CA1059537A

CA1059537A - Treatment of particulate materials

Info

Publication number: CA1059537A
Application number: CA268,898A
Authority: CA
Inventors: Norman O. Clark
Original assignee: English Clays Lovering Pochin Co Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1975-12-30
Filing date: 1976-12-30
Publication date: 1979-07-31
Also published as: DE2659218C2; SE7614709L; BE850008A; DE2659218A1; FR2337112A1; NL7614562A; FR2337112B1; GB1546588A; DK586776A; SE432095B

Abstract

IMPROVEMENTS IN OR RELATING TO THE TREATMENT
OF PARTICULATE MATERIALS

ABSTRACT OF THE DISCLOSURE
A cement clinker is produced by forming a raw cement slurry; dewatering the raw cement slurry in a pressure filter at a pressure in excess of 1450 psig to form a filter cake containing less than 15% by weight moisture; and subjecting said filter cake to a heat treatment at an elevated temperature for a time sufficient to produce therefrom a cement clinker.

Description

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~CKGROUND OF T~l~ INV~NTlON
Tbis invention relates to the dewatering of slurries of ~articulate materi~ls and the subsequent heat treatment, or calcination, thereof in a furnace and, more particularly, is concerned with a process for manufacturing cement clinker.
In the manufacture of Portland cement, an alumino-siliceous material is reacted in appropriate proportions with a calcium carbonate material. By the term "aluminosiliceous material" there is meant herein a material containing silica and alumina either in the free state or combined with other chemical compounds or with each other, e.g. as an alumino-silicate. In the so called "wet process" for manufacturing Portland cement, there are generally used raw materials which are soft and contain a high proportion of water: examples of such soft, moist raw materials are natural chalks, especially those from Southern England and parts of France and Belgium, and alluvial clays. Natural chalk normally contains about 15%
to,25% by weight of water and is often closely associated with hard impurities, such as nodules of flint, when quarried.
Conventionally lumps of natural chalk are mixed with larage volumes of water in a wash mill, or levigator, in which rotating harrows cause the lumps of chalk to abrade on themselves and - on the walls of the mill. The chalk particles pass into suspension leaving the flint nodules on the floor of the levigator; this enables the flint nodules originally present in the crude chalk to be separated from the chalk suspension.
i The suspension overflowing from the levigator generally .

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;1~59537 comprises about 75~ to 85C,J by weight of wa-ter and must be thickened by means of, for example, a continuous t}lickener, before it is blencled lvith an alu~inosiliceous material, e.g.
an alluvial clay, which is i-tself frequerltly in the form of a slurry. The hi~hest solids content of the blended slurry consistent with suf~icient fluidity to enable the slurry to be fed to a cement kiln has conventionally been about 50 to 60%
by weight. Feeding a blended slurry of this consistency straight to the cement kiln means that additional fuel has had to be consumed in order to ren~ove the large quantity of water present.
hlore recently, the step of dewatering the blended slurry by filtration using a rotary vacuum filter or a conventional plate and frame filter press, which operates at pressures below 200 p.s.i.g., has been intro~u-c~d and the resulting filter cake is then fed to the cement kiln. The filter cake produced by such means has the consistency of a paste or dough and contains more than 20% by weight of water. The moist cake is either introduced directly into the calcining kiln with the consequent ; waste of heat energy in evaporating still large quantities of water or, alternatively, an additional thermal drying step is - introduced into the process in order to reduce the moisture content o~ the feed to the kiln, again with consequent increased energy and capital costs.
A blended slurry for feeding to a cement kiln to produce a cement clinker is a difficult material to dewater because it is composed of relatively large particles of a chalk (the particles ranging in size from about 1 to about 5 .. ,.. ~ . .. . - . . .

