GB1572351A

GB1572351A - Dewatering of aqueous suspensions of fine particulate solids

Info

Publication number: GB1572351A
Application number: GB21216/76A
Authority: GB
Original assignee: English Clays Lovering Pochin Co Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1976-05-21
Filing date: 1976-05-21
Publication date: 1980-07-30
Also published as: ES458966A1; AU507341B2; ZA772802B; JPS6013726B2; DE2722913A1; AU2524677A; JPS537588A; FR2351689A1

Description

(54) IMPROVEMENTS IN OR RELATING TO THE DEWATERING OF AQUE OUS SUSPENSIONS OF FINE PARTICULATE SOLIDS (71) We, ENGLISH CLAYS LOVERING POCHIN & COMPANY LIMITED, a British company, of John Keay House, St. Austell, Cornwall, PL25 4DJ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the dewatering of aqueous suspensions of fine particulate solids, for example a mineral by pressure filtration.

Dry methods of mineral beneficiation are generally only applicable to minerals comprising particles which have a diameter predominantly larger than about 10 microns (10 tom).

When the particle size of the mineral is substantially smaller than about 10 ,um, it becomes necessary to beneficiate the mineral in suspension in a liquid, generally water, in order to achieve a sharp separation of impurities or gangue from the desired product. Unfortunately, if the mineral particles are very fine it is difficult to dewater them mechanically by conventional means, i.e. by filtration or gravitational or centrifugal sedimentation, because the particles pack together to form a bed or cake which is very impermeable to the suspending liquid. Examples of materials which behave in this way are very fine clays of the kandite group, e.g. kaolinite, dickite, nacrite or halloysite, or of the smectite group, e.g. montmorillonite, fuller's earth, bentonite or saponite, and mineral residues which contain substantial proportions of these clays.

According to the present invention there is provided a method of mechanically dewatering an aqueous suspension of a fine particulate solid containing at least 40% by weight of particles having an equivalent spherical diameter smaller than 1 micron by pressure filtration at a pressure greater than 250 pounds per square inch which method comprises mixing with the aqueous suspension of the fine particulate solid prior to the dewatering thereof a minor amount of a water-soluble salt of a di- or multi-valent metal, adjusting the pH of the aqueous suspension to a value, in the range 3.5 to 9.0, such that there is formed in situ in the aqueous suspension a precipitate of a substantially water-insoluble hydroxy polymer of the di- or multivalent metal, and maintaining the pH of the aqueous suspension at a value in said range of from 3.5 to 9.0 so that the precipitate of said hydroxy polymer of the di- or multivalent metal does not dissolve in the water of said aqueous suspension ; and thereafter mechanically dewatering said aqueous suspension by pressure filtration at a pressure greater than 250 pounds per square inch.

Information about hydroxy polymers of di- or multivalent metals is given in the following literature references: (i) "Fixation of Phosphate by Aluminium and Iron in Acidic Soils" by Pa Ho Hsu, Soil Science, Vol. 99, No. 6, pages 398-402 (1965); (ii) "Formation of X-ray amorphous and Crystalline Aluminium Hydroxides" by Pa Ho Hsu and T.R. Bates, Mineralogical Magazine, Vol. 33, pages 749-68 (1964); and iii) British Patent Specification No. 1,481,118.

The inorganic hydroxy polymer is precipitated in situ in the aqueous suspension, i.e in the presence of the fine particulate solid, by adding thereto a water-soluble salt of the di- or multi-valent metal and adjusting the pH of the aqueous suspension to a value at which precipitation of the hydroxy polymer occurs. The water-soluble salt of the element may be a salt in which the element is the cation, for example the sulphate, nitrate or chloride of the element, or a salt in which the element is present in the anion, for example sodium aluminate or sodium zincate.

