GB2283014A

GB2283014A - Soil conditioning agent

Info

Publication number: GB2283014A
Application number: GB9321789A
Authority: GB
Inventors: Joseph George Doidge; Andrew Whorton
Original assignee: Camas Aggregates Ltd; Ecc Quarries Ltd; ECC Quarries Ltd
Current assignee: Camas Aggregates Ltd; Ecc Quarries Ltd; Camas UK Ltd
Priority date: 1993-10-22
Filing date: 1993-10-22
Publication date: 1995-04-26
Anticipated expiration: 2013-10-22
Also published as: GB9321789D0; GB2283014B

Abstract

There is disclosed a process for treating an aqueous suspension of a fine particulate material. The process comprises the steps of: (a) partially dewatering the aqueous suspension to form a cake; and (b) mixing the dewatered cake with a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all the particles have a diameter smaller than 75 mu m. The final product has a granular form.

Description

SOIL CONDITIONING AGENT This invention relates to a process for providing a soil conditioning agent which incorporates a finely divided particulate material such a waste quarry material, and also to the soil conditioning agent itself.

In quarries which produce crushed stone for use as an aggregate for concrete or as road stone, the stone as initially removed from the quarry face is generally comminuted by crushing or grinding to produce a product having the required particle size distribution. The stone which is fed to the comminution equipment seldom comprises pure rock material, but is nearly always contaminated with soil or clay materials. Also the comminution operation inevitably generates a proportion of dust. As a product which is contaminated with rock dust, earth or clay is considered undesirable for most purposes, the product is washed with water. As a consequence of this washing operation, there is formed a relatively dilute aqueous slurry of dust-sized waste material particles.This slurry is difficult and expensive to dewater, and, even when the material has been successfully dewatered, there has hitherto been no way of profitably exploiting the dewatered material.

Because, for environmental reasons, it is unacceptable to discharge the slurry to a river, it has been necessary to confine it in a lagoon on land, where partial dewatering occurs and a silt forms. This method of disposing of the dust-sized particles is unsightly and occupies large areas of land which could be used more profitably for other purposes. Also a significant proportion of the quarried stone is lost as dust. The dust can amount to up to about 5% of the total weight of the quarried stone.

This invention provides an improved process for disposing of fine material such as fine waste material and rock dust particles which are liberated during the comminution of quarried stone. The resultant product is useful in agriculture.

According to the present invention, there is provided a process for treating an aqueous suspension of a fine particulate material comprising the steps of: (a) partially dewatering the said aqueous suspension to form a cake; and (b) mixing the dewatered cake with a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all the particles have a diameter smaller than 75pom, to form a granular product.

The resultant granular product may be used as a soil conditioning agent for correcting acidity in soil.

The aqueous suspension or silt is preferably partially dewatered to form a cake having a dry solids content of from about 50% to about 85% by weight.

The aqueous silt is preferably one containing a fine particulate waste material derived from comminuted quarried stone.

The fine particulate waste material preferably consists predominantly of an alkaline earth metal carbonate material and will most suitably be the fine dust which is washed from crushed aggregate and road stone which is produced at a limestone quarry.

Typically this fine dust will consist of from about 60% by weight to about 80% by weight of limestone particles, the remainder being predominantly particles of clay. The particle size distribution of the fine particulate waste material is generally such that substantially all of the particles have a diameter smaller than 75pm. This will normally mean that at least 90% by weight, more preferably from 94 to 98% by weight, have a diameter smaller than 75pm.

The aqueous suspension or silt, which will generally be recovered from a lagoon or pond, is preferably dewatered by means of a filtration device which operates at a pressure differential in excess of 85 psig (590kPa). A convenient filtration device for this purpose is a recessed plate filterpress or a membrane plate filter. Alternatively a high pressure dewatering device such as a tube pressure filter of the type described in British Patent Specification No.

1240465 may be used. A device of this type may be operated at pressure differentials up to about 1,500 to 2,000 psig (10.3 - 13.8MPa). The dry solids content of the cake produced by the filtration device will generally be in the range of from about 75 to about 85% by weight.

The cake of partially dewatered fine particulate material is preferably mixed with the dry alkaline earth metal carbonate material (which is typically natural in origin) in a weight ratio of partially dewatered waste material to dry alkaline earth metal carbonate of from 1:1 to 5:1. The cake of the fine material and the dry alkaline earth metal carbonate material are conveniently mixed in equipment which imparts a tumbling motion to the components to be mixed and is also provided with a high speed impeller which is effective in disintegrating the lumps of waste material cake.

The dry alkaline earth metal carbonate, when mixed with the partially dewatered material, reduces the overall moisture content of the mixture to below about 15% by weight, generally to within the range of from 5 to 10 by weight, and forms a granular material which is easily spreadable on the land, and which does not disintegrate easily on spreading to form dust.

According to a second aspect of the present invention, there is provided a granular product whenever prepared by the process of the first aspect of this invention.

According to a third aspect of the present invention, there is provided a granular soil conditioning agent prepared by mixing together a moist cake of a fine particulate material and a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all the particles have a diameter small than 75pm.

EXAMPLE Waste silt from a lagoon adjacent to a lime stone quarry was dewatered by means of a membrane plate filter operating at a pressure of 90psig (621kPa) to give a cake having a dry solids content of 76% by weight. The silt was of variable composition but, on average, contained approximately 60-80% by weight of limestone, the remainder being predominantly clay particles. The particle size distribution of the silt was such that from 90 to 100% by weight consisted of particles having a diameter smaller than 63pm.