i , ~ . ,, ~ ' ' ' ~-` lVS5~537 ~m) and very much finer particles of an aluminosiliceous material of which a substantial portion are colloidal (i.e., the particles are finer than 0.5 ~m). The fine particles fill the spaces between the coarse particles , 5 and the resultant mixture forms a very impermeable filter cake with the consequence that a raw cement slurry is extremely difficult to filter especially when the slurry has a high solids content.
According to the present invention there is provided a process for producing a cement clinker which process comprises the following steps:
(,a) forming a raw cement slurry consisting ~ essentially of a particulate aluminosiliceous mat,erial ,,' blended with a particulate calcium carbonate material in water in proportions suitable for the formation therefrom by heating to an elevated temperature of a cement clinker;
l (b) dewatering the raw cement slurry in a ,' ' pressure filter at a pressure in excess of 1450 psig to ; form a filter cake containing less than 15~ by weight . . .
, 20 moisture; and (c) feeding said filter cake, without an inter-' ~ vening thermal drying step, to a furnace and subjecting ... .
; said filter cake to a heat treatment at an elevated A, ~ ' . temperature for a time sufficient to prPduce therefrom '1 25 a cement clinker.

, The calcium carbonate material will preferably ,l , comprise a natural chalk. The aluminosiliceous material `I~ will generally comprise, in addition to oxides of silicon , ~ . .

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4a and aluminium, oxides of iron, together with traces of other oxides, '.
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~(~55~537 e.g. the oxi~es of titani~l and ma~nesium; and suit~ble alumino-siliceous ma-terials include primary and sedimentary classes of clay, whether alluvial or glacial, e.~. sha]es, alumino-siliceous stones, e.g. sandstone, and aluminosiliceous muds, for example Medway mud. The relative proportions of the components will generally be such that, by reaction of the aluminosiliceous material with the calcium carbonate material e.g. natural chalk particles, at elevated temperature, a cement clinker having a composition within the following range is obtainable:
Constituent ~ by wei~ht CaO 60 - 67 SiO2 17 - 26 ~-~ Al2O3 2 - 28 Fe23 0.5 - 6.0 MgO 0.1 - 4.0 Na2O+K2O0.5 - 1.5 Prior to carrying out step (a) of the process of the invention the particulate materials may be ground, for example in a wet ball mill or in a sand grinding mill or by -~ agitation in a vessel equipped with a high speed stirrer.
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Additionally or alternatively, the particulate materials may be subjected to a washing process, a particle size separation process by gravitational or centrifugal sedimentation, a froth flotation process or other separation process. For example, if the aluminosiliceous material is an alluvial clay, it may be advantageous to form it into a slurry with water and to .~ ~
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105!:~537 allow coarse i~l~urities, such as par-ticles o~ grit, to sediment out; and if the calcium carbonate material consists of lumps oI natural chalk having a natural water content in the range 15-25C~o by weight it may be advantageous to ~ix them with water and subject them to agitation by means of a high speed stirrer to break up the lumps and release any hard impurities, mainly particles of flint, which sediment under gravity.
In step (a) of the process of the invention the slurry of aluminosiliceous material is blended with the slurry of chalk in the proportions required to give the c~emical compounds which are needed to produce, on calcination and vitrification, Portland cement; generally there will be used about 3 parts by weight of chalk to 1 part by weight of alumino-; siliceous material. Alter~tively, separation of coarse impurities from the aluminosiliceous material and from the calcium carbonate material can be effected after the blended slurry has been formed. Advantageously, the blended slurry will have a solids content ranging from about 50~0 to about 60~o by weight.
In step (b) of the process of the invention the --~ pressure filter is conveniently a tube pressure filter, for example a tube pressure filter of the type described in British Patent Speci~ication No. 1,240,465, but it may be a pressure filter of the high-pressure plate filter type such as is described, for example, in British Patent Specification No.
; 1,389,003. The pressure filter should preferably be capable of operating continuously under fully automatic control so . , . . .. , - : :