In one embodiment of the present invention, a positively charged inorganic hydroxy polymer of aluminium, iron, magnesium, chromium, manganese, colbalt or zinc is formed in situ in an aqueous suspension of a fine mineral in a quantity such that there is present in the aqueous suspension from 0.5 to 30.0 mg. of the metal per gram of the fine mineral, calculated on a dry weight basis, and under conditions such that the pH of the aqueous suspension containing the fine mineral and the inorganic hydroxy polymer is at a value such that the inorganic hydroxy polymer does not dissolve in the water of said aqueous suspension.

The amount of the inorganic hydroxy polymer precipitated in situ in the suspension of the fine particulate solid is preferably such that there is present in the suspension a quantity of the metal in the range of from 0.1 to 15.0 mg of metal per gram of the fine particulate solid.

The aqueous suspension containing the fine particulate solid and the hydroxy polymer is dewatered by filtration at a pressure greater than 250 pounds per square inch.

When the mineral is the desired product the di- or multi-valent metal is preferably aluminium, as hydroxy polymers of aluminium introduce no undesirable impurities. When a hydroxy polymer of aluminium is employed the average ratio of hydroxide ions to aluminium ions is preferably in the range of from 0.5 to 2.8 and the pH at which the hydroxy polymer is precipitated in the aqueous suspension of the particulate solid is preferably in the range from 5 to 7. When however, the mineral is a waste material which it is required to dewater so that it can be disposed of and the water in which it was suspended can be recovered, the di- or multi-valent metal may be iron, magnesium, chromium, manganese, cobalt, zinc or aluminium.

The fine mineral will frequently be or comprise a substantial proportion of a clay mineral of the kandite group, e.g. kaolinite, nacrite, dickite or halloysite, or a clay mineral of the smectite group, e.g montmorillonite, fuller's earth, bentonite or saponite. The method of the invention is especially suitable for treating very fine mineral raw materials, such as ball clay, and for treating fine waste products of mineral beneficiation processes such as Florida pebble phosphate slimes (which containapatite and montmorillonite) and kimberlite slimes from the diamond mines of the Kimberley area (which consist predominantly of saponite).

Such fine minerals will generally have a particle size distribution such that at least 60% by weight of the particles have an equivalent spherical diameter smaller than 1 micron.

The follow experiments demonstrate the improvement in the permeability of a filter cake of a fine particulate solid which is achieved when a hydroxy polymer of a di- or multivalent metal is precipitated in situ in an aqueous suspension of the fine particulate solid.

Experiment clay having 1 A Dorset ball clay having a particle size distribution such that 4% by weight consisted of particles having an equivalent spherical diameter larger than 5m, m, 79% by weight consisted of particles having an equivalent spherical diameter smaller than 1calm and 62% by weight consisted of particles having an equivalent spherical diameter smaller than 0.5clam, was mixed with water to form a suspension containing 50% by weight of dry ball clay. Sufficient sodium hydroxide was added to the suspension to raise its pH to 8.5. The suspension was subjected to agitation in a high speed mixer for a time sufficient to dissipate in the suspension about 50KJ of energy per kg. of dry clay.

The suspension was then diluted with water to 20% by weight of dry ball clay, screened through a No. 300 mesh B.S. sieve (nominal aperture 53clam) and subjected to a magnetic separation step to remove ferromagnetic and paramagnetic impurities.

A portion of the screened and magnetically beneficiated ball clay suspension was then treated with sulphuric acid to reduce the pH to 4.0 and the permeability to water of a filter cake formed from the ball clay suspension at a pressure differential of 100 psi (689 kNm-2) was measured by the following method: The apparatus used consisted of a cylindrical vessel in which a closely fitting piston could slide, the ends of the cylindrical vessel being closed by screw-threaded end caps. In one end cap there was provided an outlet conduit for filtrate the opening to which conduit was covered by a piece of wire gauze surmounted by a piece of filter cloth which together acted as a filter medium on which a filter cake could be built up. The other end cap was provided with a conduit through which hydraulic fluid could be supplied at an elevated pressure. In operation, the end cap comprising the filter medium was unscrewed and a sample of the suspension to be tested was introduced into the cavity between the upper end of the piston and the filter medium. Hydraulic fluid at the required pressure was then introduced into the cavity between the other end of the piston and the other end cap, pressure being thereby applied to the suspension. Filtrate expressed through the filter medium was collected in a graduated cylinder and the volume of filtrate collected aften given intervals of time was observed. A graph was then plotted of t/V against V, where V is the volume of filtrate collected after time t. A straight line was obtained and the slope, m, was estimated.

