Note: Descriptions are shown in the official language in which they were submitted.
1200V76
This invention relates to an improved method of
producing high grade magnesium oxide (magnesia) from crude
magnesite or from other crude magnesium-containing
compounds, such as magnesium hydroxide, magnesium carbonate,
basic magnesium carbonate etc., all of which can be
thermally decomposed (calcined) to crude magnesium oxide.
Magnesium oxide is used for making furnace bricks for
roasting and smelting furnaces, as well as in numerous other
applications. For many furnaces and smelters, for example
the basic oxygen steelmaking furnace, the purity of the
magnesium oxide used to make the Eurnace bricks is
critical. In particular, the iron, aluminium, calcium,
silicon and boron contents must be below specified limits.
There a~e a number of grades of magnesium oxide used for
making furnace bricks, each grade having its own
specification limits.
In known methods of treating crude magnesium-containing
compounds to produce magnesium oxide of a particular grade,
the magnesium-containing starting material is normally
heated under controlled conditions to induce decomposition
of the magnesium-containing compound to crude magnesium
oxide. High grade maynesium oxide can be formed by removing
the impurities by physical beneficiation techniques and/or
by dissolving the crude magnesium oxide, purifying the
resultant solution, precipitating or crystallizating a
magnesium compound from the purified liquor, and thermally
decomposing (calcining) the magnesium salt to magnesium
oxide. Proposed leachants or dissolving media include
hydrochloric acicl, nitric acid, sulphuric acid, and aqueous
carbon dioxide (carbonic acid).
Physical beneficiation techniques are unsatisfactory in
cases where the major impurity, in particular iron oxide, is
present in the initial magnesium-containing compound in a
solid solution state. For example, the starting magnesium-
containing compound might be crude magnesite which contains
iron carbonate (siderite) in solid solution with the
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00~)~6
,
magnesium carbonate (magnesite). When the crude maynesium-
containing compound is thermally decomposed (calcined) the
product is crude magnesium oxide with iron oxide in solid
solution with the magnesium oxide.
Dissolution of crude magnesium oxide, derived from the
crude magnesium-containing compound, in hydrochloric acid,
nitric acid or sulphuric acid, results in the simultaneous
dissolution of the impurity in the crude magnesium oxide.
This impurity must be removed before the magnesium salt is
recovered. When the pure magnesium salt is heated to form
pure magnesium oxide by a decomposition reaction, acid gases
are evolved and these must be collected, purified and
reconverted to a form suitab~e for recycling to the
dissolution circuit.
An aqueous slurry of crude magnesium oxide reacts
readily with carbon dioxide to form soluble magnesium
bicarbonate. The latter is stable only in the presence of
excess carbon dioxide. If the excess carbon dioxide i5
removed by air sparging and/or heating, an insoluble
hydrated magnesium carbonate or basic magnesium carbonate is
precipitated, the nature of the product depending upon the
slurry temperature. The insoluble hydrated magnesium
carbonate or basic magnesium carbonate is readily thermally
decomposed (calcined) to magnesium oxide, carbon dioxide and
~ater vapour. After removing the water vapour, the carbon
dioxide can be led directly back to the dissolution
tleaching) circuit. Thus the recycled carbon dioxide is the
leachant for fresh slurry of crude magnesium oxide
introduced into the leaching circuit. Because a complex
leachant recovery circuit is not required, dissolution with
carbon dioxide has a significant advantage compared with
dissolution with hydrochloric acid, nitric acid or sulphuric
acid.
Although ferric compounds are not normally soluble at
pH values greater than 3 or 4, it was found that when an
aqueous slurry of crude maynesium oxide, derived from the
-- 2
1;~0~ 6
crude magnesium-containing compound, is dissolved by the use
o~ carbon dioxide, considerable and undesirable iron
dissolution also occurs, particularly when the crude
magnesium compound contains iron oxide or iron carbonate
present in solid solution. Unless the iron dissolved during
dissolution of the crude magnesium oxide is removed, it will
contaminate the hydrated magnesium carbonate or basic
magnesium carbonate precipitated when the excess carbon
dioxide is removed by air sparging and/or heating. As a
consequence, the magnesium oxide derived from the
precipitated hydrated magnesium carbonate or basic magnesium
carbonate by thermal decomposition (calcination), will also
be contaminated by iron oxide. Until nowr no adequate means
of removing such iron, when present, has been available.
