Note: Descriptions are shown in the official language in which they were submitted.
~L~ 3'~9
- 2 -
The present invention relates to a method for :~
dumping thixotropic m~lds', especial'ly "red mud" ~hich ';
is formed when sodium aluminate is extracted by the Bayer ~:
~ process,
S In the Bayer process', which ls a chernical extraction
- process', al~minium oxide is made from bauxite. At
temperatures between 90 and 300C, crushed bauxite is
brought in contact with sodium aluminate solution in
cir~ulation', the various aluminium hydrates contained in
10 the bauxike being dissolved with the formation of sodium
aluminate', while impurities', such as iron oxides and
titani~m oxides, as well as by-products fonmed during the . . ~::
e~tractlon opera~ion', e,g~ sodium aluminium silicates, ~'
remain suspended in the decomposition solution
: 15 Red mud (which is in most cases separate~ from the .
~: decomposition solution by sed'imentation with the aid'of
. settling thickeners and', to remove the sodium and aluminium com-
: pounds present, is washed with hot water in counter.-current
manner in up to` 10 washîng thickener machines in ~ :
20 succession) is a troublesome waste product in the
extraction of aluminium oxide by the Bayer process.
::; Depending on the quantity of the bc~uxite, between 002 and
1.3 tons of red mud, calculated as dry substance, have ~:
to be dumped or put to other use for each ton of~aluminium-
3 3~ 3.~
oxid~ produced, An analysis of the dried red mud
occurring during the processing of lateritic bauxite
shows, by way of example~ the following composition:
SiO2 - 14.03% by weight
TiO2 - 7.20% by weight
Na2~ ~ 10.20% by weight
Loss on ignition9.04% by weight ...
2 3 29.56% by weight
A1203 ~ ~3.90% by weight
CaO _ .~6 30% by weigh~
Despite considerable effort it has not been possible
hitherto to put red mud, approximately 25 million tons
of which are at present produced annually throughout the
world, to any useful purpose to the necessary extent. The
major part of the red mud has~ therefore, even today5
still to be stored in dumps (e.gO in lakes or reservoirs)
or pumped into the sea as diluted mud.
When red mud is dumped, the mud from the final
washing stage, with a concentration of approximately 150
tc 360 g of solids per litre, is pumped into red-mud lakes
where the solid substance set~les out~ Depending on the
~ . ~umber of washing stages used, the red mud still contains
: 1.2 to 4% by weight of soluble Na20 relative to the~ dry
: substance. After~sedimentation has taken place,~the
supernatant liquid containing Na20 is fed back into the
production process where it is used in the manufacture of
alumina with 0.3 to O~S tons of C02 per ~on of A12o3.
Owing to the conversion of sodium aluminate into
Na~C03 during storage in the red-mud lake causticisation
: 30 of the soda with quicklime or removal thereof by
separation of foreign salts ~separation of common salt)
:
4 ~ 33~
is necessary. However, other inorganic salts, such
as sulphates and phosphates, as well as organic
substances', also are trapped in the liquor circulation
system during the above-mentioned feedback o the dumped ' ~-
liquid, contaminate the decomposition liquor and con- ;
sequently increase the costs.o~;purifying the liquor.
It is known that when red-mud lakes are established
many factors must be taken into account. Thus'l to prevent
alkaline water from ~ntering the ground water', the dikes
are sealed with suitable building material. Where
necessary the tanks are also lined with a rubber film or
with a thick layer of compressed silt4 Neither proposal
has'~ however'~ yet assumed any ;mportance'9 as', on the one
~ hand', plastic films undergo ageing and become penmeable ~ '~
in time and', on the other'~ very high layers of silt are
required for sealing purposes.
In red-mud lakes the height of the dike is kept
below 10 m for reasons of cost and the risk of a breach
in the dike~ These lakes are~rarely made deeper than
6 m', so that large areas of land are necessary ~or
depositing the red mud~
To increase the solids content of the red-mud lake',' :
: a system has~been~developed whereby the bed of the
dumping site is p~ovided with a~sand filter layer~and a
network of drainpipes. This method', also-9 is ~ howe~er',
considered by many red~mud operators to be uneconomical ~:
and laborious. Also', it can be applied only in regions ~:
which have a subsoil impervious to water.
A need exists', therefore', for an improved method
30 for dumpin~ thixotropic muds, especi;~lIy red mud as ~-
a by-product of the manufacture of aluminium oxide, ~ ;
- - 4
'
' ~
. , .
5 ~ 3~-~
wherein the above-mentioned disadvantages are eliminated
and storage heights depths up to at least lS m can be
achieved.
This invention provides a method of depositing and
solidiying thixotropic mud', which comprises filtering',
washing and mechanically liquefying the mud and then
transerring it to a d~mp for solidi~ication; the
mechanical liquefaction being performed on moist m~d
having a solids content of over 30% by weight and in
the presence of an electrolyte; the dump being provided
with at least one retaining embankment or discharging
meteoric water by hydraulic action', the embankment
containing at lea~t one substance having cation exchanging
properties whereby alkali metal ions absorbed by the water
from the mud are removed ~rom the water before it is discharged
The mechanical liquefaction may be effected by
:: means of apparatus applying shearing forces to the mud.
The electrolyte is generally used in an amount of 0.2 ¦
to Go6% by weight', especially 0~25 to 0.4% by weight',
rela~ive to the dry weight of the mud', and may or ~ :
example be sulphite waste liquor', NaOH', Na2C03', or a
polyphosphate or polyacrylate or other polyelectrolyte~ ~ '
: The d~mp may for example be in the form of a
; reservoir', at least one of whose banks possesses the
necessary cation-exchanging capabilityO Other parts
of the dump may, if desired, also contain a ca~ion
exchanging material.