The permeability, p, was calculated from the expression p= v 2A2 AP.m where AP is the pressure drop across the filter medium and the cake; A is the surface area of the filter medium perpendicular to the direction of flow of filtrate, and v is the volume of cake deposited per unit volume of filtrate and is given by the expression v = ds - df dc - ds where ds is the specific gravity of the feed suspension, df is the specific gravity of the filtrate and dc is the specific gravity of the filter cake.

The permeability is expressed as cubic inches of filtrate per hour passing through a cube of filter cake of side one inch under a pressure differential of one pound per square inch()psi).

Further portions of the screened and magnetically beneficiated ball clays suspension were then subjected to treatment with different amounts of an aluminium hydroxy polymer by reducing the pH to 4.0 with dilute sulphuric acid, adding the amount of aluminium sulphate which would provide the desired dose of aluminium per unit weight of dry ball clay and, finally, adjusting the pH to 6.8 with sodium carbonate to precipitate a hydroxy polymer of aluminium.

The permeability of the filter cake at a pressure differential of 100 psi was measured in each case as described above and the percentage by weight of water remaining in the cake was also measured. The results obtained are set forth in Table 1 below.

TABLE 1 Aluminium dose (mg Al per g. of Permeability % by wt. of water dry ball clay) x 10-4 retained in cake 0 3.5 37.6 2 10.8 40.8 4 32.3 45.8 8 68.9 48.3 16 114.5 51.5 It will be appreciated that, although the amount of water retained in the cake increases with increasing aluminium dose, the low water content corresponding to zero aluminium dose could not be achieved commercially because filtration would be prohibitively slow on account of the low filter cake permeability.

The portion of the suspension which had been treated with 4 mg of Al per g. of dry ball clay exhibited a good improvement in permeability which rendered it easily dewatered by filtration at high pressures and at the same time the water content of the cake was not increased to an undesirable level and the dose of aluminium was not excessive.

Experiment 2 Experiment 1 was repeated except that the portions of the screened and magnetically beneficiated ball clay suspension were not treated with dilute sulphuric acid to reduce the pH from 7.5 to 4.0 before addition of aluminium sulphate but the aluminium sulphate was added to the suspension at its pH of about 7.5. The addition of the aluminium sulphate itself reduced the pH to about 3.5 and the pH was of the suspension then adjusted to 6.8 with sodium carbonate to precipitate a hydroxy polymer of aluminium.

The permeability of the filter cake at a pressure differential of 100 psi and the percentage by weight of water retained in the cake were measured in each case and the results are set forth in Table II below.

TABLE II Aluminium dose (mg Al per g. of Permeability % by weight of water dry ball clay x 10-4 retained in cake 0 3.5 37.6 2 16.8 44.8 4 36.8 44.9 8 58.2 49.8 Experiment 3 Further portions of the same suspension of screened and magnetically beneficiated ball clay as was used in Experiments 1 and 2 were treated at their pH of 7.5 with the same dose of aluminium, namely 8 mg. of Al per g. of dry ball clay, but subsequently different amounts of sodium carbonate were added to give different values of the final pH and to give different hydroxy polymers of aluminium.

The permeability and water content of the cake after filtration at the different pH values were measured by the method described in Experiment 1 and the results are set forth in Table III below.

