as sulphates and carbonates, before draining. This procedure permits increasing the content of clayey material, removing the unwanted large particles, increasing the whiteness of the clays after baking and reducing the percentage of defects after baking.

Description

PROCEDURE TO IMPROVE BALL CLAYS TO BE USED IN THE MANUFACTURE OF CERAMIC PRODUCTS.

The present invention is related to a procedure to improve the different types of ball clays, hereinafter clays, which allows to obtain a proper product to be used in the manufacture of ceramic and porcelain floor tiles, specifically for their use in ceramic bodys and engobes.

This invention applies to the ceramic materials sector, and to be more specific, is aimed at improving white ball clays for the manufacture of this kind of materials.

BACKGROUND OF THE INVENTION

At present, clays and washed kaolin, amongst other components, are used in the manufacture of supports and engobes for ceramic and porcelain tiles. A series of procedures have been developed to obtain and purify them, some of them are mentioned following:

(a) Methods for purifying kaolin by magnetic separation, so that it can be used as a coating or paper filler. Some of the most outstanding ones are described in the following documents:

- WO9958613, where a procedure is described for obtaining a very shiny kaolin pigment. The procedure is based upon selecting kaolin that has a volume of particles less than 2 μm, ranging from

30% to 60% of the total, which is then used to prepare a suspension that is treated magnetically. The kaolin is then subjected to a grinding process and classified with a view to obtaining a certain shape factor for its particles. This procedure is applied to obtaining a coating for paper products.

- GB1 122523, where a process is described for improving the shine on raw kaolin by preparing a suspension that is subjected to a high-intensity magnetic field (between 8,500 and 18,000 Gauss) for a period of time ranging from 30 seconds to 8 minutes. The size of the particles in the kaolin suspension is less than 44 μm, to be specific, 90% of the particles are smaller than 2 μm. Although it is not expressly mentioned, this procedure is applied to the mineral loads in a variety of products (paper, plastic, etc.).

- US6312511 , where a procedure is described for obtaining a high brightness kaolin, which involves applying a magnetic treatment to a kaolin suspension followed by a separation process to ensure that 90% of the particles are less than 2 μm. The product obtained is used as paper filler or as a coating for paper.

(b) Methods for improving raw materials or mixtures of raw ceramic materials by applying magnetic treatment to these materials when they are in suspension. The following are some of the outstanding examples of these methods:

- GB1469766 where a method is described for the production of white-fired ceramic compositions that consists of preparing a suspension of the entire composition (mixture of raw materials), which is treated magnetically with field strengths that range from 10,000 to 60,000 Gauss.

- GB1004570 where a method is proposed for manufacturing a white-fired kaolinitic clay whose particle size ranges from 5 to 75 μm, by means of a magnetic separation (magnetic field greater than

10,000 Gauss).

- DD268235 where a procedure is described for removing magnetic minerals of iron and titanium by means of a high-intensity magnetic separator. The procedure involves preparing suspensions from mixtures of raw materials (mainly kaolin and feldspars), which are screened in a sieve at 90 μm, treated magnetically and then mixed with the rest of the components of the composition. They are finally subjected to a filtering and pressing process.

To date, the process used for benefiting or improving kaolin is not used to purify fired white clays for use in the composition of ceramic tiles, engobes, as the concentration and purification of kaolinite is not possible using economically profitable methods and are therefore used directly in the industry without any purification treatment.

The factors that hamper the processing of fired white clays include the smaller particle size of philosilicates compared to that of kaolins and their higher content of small-sized impurities, which prevents adequately executing the size-based separation, magnetic separation and suspension drying stages.

SUMMARY OF THE INVENTION

The present invention is related to a procedure for enhancing ball clays with a composition and a purity grade so that they can be used in the manufacture of ceramic floor tiles. To be more specific, the ball clays obtained by the procedure of the present invention can be used for obtaining the supports and engobes for ceramic floor tiles and wall tiles

In a first aspect, the present invention is related to a procedure to improve ball clays which comprises the following steps:

(i) Preparing an aqueous suspension with the kaolinitic clay or clays either by grinding or by dispersion; and, occasionally, flocculants of an organic and/or inorganic nature can be used, treated to reduce viscosity, as well as reactive additives such as peroxides or weak acids which improve the decomposition of organic material and reduce the colloid particles of soluble iron oxide.