The thermal decomposition or calcination of magnesium-
containing compounds to magnesium oxide is a well understood
art - see for example ~.C. Mackenzie, editor, Differential
Thermal ~nalysis, Volume 1, Fundamental Aspects, Academic
Press, London, 1970. However, it has not been previously
appreciated that control of the thermal decomposition
(calcination) conditions is of importance when leaching an
aqueous slurry of crude magnesium oxide, derived ~rom the
crude magnesium-containing compound, with carbon dioxide.
The calcination conditions control the surface area of the
crude magnesium oxide which in turn controls the rate at
which the aqueous slurry of the crude magnesium oxide reacts
with the carbon dioxide. It is commercially desirable to
achieve the hi~hest practical reaction rate.
The amount of iron dissolved, while reacting the
aqueous slurry of crude magnesium oxide with the carbon
dioxide also depends on the dissolution or leaching
conditions, particularly the leaching temperature and the
time between the formation of the aqueous slurry of crude
magnesium oxide and the introduction of the carbon
dioxide.
^- - . .. ,. . .... ~
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lZOV(~76
In its broadest aspect the invention provides a process
for preparing substantially pure magnesium oxide from crude
magnesium-containing compounds which have an iron impurity,
which process comprises calcining the crude magnesium-
containing compounds to crude magnesium oxide, forming a
slurry of the crude magnesium oxide and reacting it with
carbon dioxide, removing the unreacted solid from the iron-
containing pregnant magnesium bicarbonate solution so
produced and adding a water-soluble aluminimum salt to the
pregnant solution to precipitate out the iron, air sparging
and/or heating the solution after removal of the
precipitated iron to produce a precipitate of hydrated
magnesium carbonate and/or basic magnesium carbonate,
separating the precipitate and decomposing it to produce
substantially pure magnesium oxide. As an alternative
sequence in the process, the aluminimum salt is added to the
slurry prior to or during the reaction of the slurry with
carbon dioxide~
In one particular aspect of the invention, the
calcining of the crude magnesium-containing compounds having
the iron impurity is effected by heating the crude
magnesium-containing compounds to a temperature and for a
time such that from 85% to 95~ by weight of the magnesium
present in the crude magnesium-containing compounds are
transformed into crude magnesium oxide with a high surface
area.
In a further aspect of the invention, the slurry of
crude magnesium oxide is reacted with the carbon dioxide at
a temperature within the range of 10C to 45C using a
slurry agitation rate and reaction time sufficient to ensure
that more than 95~ of the magnesium present as magnesium
oxide has reacted to form soluble magnesium bicarbonate.
Preferàbly, the crude magnesium oxide is dry ground (e.g. to
such a size that 100~ passes through a 400 micron mesh and
80~ passes through a 150 micron mesh) prior to slurrying
with water and the pulp density is adjusted to a value in
-- 4 --
.... ~.~ .. ,, .... . ~
.
` 1200076
the range o~ 2% to 5% solids wi.th recycle liquor having a mag-
nesium content of less than 0.5 gpl and preferably less than
0.2 gpl. The reaction with carbon dioxide may be effected at a
partial pressure within the range of 175 kPa to 700 kPa, the
time taken between the formation of the slurry and the contact
of the latter with the carbon dioxide being less than O.5 hour.
The preferred aluminium salt employed to precipitate
out the iron is aluminium sulphate. Precipitati.on may be
carried out under a carbon dioxide partial pressure of. 175
kPa to 700 kPa, the amount of aluminium sulphate or other
water-soluble aluminium salt being such that the
[Fe x lOO/Mg] concentration ratio of the resultant solution
is in the range of 0.0 to 0.2.
The process may be effected in such a manner that the
carbon dixide evolved during any of the process steps is
recovered, purified, compressed and recycled to the leaching
circuit.
Preferred embodiments of the .invention will now be
described with reerence to the accompanying drawings, in
which:-
Figure l is a flow chart illustrating the process stepsof the invention in which the iron impurity is removed from
the pregnant liquor, which has been separated from unreacted
solid material prior to the precipitation of hydrated
magnesium carbonate or basic magnesium carbonate, and
Figure 2 is a flow chart illustrating the process steps
of the invention in which the iron impurity is removed
simultaneously with the dissolution of the magnesium, as
magnesium bicarbonate, from the crude magnesium oxide.