TABLE III pH Permeability x 10-4 %wit. of water retained in cake 3.5 12.7 41.4 4.0 52.8 44.8 5.0 69.9 49.5 6.0 62.4 50.7 6.8 63.6 49.1 8.0 36.2 48.6 9.0 23.7 47.4 It can be seen that when the ball clays are treated with a hydroxy polymer of aluminium, it is important that the final pH is in the range from 4 to 8 and preferably is in the range from 5 to7.

Experiment 4 A sample of Florida pebble phosphate slime had a particle size distribution such that substantially all of the particles were smaller than 100 cam (i.e. passed a No. 150 mesh B.S.

Sieve), 27% by weight consisted of particles having an equivalent spherical diameter larger than slum, 60% by weight consisted of particles having an equivalent spherical diameter smaller than 1,um, and 51% by weight consisted of particles having an equivalent spherical diameter smaller than 0.5,us. The slime consisted of about 30%by weight of apatite, about 60% by weight of montmorillonite, about 5% by weight of quartz and about 5% by weight of dolomite.

The phosphate slime was mixed with water to form a suspension containing about 10% by weight of dry solids and the suspension was divided into three portions. The first portion was tested to determine the permeability of the filter cake under a pressure differential of 100 psi according to the method described in Experiment 1, and the percentage by weight of water retained in the cake was measured. The second and third portions were treated with aluminium sulphate solutions in amounts such that there were added to the phosphate slime 5 mg and 15 mg of aluminium respectively per gram of dry solids. The pH of each portion of treated suspension was then adjusted to 6.0 with sodium carbonate to precipitate a hydroxy polymer of aluminium, and the permeability and percentage by weight of water retained by the filter cakes were then measured. The results obtained are set forth in Table IV below.

TABLE IV Aluminium dose (mg Al per Permeability % by weight g. of dry solids) x 10-4 water retained in cake 0 1.2 55.1 5 23.2 58.2 15 25.8 59.4 Experiment 5 A further sample of the same Florida pebble phosphate slime as was used in Experiment 4 was mixed with water to form a suspension containing about 10% by weight of dry solids.

The suspension was treated with ferric chloride solution in an amount such that there were added to the phosphate slime 10 mg of iron per gram of dry solids. The pH of the treated suspension was then adjusted to 4.5 with sodium carbonate to precipitate a hydroxy polymer of ferric iron, and the permeability and percentage by weight of water retained by the filter cake were then measured. The permeability was 15.4 x 10-4 cubic inches of water per hour per one inch cube of cake per psi and the percentage by weight of water retained in the cake was 58.0.

WHAT WE CLAIM IS: 1. A method of mechanically dewatering an aqueous suspension of a fine particulate solid containing at least 40% by weight of particles having an equivalent spherical diameter smaller than 1 micron by pressure filtration at a pressure greater than 250 pounds per square inch, which method comprises mixing with the aqueous suspension of the fine particulate solid prior to the dewatering thereof a minor amount of a water soluble salt of a di- or multi-valent metal, adjusting the pH of the aqueous suspension to a value, in the range 3.5 to 9.0, such that there is formed in situ in the aqueous suspension a precipitate of a substantially water-insoluble hydroxy polymer of the di- or multi-valent metal, and maintaining the pH of the aqueous suspension at a value in said range of from 3.5 to 9.0 so that

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Claims

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The permeability and water content of the cake after filtration at the different pH values were measured by the method described in Experiment 1 and the results are set forth in Table III below.

TABLE III pH Permeability x 10-4 %wit. of water retained in cake

3.5 12.7 41.4

4.0 52.8 44.8

5.0 69.9 49.5

6.0 62.4 50.7

6.8 63.6 49.1

8.0 36.2 48.6

9.0 23.7 47.4 It can be seen that when the ball clays are treated with a hydroxy polymer of aluminium, it is important that the final pH is in the range from 4 to 8 and preferably is in the range from 5 to7.

Experiment 4 A sample of Florida pebble phosphate slime had a particle size distribution such that substantially all of the particles were smaller than 100 cam (i.e. passed a No. 150 mesh B.S.

