WO2008152385A1

WO2008152385A1 - Dewatering kaolin

Info

Publication number: WO2008152385A1
Application number: PCT/GB2008/002007
Authority: WO
Inventors: Anthony R. Phillips
Original assignee: Imerys Minerals Limited
Priority date: 2007-06-13
Filing date: 2008-06-12
Publication date: 2008-12-18
Also published as: GB0711458D0

Abstract

A slurry of relatively fine kaolin particles (80% by weight smaller than 2μ) is dewatered in a membrane filter press. The slurry is supplied to the filter press at a feed pressure not greater than 1.5 MPa and subsequently subjected to a membrane pressure which is not less than 0.5 MPa greater than the feed pressure. For example, the feed pressure may be approximately 1 or 1.2 MPa, and the membrane pressure may be in excess of 2.5 MPa, for example 3.0 MPa. The process achieves a dry final press cake which can be detached from the filter cloths, and subsequently handled, relatively easily.

Description

DEWATERING KAOLIN

This invention relates to the dewatering of kaolin slurries.

It is known to dewater kaolin slurries using filter presses, for example plate filter presses. In such a press, filter plates are stacked, either vertically or horizontally, to form a cavity between each pair of plates. Filter cloths are placed over the plates to define the cavities, and feed slurry is fed under pressure into the cavities so that the liquid component of the slurry is forced through the filter cloths, leaving the solid component as a filter cake which is discharged by separating the plates from one another. In this specification, the expression "slurry" is used to refer to suspensions of the particulate kaolin irrespective of the actual solids content or the nature of the suspending liquid (usually water).

In a modified form of filter press, known as a membrane filter press, a membrane is disposed in the cavity between each pair of filter plates, and, after the filter press has been filled with feed slurry under pressure, the membrane is displaced, for example by water or air under pressure, to apply a squeeze to the filter cake accumulated during the slurry feed process. This squeeze extracts more moisture from the filter cake, leading to a drier product.

Chamber filter presses, (ie presses which do not include a squeezing membrane, and so rely solely on the slurry feed pressure to force a liquid component through the filter cloths) operate at a pressure limited by the capacity of the slurry feed pumps. A typical pressure achieved in a chamber press is 1.5 MPa. Membrane presses are commercially available which achieve a squeeze, or membrane, pressure of 3.0 MPa.

Both chamber and membrane presses have been used for dewatering relatively coarse mineral slurries, including ceramic grade kaolin slurries having 30 to 40% of particles by weight having an equivalent spherical diameter of 2 microns (μ).

Particle sizes, and other particle size properties referred to in the present application, are measured in a well-known manner by, e. g., sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a SEDIGRAPH 5100 or 5120 instrument as supplied by Micromeritics Corporation. The size of a given particle is expressed in terms of the diameter of a sphere of equivalent diameter, which sediments through the suspension, i. e. , an equivalent spherical diameter or e.s.d. Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having an equivalent spherical diameter (e.s.d.), less than the given e.s.d. values. All particle size data measured and reported herein, including in the examples, were taken in a known manner, with measurements made in water at the standard temperature of 34.9 C. All percentages and amounts expressed herein are by weight. All amounts, percentages, and ranges expressed herein are approximate.

Experience of dewatering kaolin slurries using chamber or membrane filter presses has suggested that the quantity of water extracted from the slurry increases with increasing press (or membrane) pressure in an asymptotic manner to a maximum pressure beyond which any further pressure increase fails to extract more water. It is believed that the clay particles pack together as water is displaced from the slurry to form a relatively incompressible matrix in which water is retained in the interstices between the kaolin particles and cannot be extracted by further pressure increases.

When relatively fine clays (ie having more than 80% by weight of particles smaller than 2μ) are dewatered in chamber or membrane filter presses at the pressures conventionally used, the water content remaining in the resulting filter cake is high enough to leave the cake plastic and sticky, so the filter cake cannot easily be removed from the filter cloths when the press is opened and, when removed, is difficult to handle. The difficulty in removing the filter cake means that automatic cake discharge is difficult or impossible, so that extensive manual intervention is necessary. This significantly reduces the productivity which can be achieved when attempting to dewater fine kaolin by conventional filter press techniques.

If a kaolin product is to be supplied in a substantially dry state, it is almost always necessary to complete the drying process by means of thermal drying. Thermal drying processes have a very high energy requirement, and it is consequently desirable to maximise water removal by means of non-thermal methods. Nevertheless, as mentioned above, it had previously been considered that no further benefit, in terms of increased water extraction, could be achieved by increasing filter press pressures, particularly when processing the finer grades of clay. However, the present invention is based on the surprising discovery that a significant increase in the quantity of water extracted can be achieved by substantially increasing the pressure applied to the slurry.

Thus, according to the present invention, there is provided a method of dewatering a slurry comprising suspended kaolin particles of which not less than 80% by weight have a particle size less than 2μ, the method comprising:

(i) feeding the slurry to a membrane filter press at a feed pressure to form an intermediate press cake;

(ii) applying to the intermediate press cake a membrane pressure of not less than 0.5 MPa greater than the feed pressure to form a final press cake; and

(iii) discharging the final press cake from the filter press.