The invention as illustrated by Figure l is now
described. Crude magnesite ore is fed to a crushing circuit
(l) whereby the particle size of the crude magnesite is
reduced to a size suitable for thermal decomposition
~calcination). The optimum particle size of the crushed
crude magnesite depends upon the type of equipment used for
thermal decomposition (calcination) and will normally be
less than 4 inches and preferably less than l inch.
.
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.
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~Z00076
The crushed crude magnesite ore is now thermally
decomposed (calcined) in, for example a rotary kiln (2).
Fuel, in the form of fuel oil or LPG plus excess air are
used to fire the rotary kiln or alternative thermal
decomposition (calcination) furnace (2). The off-gases,
containing impure carbon dioxide formed by the thermal
decomposition of the crude ma~nesite, are collected,
purified and the carbon dioxide content compressed by
standard techniques ~11).
The temperature and time of calcination are controlled
by the composition of the feed material. Calcination should
be carried out at-as low a temperature and for as short a
time as is consistent with the optimum decomposition of the
erude magnesite to crude magnesium oxide, the latter having
as high a surface area as possible. For erude magnesite,
that is ore with a magnesite content of about 70% or more,
optimum ealcination conditions are of the order of 700C for
one hour, the aetual time depending to some extent on the
partiele size of the feed material. The ealeination
conditions should be sueh that about 90~ of the erude
magnesite has been thermally decomposed to erude magnesium
oxide. Caleination at a lower temperature or for a
substantially shorter time results in a reduced amount of
crude magnesite that has been thermally decomposed to crude
magnesium oxide. Calcination at higher temperatures or for
substantially longer times results in over calcination; in
particular the surface area of the crude magnesium oxide is
substantially redueed sueh that it reacts very slowly with
earbon dioxide when it is slurried with water and contaeted
with the earbon dioxide.
The hot, cr~ide magnesium oxide from the ealcination
eircuit (2) is allowed to eool to room temperature and then
passed to a grinding cireuit (3) where its partiele size is
reduced by dry grinding to a size suitable for leaching,
preferably 100~ passing through a 400 micron mesh and 80%
passing through a 150 mieron mesh. Grinding must be carried
out in the dry state sinee if wet grinding is carried out,
.
` ` lZ00~76
there is a reaction between the crude magnesium oxide and
the water (slaking) which affects the amount of iron
dissolved in the subsequent leaching stage(4).
The ground crude magnesiwn oxide is slurried with water
just before it is introduced into the leaching circuit
(4). The reaction between the crude magnesium oxide and the
water used to slurry the former so that it can be introduced
into the autoclaves, that is, the slaking reaction, is
exothermic, that is, it generates heat. The amount of iron
that is dissolved in the subsequent leaching step is
affected by the temperature of the slurry and the time
before it is contacted with the carbon dioxide. An increase
in the slurrying time and an increase in the slurry
temperature both lead to an increase in the amount of iron
dissolved in the leaching step. Preferably, the slurry
temperature should be maintained at the leaching
temperature, that i5, in the range 10C to 45C while the
slurring time should be kept below 30 minutes.
Leaching is carried out in a closed reaction vessel (~)
with suitable inlets for feed slùrry, water ~make-up and/or
recycle-liquor) and carbon dioxide. Suitable outlets for
sampling and slurry discharge are also necessary. The
reaction vessel is fitted with a suitable agitation system
and baffled such that there is adequate mixing and
dispersion of the carbon dioxide-throughout the slurry.
~ eaching conditions (time, temperature, carbon dioxide
partial pressure, pulp density and initial leachant
composition) are controlled by the solubility of magnesium
bicarbonate, the product of the reaction between the slurry
of crude magnesium oxide and the carbon dioxide. The amount
of iron that is simultaneously dissolved is also affected by
the leaching conditions, particularly the temperature, pulp
density and the initial leachant composition.
Since the solubility of magnesium bicarbonate increases
with decreasing temperature, it may be considered preferable
to leach at as low a temperature and at as high a pulp
density as possible, say of the order of 5C and 5% solids
-- 7 --
.