Sieve), 27% by weight consisted of particles having an equivalent spherical diameter larger than slum, 60% by weight consisted of particles having an equivalent spherical diameter smaller than 1,um, and 51% by weight consisted of particles having an equivalent spherical diameter smaller than 0.5,us. The slime consisted of about 30%by weight of apatite, about 60% by weight of montmorillonite, about 5% by weight of quartz and about 5% by weight of dolomite.

The phosphate slime was mixed with water to form a suspension containing about 10% by weight of dry solids and the suspension was divided into three portions. The first portion was tested to determine the permeability of the filter cake under a pressure differential of 100 psi according to the method described in Experiment 1, and the percentage by weight of water retained in the cake was measured. The second and third portions were treated with aluminium sulphate solutions in amounts such that there were added to the phosphate slime 5 mg and 15 mg of aluminium respectively per gram of dry solids. The pH of each portion of treated suspension was then adjusted to 6.0 with sodium carbonate to precipitate a hydroxy polymer of aluminium, and the permeability and percentage by weight of water retained by the filter cakes were then measured. The results obtained are set forth in Table IV below.

TABLE IV Aluminium dose (mg Al per Permeability % by weight g. of dry solids) x 10-4 water retained in cake 0 1.2 55.1 5 23.2 58.2 15 25.8 59.4 Experiment 5 A further sample of the same Florida pebble phosphate slime as was used in Experiment 4 was mixed with water to form a suspension containing about 10% by weight of dry solids.

The suspension was treated with ferric chloride solution in an amount such that there were added to the phosphate slime 10 mg of iron per gram of dry solids. The pH of the treated suspension was then adjusted to 4.5 with sodium carbonate to precipitate a hydroxy polymer of ferric iron, and the permeability and percentage by weight of water retained by the filter cake were then measured. The permeability was 15.4 x 10-4 cubic inches of water per hour per one inch cube of cake per psi and the percentage by weight of water retained in the cake was 58.0.

WHAT WE CLAIM IS:

1. A method of mechanically dewatering an aqueous suspension of a fine particulate solid containing at least 40% by weight of particles having an equivalent spherical diameter smaller than 1 micron by pressure filtration at a pressure greater than 250 pounds per square inch, which method comprises mixing with the aqueous suspension of the fine particulate solid prior to the dewatering thereof a minor amount of a water soluble salt of a di- or multi-valent metal, adjusting the pH of the aqueous suspension to a value, in the range 3.5 to 9.0, such that there is formed in situ in the aqueous suspension a precipitate of a substantially water-insoluble hydroxy polymer of the di- or multi-valent metal, and maintaining the pH of the aqueous suspension at a value in said range of from 3.5 to 9.0 so that

the precipitate of said hydroxy polymer of the di- or multi-valent metal does not dissolve in the water of said aqueous suspension; and thereafter mechanically dewatering said aqueous suspension by pressure filtration at a pressure greater than 250 pounds per square inch.

2. A method of dewatering an aqueous suspension of a fine particulate solid according to claim 1, wherein said fine particulate solid is a mineral, wherein said di- or multi-valent metal is aluminium, iron, magnesium, chromium, manganese, colbalt or zinc, and wherein the quantity of said precipitate of the hydroxy polymer which is formed in situ in said aqueous suspension is such that there is present in the aqueous suspension from 0.5 to 30.0 mg. of the metal per gram of the mineral, calculated on a dry weight basis.

3. A method of dewatering an aqueous suspension of a fine particulate solid according to claim 2, wherein said mineral contains a substantial proportion of a clay.

4. A method of dewatering an aqueous suspension of a fine particulate solid according to claim 3, wherein said multi-valent metal is aluminium and wherein the pH of the aqueous suspension is adjusted to a value in the range of from 5 to 7.

5. A method of dewatering an aqueous suspension of a fine particulate solid according to claim 1 and substantially as hereinbefore described.

6. A fine particulate solid whenever obtained by a method according to any one of the preceding claims.