(ii) Separation of the fraction with a particle size over 60 μm (dgo), through carrying out a physical separation process, by screening or sifting the suspension and/or using hydrocyclons, where the mineral particles which provide mainly iron and titanium oxides are removed.

(iii) Classification with centrifugal decanter or similar machine on the basis of the size of the clay suspension, to enrich the suspension containing the fraction with the smallest particle size; typical values are between 45 μm and 3 μm (dgo), and allowing a regulation in the percentage of alumina in respect to the silica, thus permitting the design of the melting grade of the treated clay.

(iv) Magnetic separation of the fine fraction that is obtained at the third step, with a superconducting magnet where the slip is sieved internally through the use of magnetic fields whose intensity is between 8,000 and 20.000 Gauss; in this way all the paramagnetic fine particles are removed and they are not incorporated to the final phase of the treated clay.

(v) Removing of water in the suspension to obtain a cake so that part of the soluble reactants is also removed, as well as an important percentage of the soluble salts typical of the clayey minerals.

(vi) Washing of the cake obtained in the previous step with a filtering centrifuge or similar machine for the separation of soluble salts, basically sulphates and carbonates, by carrying out at least a procedure of ongoing washing and draining steps. This procedure can be made successive times introducing low conductivity water until low levels of sulphates and carbonates are reached. Normally, clays for engobes need more washing steps than those for the body of the ceramic floor tiles.

In the present invention, the term "clay" relates to a naturally occurring material mainly composed of fine-grained minerals that is generally plastic, has the appropriate water content and is hardened by drying or firing. Although clays normally contain philosilicates, they can also contain other materials that confer plasticity and hardening by drying or firing, in addition to associated phases, which are materials that do not confer plasticity, and organic matter. The associated phases can be both crystalline (quartz, chalcite, dolomite, feldspars, iron and titanium compounds, organic phases, etc.) and non-crystalline (colloidal silica, iron hydroxide gels, organic gels, etc.). Clays are, therefore, natural clays.

In accordance with the definition of the CMS (Clay Minerals Society) and AIPEA (International Association for the Study of Clays) nomenclature committees, two types of kaolinite-bearing materials can be distinguished: kaolins and fired white clays or kaolinitic clays. Kaolin is formed by the alteration of granitic rocks or feldspar-rich sedimentary rocks, does not show plasticity due to the low kaolinite phase content and is therefore not considered clay. However, kaolinitic clays are fine-grained sedimentary rocks containing a large proportion of kaolinite, show plasticity and are, strictly speaking, clays. When kaolins are processed by means of washing, the finest fractions are enriched, which are those that contain the kaolinite, giving rise to a material that is no longer a natural material, due to which it cannot be included under the term clay.

The starting clays used in this procedure can have a variable mineral composition, although at least contain illite, mica, kaolinite and quartz. The aims of the described procedure are to increase the content of clayey minerals, to remove the impurities with respect to the high size of the particle, to improve the degree of whiteness after baking and to reduce the soluble salt content. The advantages of this procedure are as follows: - To provide clays with a higher content of clayey minerals, which increases the AI2O3 - Siθ2 ratio, thus increasing the plasticity of the clay and allows to manipulate the clay fusibility to a desired point, reducing the time needed for firing in an industrial kiln. Reducing the fusion point with treated clay is much more useful than with alkalis, due to the temperature range of firing is not reduced

- To decrease the proportion of impurities involving the presence of a high particle level, reducing the number of surface defects upon the ceramic tile.

- To reduce the proportion of minerals that contain iron, which leads to the product being whiter after it has been fired.

- To reduce the soluble salt content, improving the rheological performance of the suspension, overcoming the problem of pinholes that causes the sulphate sales on the fired tiles.

In a preferred embodiment, the aqueous suspension obtained in the step (i) posses a solid contain between 10% and 70% in mass.