The cake was mixed with pulverised limestone of the type used as a filler in asphalt. The pulverised limestone had a particle size distribution such that 98 by weight consisted of particles having a diameter smaller than 63pm. The components were mixed in the proportions of 1 part by weight of pulverised limestone to 4 parts by weight of the cake formed by dewatering the silt. These proportions corresponded approximately to 3 parts by weight of dry silt solids to 1 part by weight of pulverised limestone.

The mixing was performed in an Eirichs mixer which comprises a rotating pan and a high speed rotating agitator which is effective in disintegrating any lumps of the cake which are present. Mixing was continued for a period of 6 minutes and the mixed product consisted of granules which were sufficiently dry not to cohere and which had sizes in the range from 0.5 to 4.0mm.

A further batch of granules was prepared by mixing together, in the same way as described above, 1 part by weight of silt cake and 1 part by weight of a coarser pulverised limestone which was prepared according to the Ministry of Agriculture specification for agricultural ground limestone and which had a particle size distribution such that about 69t by weight of the particles had a diameter smaller than 300pm and 50% by weight of the particles had a diameter smaller than 150pm. The proportions corresponded approximately to 1 part by weight of dry silt solids to 1.33 parts by weight of pulverised limestone.

The granules were tested for their soil conditioning properties by applying them to plots marked out in an area of arable land. Each batch of granules was applied at the rate given in the Table below to four randomly distributed plots each of size 4m x 15m. The pH of each plot was measured at the beginning and end of a six-week period and the average change in pH for each group of four plots was determined. As control experiments four plots of the same size, randomly distributed within the same overall pattern, were treated with an appropriate amount of the agricultural pulverised limestone alone, and four plots of the same size, also randomly distributed within the same overall pattern, received no treatment.

The results are set forth in the Table below:

Treatment Neutralising Dry Solids Rate of Application Rise Value (%) Content (%) (t/ha) in pH Agricultural Limestone 54.6 100 7.15 +0.65 1.33 parts Agricultural 42.0 90.7 10.23 +0.72 Limestone/1 part Silt Solids 1 part Fine Limestone/ 33.0 86.0 13.65 +0.70 3 parts Silt Solids No treatment - - - -0.02 Notes: (i) The neutralising value of a soil conditioning agent is a measure of its calcium content as expressed as % by weight of calcium oxide (CaO). Thus pure calcium carbonate would have a neutralising value of about 56%.

(ii) The rate of application for each batch of granules was calculated to give the same calcium level per hectare as the agricultural limestone, taking into consideration the neutralising value and the dry solids content of each batch of granules.

These results show that the granules containing the waste quarry silt were at least as effective as soil conditioning agents as the agricultural pulverised limestone. However, the granules had the advantage of being substantially dust free, and therefore virtually eliminated the dust problem which is known to arise when conventional pulverised limestone is used as a soil conditioning agent. Also the incorporation of the silt material into the granules makes it possible to reduce the requirement for land for retaining the waste material in lagoons.

Claims

CLAIMS:

1. A process for treating an aqueous suspension of a fine particulate material comprising the steps of: (a) partially dewatering the said aqueous suspension to form a cake; and (b) mixing the dewatered cake with a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all the particles have a diameter smaller than 75pm, to form a granular product.

2. A process according to claim 1, wherein the aqueous suspension is partially dewatered to form a cake having a dry solids content of from about 50% to about 85% by weight.

3. A process according to claim 1 or 2, wherein the solid component of the aqueous suspension comprises a fine particulate waste material derived from comminuted quarried stone.

4. A process according to claim 3, wherein the fine particulate waste material consists predominantly of an alkaline earth metal carbonate material

5. A process according to claim 4, wherein the fine particulate waste material comprises from about 60% by weight to about 80% by weight of limestone particles, the remainder being predominantly particles of clay.

6. A process according to claim 3, 4 or 5, wherein the particle size distribution of the fine particulate waste material is such that substantially all of the particles have a diameter smaller than 75pm.

7. A process according to any one of claims 1 to 6, wherein the aqueous suspension is dewatered by means of a filtration device which operates at a pressure differential in excess of 85 psig (590kPa).

8. A process according to any one of claims 1 to 7, wherein the cake of partially dewatered fine particulate material is mixed with the dry alkaline earth metal carbonate material in a weight ratio of partially dewatered waste material to dry alkaline earth metal carbonate of from 1:1 to 5:1.

9. A process according to any one of claims 1 to 8, wherein the cake of the fine material and the dry alkaline earth metal carbonate material are mixed in equipment which imparts a tumbling motion to the components to be mixed and is also provided with a high speed impeller which is effective in disintegrating the lumps of waste material cake.

10. A granular product whenever prepared by the process of the first aspect of this invention.

11. A granular soil conditioning agent prepared by mixing together a moist cake of a fine particulate material and a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all the particles have a diameter small than 75pm.

12. A process for treating an aqueous suspension of a fine particulate material, substantially as hereinbefore described, with reference to the accompanying Example.

13. A granular product suitable for use as a soil conditioning agent, substantially as hereinbefore described, with reference to the accompanying Example.