:~- As indicated above9 at least one of the retaining emban~-
ments must be capable of hydraulically discharging water It
is prefered that this embankment should achieve this by being
: water-permeable; mechanical arrangements are unnecessary
It has been shown, surprisingly, that with red mud dumped ~
with a solids content of over 30% by weight ~e g ' 30-70%), ;
~ preferably over 45% by weight (e g 45-55%), there is
: _ 5 _ :
; , . , : '' ' '' '
33~
~ .
- 6 -
obtained within a few days due to the
thixotropic properties7a clay-like mass which is
impervious to rainwater and from which the alkali metal ions
cannot therefore be washed into the subsoil or into the ground
waterO If the red mud is dumped conventionally with
concentrations under 30% by weight by the usual method',
it is certainly concentrated'~ aEter standing ~or a long
time, as a result of sedimentation'~ but it remains soft~
impossible to walk on and permeable to water', so that on
porous subsoils the alkali metal ions contained in the red mud
penetrate the subsoil and thé ground water. The
combination of the above-mentioned measures in accordance
with the invention has enabled red mud to be dumped in
a way which does not adversely affect the environment.
It has been proven by X-ray analyses that due to a
chemical reaction of the constituents contained in the
red mud it is possible to achieve further solidi~ication ~ ~
in course of time~, which leads to an increase ;n shearing -force (storage height or depth3 and even better sealing '~
against rainwater', consequently further reducing the
20 risk that alkali metal ions'will be washed out. ' ;
According to the preerred proce~s of the
invention the liquefied mud is conveyed at pressures from
10 to 200 atmospheres', more preferably 30 to 100
atmospheres. Positive-displacement pumps have proved
most successful in this case. The liquefied mud can be
p~mped without difficulty over dis~ances of up to 15 km ;~
or beyond, if necessary.
The viscosi~y of the filter moist mud introduced ' ~-
into the reactor (by screw conveyors or other conveying ~ '
systems) is usually between 30~000 and 130,000 c.p. It ~;
ran be reduced by mechanical liquefaction down to
- approximately 10900Q c.p. and a minimum of SjO00 c.p.
~-
- 6 ~
~., ,~ , .
-
.
;. . . . .
3:~
. .
By the simultaneous use of small quantities o~ electrolytethe viscosity can even be lowered to 1,200 c.p.
It was not previously known and must be considered
as surprising that thixotropic m~lds in a liquefied state
S can be pumped over distances up to several kilometers
in a pipeline on to dumps which have at least one
hydra~lically acting embankment contai~ing a ca~ion-
exchangeable substance through which the meteoric water
is discharged. Sedimentation of the solids either in the
pipeline or in the dump has not been o~served. It has
been observed, on the other hand, that mud spreading like
lava in t~e dump has solidified after ~nly 3 to 5 weeks
to the extent tha~ it is possible to walk and driye o~er it, By
the addition ~f chemical additives [e,g, Ca ~ ~ the solidi-
f,icatiun oper~tion can be accelerated still urther, Solidiied
red mud treated in this way can be used to seal the s~lbsoil for -
~he ~stallation of a new dump, There is consequently no
need to feed back nd recover supernatant water
containing alkali metal,ions.
The particle size of the mud should be between 0.1
and 150 ~m, preferably between 0.3 and 10 ~m, although
these particle ranges do not represent critical valuesO
It has been shownj ~urther~ that it is especially
advantageous to build the dump in stages,~ For this
purpose, the dum? is divided into several sectors which
are supplied alternately with liquefied mud. In this
way, there is always suficient time for the red mud
to solidify and dry out in layers. Also, cracks due
to shrinkage occurring after solidification are filled
up with fresh red mud, Continuous Gompression and
packing of the deposited mass are thereby achievedO The
- 7 -
,
~ . .
,~ ' ' , ' ~ ' '
- `'``` - 8 ~ 3~3
intermedlate dams required to divide the dump into :'
sectors can be made from already hardened red mud.
The method of the invention will 710W be described
in detail by reference to the accompanying diagrammatic
drawing
Reference number 1 denotes th~ inlet of the mud
from the preliminary thickener into the washing thickener
2 and 3 designates the washing liquor discharged from the
thickener. The red mud is conve~éd from the washing
thickener 2 into the filter 4', e g. a d~m-type vacuum
filter', washed with washing water from the pipe 5 and
then supplied to the reactor 6 where mechanical liquefaction is
efected by means of the agitator 7 in the presence.of an elec- .
trolyte By the pump 8 the liquefied mud is conveyed into the ~.
dump according to the invention where the embanking.material
: with cation exchange capacity is designated by 9 an~ the
deposited mud by 10. Clay or silt',:for example, can be :
used as embanking material'J'although especially mixtures
of clay 9 silt', sand and rubble can also be used. Pre-
20 treatment of the bed of the dump is not necessary. The '
red mud can be conveyed directly into the dump where it
solidifies and, consequently', forms a bed impervious ;~
to water.
The height of the deposited red-mud can be', for `~
example', up to 20 m over a gro~md area of 90',000 m and ~.
can be increased considerably still further.
The advantage of the dumping method according to
the invention is', finally9 seen in the fact that with
the same ground area and the same storage height
30 approximately 1.4 times as much red mud in the moist `:~
state can be stored as.i~ conventional dumping~ Also,
. . .
.,.. . . . . ........... . .......... ,
- ~ . ~ ., : -, " :.,
- ~ ,
_ 9 ~
tlle clump hei,ght can be increased to 3 to 4 times the
height of a conventional dump, so that there results a
storage capability o~ 4 to 6 times the quantity of
red mud.
:'`
. .
. .
,', ~.
;,
' :
. ,
.