^ 120()0~76
respectively. However, the amount of iron that is dissolved
under these conditions is excessive. The amount: of iron
that is dissolved decreases as the leaching temperature and
pulp density are increased and decreased respectively.
Preferred leaching temperatures and pulp densities are in
the range of 10C to 45C and 2% solids to 5% solids
respectively. The preferred pulp density should be such
that the solubility limit of the magnesium bicarbonate,
formed by the interaction of the magnesium oxide with the
carbon dioxide, is not exceeded at the operating carbon
dioxide partial pressure and leaching temperature.
The preferred leaching time is such that greater than
95~ of the available magnesium, present as magnesium oxide,
reacts to form soluble magnesium bicarbonate. The preferred
leaching time depends upon the leaching temperature, carbon
dioxide partial pressure r pulp density and agitation rate
and also on the calcination conditions used to thermally
decompose the crude magnesite to crude magnesium oxide. The
preferred leaching time should be less than two hours and
preferably less than one hour.
The preferred carbon dioxide partial pressure, which is
dependent upon the leaching temperature, should be as low as
practical so as to avoid the use of expensive and complex
high pressure reaction vessels (autoclaves). The preferred
carbon dioxide partial pressure-is in the range 175 kPa to
700 kPa.
It is normal hydrometallurgical practice to recycle
liquor which has been treated to recover the desired
product(s) and t:o remove undesirable impurities, back to the
leaching circuit:. In this way the amount of make-up process
water that is required is substantially reduced. In the
calcinat~ion-carbon dioxide lèach process, the filtrate
obtained after separation oE solid hydrated magnesium
carbonate from the slurry of hydrated magnesium carbonate
(~) is used to adjust the pulp density o the slurry of
crude magnesium oxide being introduced into the leaching
circuit (4). The Eiltrate is termed the recycle liquor in
Figure 1.
-- 8 --
. . ~ .
` lZVO(~6
The composition oE the recycle liquor, particularly the
magnesium content, has an important bearing on the amount of
iron dissolved in the leaching circuit. As the magnesium
content of the recycle liquor is increased, so the amount of
iron that is dissolved simultaneously with the formation of
soluble magnesium bicarbonate from the slurry of crude
magnesium oxide also increases. The preferred magnesium
content of the recycle liquor is in the range 0.0 gpl to 0.5
gram per litre (gpl), preferably in the range 0.0 gpl to 0.2
gpl .
AEter the aqueous slurry of crude magnesium oxide has
been reacted with carbon dioxide to form soluble magnesium
bicarbonate, any unreacted solid material is separated from
the magnesium bicarbonate slurry by solid/liquid separation
techniques (5). The preferred solid/liquid separation
technique is pressure filtration, using carbon dioxide as
the pressurization atmosphere. By this means all of the
magnesium bicarbonate remains in solution. With other
solid/liquid separation techniques such as counter-current
decantation and vacuum filtration, there is the possibility
that an insoluble hydrated magnesium carbonate such as
MgCO3.3H20 (nesquehonite) and/or an insoluble basic
magnesium carbonate such as Mg5(co3)4(OH)2.~H2o
(hydromagnesite) will precipitate out from the slurry and/or
out from the clarified liquor during solid/liquid separation
by virtue of the fact that the bicarbonate concentration in
solution is reduced by loss of carbon dioxide to the
atmosphere to such an extent that the solubility product of
the nesquehonite and/or hydromagnesite is exceeded.
The clarified iron-containing magnesium bicarbonate
solution issuing from the solid/liquid separation circuit
(5) is transferred to a further reaction vessel (6) where
the iron is removed by precipitation on addition of
aluminimum sulphate or another water-soluble aluminium salt
or a solution of such a salt. Precipitation is carried out
under a carbon dioxide partial pressure similar or identical
to that used in the leaching circuit (4), that is, in the
_ g
~200Q~76
range 175 kPa to 700 kPa. In this way, the possibility of
the undesirable precipitation of hydrated magnesium
carbonate and/or basic magnesium carbonate is avoided. The
amount of aluminium sulphate added is such that the soluble
iron content of the resultant slurry is below the desired
level. The preferred soluble iron content of the resultant
slurry i5 such that the [Fe x 100/~g] concentration ratio of
the clarified liquor obtained from the slurry formed on
addition of the aluminium sulphate to precipitate the iron
is less than 0.2 and preferably less than 0.1. For a
clarified liquor containing 10 gpl magnesium, the
corresponding iron contents are 0.020 and 0.010 gpl
respectively, or 20 ppm and 10 ppm respectively.