Clays with a higher content of clayey minerals can be obtained thanks to the invention thus described, which increases the AI2O3 - Siθ2 ratio, giving them a higher degree of plasticity. Along the same lines, this clay yields a lower percentage of impurities involving high particle sizes, which reduces the amount of surface defects in the end product. Finally, the invention also yields a lower proportion of minerals that contain iron, which leads to an improvement in the degree of whiteness after baking, and reduces the content of soluble salts, improving the rheological performance of the suspension, preventing certain defects from affecting the fired floor or wall tiles.

During stage (v), wherein water is eliminated from the suspension, which is preferably carried out by means of filter pressing, a large part of the water is eliminated, thus removing a portion of the soluble salts present in the clay. The small size of the clays hampers this operation, which provides cakes with a humidity of between 20% and 25% and, therefore, having a considerable amount, although less than the original, of soluble salts. The newly formed cake is washed once or twice in order to reduce the amount of soluble salts therein, given that it is carried out using water with a scarce salt content. This cake-washing process is not normally carried out in kaolins due to their lower content of soluble salts and lower humidity in the newly formed cake.

Another aspect of the present invention is related to a material obtained by the procedure described above.

A last aspect of the present invention is related to the use of the material described above for the obtaining of engobe for the manufacture of ceramic and porcelain floor tiles.

In the present invention, "engobe for bodys" means glazes developed for each type of body keeping in mind the expansion properties of the body in relation to temperature, the firing cycle employed and the plastic conditions as effected by the glazing lines.

Throughout the description and the claims, the use of the word "includes" and other synonyms in this context, is not to be interpreted as excluding other technical characteristics, additives, components or steps to be taken. For experts on the subject, other objects, advantages and characteristics of the invention will be apparent, partly from the description and partly from practical aspects of the invention. The following example is provided by way of illustration, but in no way is it to be interpreted as being in any way restrictive where this invention is concerned. Furthermore, this invention covers all the potential combinations of specific and preferred realisations that are indicated here.

EXAMPLES

This invention is also illustrated by means of the following examples, whose application is likewise non-restrictive:

Example 1 : Clay used in the manufacture of white-fired ceramic supports is used, with a content of 3.80% in weight of Fe₂O₃, 0.87% in weight of Tiθ2, 22% AI₂O₃ and 3,478 ppm. of sulphur. The clay is dispersed with water and deflocculant (1 % in weight of sodium silicate) in a turbo dilution device until a solid content of 41 % in weight is achieved. The average size of the particles of clay in suspension (d50) is 9.2 μm.

After that, and first of all, a first grain-size cut of the clay suspension is made at 100 μm, with a view to obtaining a sieving with 2.68% in weight of Fe₂O₃, 0.93% in weight of TiO₂ and 23% in weight of AI₂O₃. The d50 of the clay suspension is 9.1 μm. A second grain-size cut is then made, thus obtaining a fine fraction d50 = 7.9 μm. The content of this fraction is 2.85% in weight of Fe₂O₃, 0.93% in weight Of TiO₂ and 26.4% in weight Of AI₂O₃.

After that, the one single step of magnetic separation is carried out using a magnetic field of 8,000 Gauss. After this treatment, the clay in suspension has a Fe₂O₃ content of 2.60% in weight.

The magnetically treated suspension is then washed several times with water to entrain the soluble salts, after which it is dried to obtain cakes whose moisture content is 20% in weight. At the end of this process, the total sulphur content in the clay is 640 ppm.

Clay with a greater plasticity is obtained after this enhancing procedure has been completed, this being due to the increase in the content of clayey minerals while at the same time there is a lower proportion of impurities (Fe2O3 and sulphur). The table 1 below shows the chemical composition of the original clay and the treated clay, as well as the plasticity ("Perfrekorn" Method) and the Hunter Lab chromaticity coordinates after the clay has been fired at 1 ,190 ⁰C:

Table 1

Example 2:

In this case, the initial clay is sufficiently plastic for manufacturing ceramic engobes but has a chromatic L coordinate of 60, which does not serve to achieve the desired purpose.

By applying the steps described in the patent, turbo dilution, sieving, sifting, magnetic separation and washing, the average size of the suspended clay particle can be increased from a (dso) of 4.7 μm and (dgo) of 24.5 μm, to a (dδo) of 4.5 μm and (dgo) of 18.1 μm.