In a preferred process in accordance with the present invention, the feed pressure is not greater than 1.5 Mpa. The feed pressure may be not less than 1 MPa and may be, for example, 1 , 1.2 or 1.5 MPa.

The membrane pressure may be up to 2 MPa greater than the feed pressure, or greater if the feed pressure is relatively low (below 1.0 Mpa). It has been found to be desirable for the membrane pressure to be in excess of 2.0 MPa, more preferably 2.5 MPa. In a preferred process in accordance with the present invention, the membrane pressure is 3.0 MPa.

The present invention has been found to be particularly effective when used for dewatering slurries of kaolin particles having a relatively large shape factor. In the context of the present invention, the shape factor is a measure of the "aspect ratio" of the clay particles as determined using the method and apparatus described in

EP 0528078. A kaolin having a high shape factor will contain particles in the form of plates or sheets, thus being relatively thin by comparison with their area measured in a direction perpendicular to their thickness. By contrast, a kaolin having a relatively low shape factor is one in which the particles are more block-like. The present invention has been found to be particularly suitable for dewatering kaolin slurries having a shape factor not less than 20, more particularly not less than 25, and even more particularly not less than 30. Such shape factor characteristics apply particularly to kaolin clay present in deposits in Cornwall, England. It is believed that, when a kaolin slurry containing kaolin with a relatively high shape factor is dewatered in a filter press, the plate-like kaolin particles overlap one another in the filter cake as it is formed, to form block-like agglomerations. Finer particles (less than 0.25μ) accumulate in the interstices between the plate-like particles and the aggregated blocks, and the resulting structure provides an array of narrow, tortuous paths through which water must flow in the dewatering process. Without wishing to be bound by theory, it is believed that these narrow, tortuous paths provide substantial resistance to the flow of water out of the forming filter cake, and consequently the extraction of water proceeds slowly, if at all, at filter press pressures conventionally used for dewatering kaolin slurries. However, at significantly increased pressures, for example in the region of 3.0 MPa, it has been found that a surprisingly substantial additional quantity of water can be extracted.

In a method in accordance with the present invention, the slurry may be fed to the membrane filter press at a temperature above the ambient temperature, for example at a temperature of not less than 3O⁰C and not more than 5O⁰C. If the final press cake is subsequently fed to a thermal drier, the waste heat from the thermal drier may be used to raise the temperature of the feed slurry.

The pH of the feed slurry may be controlled to achieve a desired degree of flocculation. Preferably, the pH of the slurry may be not less than 3.5 and not greater than 4.5. In some processes, this will require the addition of an alkaline composition to the feed slurry in order to increase a small pH value. For example, adjustment of the pH value may be desirable if the feed slurry is the output of a kaolin refining process, in which the kaolin is subjected to reductive bleaching which typically results in a pH value for the slurry of approximately 2.5.

The degree of flocculation of the feed slurry is preferably minimised. Excessive flocculation can result in aggregates of kaolin particles which form a water-containing enclosure, making it difficult to extract the enclosed water by pressure alone. The solids content of the feed slurry may be not less than 10% and not more than 30% by weight. For example, the solids content may be not less than 14% and not more than 20%. The feed slurry may have been subjected to processing, for example comminuting and beneficiating, and in particular may have been comminuted to achieve a particular size distribution in which not less than 80% of the particles by weight have an equivalent spherical diameter less than 2μ. A process in accordance with the present invention has proved suitable for dewatering slurries in which not less than 90%, and possibly approximately 92%, of particles, by weight have an equivalent spherical diameter less than 2μ.

The present invention will be described with reference to the following Examples:

Example I

Samples of paper coating clays having a particle size profile such that at least 90% of particles have an equivalent spherical diameter less than 2μ were dewatered in a series of different membrane filter presses operating at different maximum pressures. The feed slurries in each case had a solids content by weight of 15%. The results are summarised in Table I:

TABLE I

0

It will be appreciated from these tests that Press A with a maximum membrane pressure of 1.6 MPa achieved a filter cake with a solids content of 68.5% by weight. The energy consumption was 15 kWh/t. It is noted that the remaining high moisture content of the filter cake resulted in poor cake discharge.

The sample processed in Press B was subjected to a maximum membrane pressure of 2.5 MPa, ie an increase of 0.9 MPa over Press A, with a reduced energy consumption of 10 kWh/t. The additional pressure increased the solids content by 1.5% which, while significant, suggests that small increases in pressure beyond 1.6 MPa do not yield major advantages in terms of additional water extracted. Even at the solids content of 70% from Press B, the filter cake remained plastic and sticky, making cake discharge difficult.

