Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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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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
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~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
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; 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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and aluminium, oxides of iron, together with traces of
other oxides,
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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
SO 1 - 3
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.
. .
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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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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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
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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
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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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