Next, a single magnetic separation stage is carried out using a magnetic field of 15000 Gauss. The clay, after this treatment, has a Fe₂O₃ content of 1 .05% by weight.

The magnetically treated suspension is subsequently washed several times with water in order to remove the soluble salts and defloculation additives, and then dried until obtaining cakes having a humidity of 20% by weight. On completing this process, the clay has a total sulphur content of 50 ppm.

On completion of the benefiting process, clay with greater plasticity is obtained, due to the increased content of clay minerals with a smaller proportion of impurities (Fe₂O₃ and sulphur). The following table shows the chemical composition of the original and treated clay, while table 2 shows the Hunter Lab chromatic coordinate L after firing the clay at 1 ,19O⁰C.

This treated clay now fulfils the required standard to be used as clay for engobes.

Table 2 Example 3:

In this case, it is clay with high AI2O3 and low Fe2U3 values but with a sulphur (S) value of 1 ,938 ppm, which prevents its use in ceramics.

The steps described in the patent, turbo dilution, sieving, sifting, magnetic separation and washing, are applied. The washing process is carried out intensively and is repeated up to five times, using washing water of 360 μS.

According to table 3, conductivity is reduced, being proportional to the soluble salt content, mainly SO2 ions.

Table 3

The washing process is repeated until the variation in conductivity is less than 100 (μS) and the final sulphur (S) content value is 316 ppm.

In this clay, the sulphur is in the form of pyrite and soluble salt, due to which elimination is produced especially during the sieving, sifting and washing phases.

Table 4

Example 4

In this case, different classifier adjustments are applied to plastic clay in order to obtain decreasing particle sizes, in such a manner that the firing temperature is reduced to obtain the same Water Absorption (WA%) percentage. In the example, a 2% adjustment is made.

As can be observed in table 5, the temperature can be regulated by 100⁰C, depending on the particle size obtained in the classifier. The rest of the processes, such as use of magnets or washing, are not of special relevance to the final clay processing temperature.

Table 5 Example 5

This example consists in the introduction of additives and peroxides in the phase of turbo dilution device, so that the percentage of organic carbon in the clay is significantly reduced.

Clay with hot water at 7O⁰C and with a solids content of 29% is set into the turbo dilutor, and the reactant, hydrogen peroxide, is added with a percentage of 25% with respect to the liquid fraction.

Stirring is kept for different times, 1 hour, 6 hours and 24 hours before continuing with sieving, sifting, magnetic separation and washing treatment. As table 6 shows, reductions up to 46% of the initial percentage of organic carbon are obtained.

Table 6

Claims

1. A procedure for enhancing kaolinitic clays characterised in that it comprises the following steps: (i) Obtaining of an aqueous suspension of clay;

(ii) Removal of the fraction with a particle size greater than 60 μm in dgo using physical separation procedures; (iii) Classification of the clay in suspension on the basis of the particle size, using mechanical means; obtaining a particle size between 45 μm and 3 μm.

(iv) Magnetic treatment of the fraction obtained in the step (iii) (v) Removing the water in the suspension in order to obtain the cake; and

(vi) Carrying out at least a process of washing the cake with water and subsequent draining.

2. Procedure according to previous claim wherein the composition of the starting clay comprises the following minerals: illite, mica, kaolinite and quartz.

3. Procedure according to any previous claims wherein the solid content in the aqueous suspension of clay ranges from 10% to 70% in weight.

4. Procedure according to any previous claims wherein deflocculants of an organic and/or inorganic nature and oxidant and/or reducing additives are used at the step where the aqueous suspension of clay is prepared.

5. Procedure according to any previous claims wherein the step of magnetic treatment of the fraction with the smallest particle size is carried out in one or more ongoing steps, through the use of a high- density magnetic separator, with field strengths that are equivalent to or greater than 8,000 Gauss.

6. Procedure according to any previous claims which comprises a step

(vii) of drying to obtain a cake whose moisture content ranges from 10% to 40%.

7. Material obtained by the procedure according to any of previous claims.

8. Use of the material according to claim 7 for the obtaining of engobe for the manufacture of ceramic and porcelain floor tiles.