By contrast, Presses C, D and E, in which the maximum membrane pressure was 3.0 MPa, achieved 72.5% cake solids by weight, a substantial increase over the 70% achieved by Press B, from a comparatively small pressure increase of 0.5 MPa. In addition, the energy consumption per tonne of filter cake was reduced still further to 8 kWh/t . The filter cake produced in Presses C, D and E was of substantially different quality from those of Presses A and B, in that they were dry and solid, separating easily from the filter cloths when the press plates were separated. Furthermore, the reduction in water content from 31.5% in Press A to 27.5% in Presses C, D and E, a reduction of more than 12%, results in a substantial reduction in the energy requirements for any subsequent thermal drying process.

In practice, Press C is a 30 bar (3.0 MPa) horizontal overhead beam press available from Andritz AG of Graz, Austria.

Example Il

Samples of different kaolin slurries were dewatered in a pilot press, the results from which are scalable to give predicted performance for an Andritz filter press of the size represented as Press C in Table I, the characteristics of which would be as follows:

Press size metres: 2 x 2

No of chambers: 108

VoI per chamber m3: 114

Chamber depth mm: 40 The samples comprised processed slurries of Cornish clay known as ND 3090, SGL 95 and SPS. In addition, for comparison purposes, two coarser ceramic grade kaolin slurries were also processed in the pilot press, these being known as REMC and STP.

5 The results are represented in Table II:

TABLE Il

Table Il demonstrates the difficulty of dewatering finer kaolin products in a filter press. It will be appreciated that, although the ceramic grades REMC and STP were subjected0 to a membrane pressure of 1.6 MPa, the resulting water content of the filter cake was comparable to, or lower than, the water content achieved for the finer paper coating clays ND 3090, SLG 95 and SPS, despite the fact that the finer products were subjected to almost double the membrane pressure. 5 It will be noted that the fill time for the press fell in the range from 40 to 60 minutes, and the membrane squeezing time was 15 minutes. In practice the membrane squeezing time is determined by the time taken to reach the maximum pressure of 3.0 MPa, and can vary between 15 and 30 minutes. 0 In the tests summarized in Table II, the feed slurries had a solids content ranging between 14.5 and 17.2%, and a feed slurry temperature ranging between 39°C and 65°C. The water content of the intermediate press cake after completing of the slurry feed but before the membrane squeeze fell in the range 37% to 46.5%.

Claims

1. A method of dewatering a slurry comprising suspended kaolin particles of which not less than 80% by weight have a particle size less than 2μ, the method comprising: (i) feeding the slurry to a membrane filter press at a feed pressure to form an intermediate press cake; (ii) applying to the intermediate press cake a membrane pressure of not less than 0.5 MPa greater than the feed pressure to form a final press cake; and (iii) discharging the final press cake from the filter press.

2. A method as claimed in claim 1 , in which not less than 90% by weight of the particles of the slurry have a particle size less than 2μ.

3. A method as claimed in claim 2, in which approximately 92% by weight of the particles of the slurry have a particle size less than 2μ.

4. A method as claimed in any one of the preceding claims, in which the particles of the slurry have a shape factor not less than 20.

5. A method as claimed in claim 4, in which the particles of the slurry have a shape factor not less than 25.

6. A method as claimed in claim 5, in which the particles have a shape factor not less than 30.

7. A method as claimed in any one of the preceding claims, in which the temperature of the slurry fed to the membrane filter press is not less than 3O⁰C.

8. A method as claimed in claim 7, in which the temperature of the slurry fed to the membrane filter press is not greater than 5O⁰C.

9. A method as claimed in any one of the preceding claims, in which the slurry fed to the membrane filter press has a pH of not less than 3.5 and not more than 4.5.

10. A method as claimed in any one of the preceding claims, in which the slurry fed to the membrane filter press has an initial solids content of not less than 10% and not more than 30%.

11. A method as claimed in claim 10, in which the feed slurry to the membrane filter press has a solids content of not less than 14% and not more than 20%.

12. A method as claimed in any one of the preceding claims, in which the duration of step (i) is not less than 40 minutes and not more than 60 minutes.

13. A method as claimed in any one of the preceding claims, in which the duration of step (ii) is not less than 15 minutes and not more than 30 minutes.

14. A method as claimed in any one of the preceding claims, in which the maximum feed pressure is not less than 0.8 MPa.

15. A method as claimed in claim 14, in which the maximum feed pressure is not less than 1 MPa and not more than 1.5 MPa.

16. A method as claimed in any one of the preceding claims, in which the maximum membrane pressure is not less than 2.0 MPa.

17. A method as claimed in claim 16, in which the maximum membrane pressure is not less than 2.5 MPa.

18. A method as claimed in claim 17, in which the maximum membrane pressure is approximately 3.0 MPa.

19. A method as claimed in any one of the preceding claims, in which the feed slurry is obtained by comminuting a raw clay product.

20. A method as claimed in any one of the preceding claims, in which the final press cake is subjected to a thermal drying operation.

21. A method as claimed in claim 20, in which the feed slurry is heated by means of waste heat from the thermal drying process.

22. A method of dewatering a slurry comprising suspended kaolin particles as claimed in claim 1 and substantially as described herein.