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~OS~537 that expenditure on labour and supervision can be reduced to a low level. The filter cake produced by the pressure filter should have a moisture content not exceeding 15%
by weight, but it will not normally be less than 10% by weight. The blended slurry is dewatered at a pressure in excess of 1450 psig.
At the end of step (b) of the process of the invention there is formed a filter cake which constitutes the feed material for the cement kiln. The filter cake is non-sticky and will break down on gentle crushing or even on handling in, for example, a screw or ribbon conveyor to give a crumbly material with very little dust which provides a consistent feed in the best statç of ; division for a conventional cement kiln. Thus, in step (c) of the process of the invention the filter cake produced by the pressure filter is preferably divided into granules of suita~le size for feeding to the furnace, or kiln, for example by light crushing, pellet-izing, nodulizing or briquetting. After the fiiter c~ke has been fed to the cement kiln it is heated therein under conventional conditions to form the desired cement clinker. Generally, the cement kiln is operated at a temperature in the range of from 1300C to 165QC.
The invention is illustrated by the following Example.
EXAMPLE
~ raw cement slurry was prepared by mixing together L~

l~S5~537 in water 1 part by ~veight oI r.ledway mud and 3.2. parts by weight ol white Kenl~ chalk, both calculated as dry mat~rial, the proportions being those which were calculated frorn the properties o the component materials to give an ordinary Portlancl cement clinker conIorming to 13ritish Standarcl Specification No 12: 1958.
A mixer equipped with a high speed stirrer was used to break up lumps, principally o:E chalk, and to release the hard flint impurities which sedimented under gravity. The slurry which had a solids content of 58~ by weight was passed through a sieve of aperture 0.5 mm to remove any grit or flint particles still in suspension.
The blended slurry was supplied as feed to a battery oi ninety tube pressure filters of the type described in British Patent Specification No. 1,240,465 each OI which was operated at a hydraulic fluid pressure of 1500 p.s.i.g. and gave a filter cake having a water content of 14.5% by weight.
The filter cake produced by the tube pressure filters was non-sticky and consisted of fragments of a cylindrical surface of average thickness about 5 mm; the -- material was conveyed in a screw conveyor which also served to break up the cake into granules of suitable size for feeding to a long rotary cement kiln in which it was calcined and vitrified ,at a temperature of about 1400C.
As a comparison the same blended chalk/clay slurry was dewatered in a conventional recessed-plate type filter press at a` pressure of 120 p.s.i.g. (825 kN m 2) but the water -, -- 8 ---.: - .... , . : .

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content of the cake could not be reduced by thls method to less th~n 20. 2(,J by wei~ht which was too moist for direct feeding to a rotary cement kiln withou-t some preliminary thermal treatment.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a cement clinker which process comprises the following steps:
(a) forming a raw cement slurry consisting essentially of a particulate aluminosiliceous material blended with a particulate calcium carbonate material in water in proportions suitable for the formation there-from by heating to an elevated temperature of a cement clinker;
(b) dewatering the raw cement slurry in a pressure filter at a pressure in excess of 1450 psig to form a filter cake containing less than 15% by weight moisture; and (c) feeding said filter cake, without an inter-vening thermal drying step, to a furnace and subjecting said filter cake to a heat treatment at an elevated temperature for a time sufficient to produce therefrom a cement clinker.

2. A process according to claim 1, wherein the calcium carbonate material comprises a natural chalk.

3. A process according to claim 1, wherein the relative proportions of the calcium carbonate material and aluminosiliceous material are such that on reaction thereof, at elevated temperature, a cement clinker having a composition within the following range is obtained:

Constituent % by weight CaO 60 - 67 SiO2 17 - 26 Fe2O3 0.5 - 6.0 MgO 0.1 - 4.0 Na2O+K2O 0.5 - 1.5

4. A process according to claim 1, wherein the blended raw cement slurry has a solids content ranging from about 50% to about 60% by weight.

5. A process according to claim 1, wherein the filter cake fed to the cement kiln is heated therein to a temperature in the range of from 1300° to 1650°C.