Precipitation, by addition of aluminimum sulphate, of
the iron present in the clarified magnesium bicarbonate
solution takes place rapidly, so that the preferred
retention time in the precipitation vessel (6) is in the
range S minutes to 10 minutes.
It is apparent that the interaction of the iron-
contaminated magnesium bicarbonate solution with the
aluminium sulphate in the reaction or precipitation vessel
(6) results in the precipitation of a complex magnesium-
iron-aluminium compound since both the magnesium and the
iron contents of the clarified liquor derived after removal
of the magnesium-iron-aluminium precipitate are lower than
those of the liquor being treated with the aluminium
sulphate in the reaction or precipitation vessel (6).
The slurry of the magnesium bicarbonate solution and
the magnesium-iron-aluminium precipitate is passed to a
pressure filtration circuit t7) and the magnesium-iron-
aluminium precipitate removed. Carbon dioxide is used as
the pressurizing atmosphere to prevent precipitation of
hydrated magnesium carbonate and/or basic magnesium
carbonate.
The iron-free magnesium bicarbonate solution issuing
from the pressure filtration circuit (7) is fed to a
precipitation vessel (8) where the magnesium is precipitated
-- 10 --
~Z000'76
by air injection (~parging) and/or heating such that the
carbon dixoide content of the iron-free magnesium
bicarbonate solution, in the form of dissolved carbon
dioxide and/or as the carbonate anion and/or as the
bicarbonate anion, is rapidly reduced so that hydrated
magnesium carbonate (nesquehonite, MgCO3.3H2O) and/or basic
magnesium carbonate (hydromagnesite, ~Ig5(CO3)4(OH)2.4H2O) is
preciptated. The rate at which the magnesium is
precipitated depends upon the -temperature of the pure
magnesium bicarbonate solution and the rate of air
injection. The rate o precipitation is increased by
increasing the solution temperature and by increasing the
rate of air injection. The temperature of precipitation and
rate oE air injection should be such that the magnesium
content of the resultant slurry solution is reduced to less
than 0.5 gpl and preferably less than 0.2 gpl within 1 hour
to 2 hours.
The rate o air injection should not be too high since
the carbon dioxide evolved during precipitation must be
collected, purified and compressed (11) before being
utilized in the leachi~g circuit(4). I the rate of air
injection during precipitation (8) is too high, the carbon
dioxide will be excessively diluted with air, leading to
complications with the collection, purification and
compression circuit tll).
The temperature at which precipitation takes place (8)
should not be too high since the bulk density of the
precipitate formed decreases with increasing precipitation
temperature. The precipitated hydrated magnesium carbonate
(nesquehonite) and/or basic magnesium carbonate
(hydromagnesite) and the magnesium oxide derived from it
should have as high a bulk density as possible. The
preferred precipitation conditions (8) are in the ranges of
20C to 45C and 1 hour to 2 hours respectively.
The slurry o precipitated hydrated magnesium carbonate
and/or basic magnesium carbonate (only the former is
indicated in Figure 1) is transferred to a conventional
. .
` lZ000~76
solid/liquid separation circuit (9) where the solid hydrated
magnesium carbonate and/or basic magnesium carbonate is
separated from the solution. Counter-curren~ decantation
and rotary vacuum filtration are suitable techniques for
carrying out this solid/liquid separation. The separated
solution, which contains 0.0 gpl to 0.5 gpl magnesium and
preferably 0.0 gpl to 0.2 gpl magnesium, forms the recycle
liquor to the leaching circuit (4).
The solid hydrated magnesium carbonate and/or basic
magnesium carbonate is transferred to a suitable furnace
(10) where it is thermally decomposed (calcined) to
magnesium oxide, water vapour and carbon dioxide. Evolution
of water vapour and of carbon dioxide may be carried out in
two essentially separate stages, so that carbon dioxide
recovery is more readily performed.
The water vapour is removed from the off-gases, and
after purification and compression ~11), the carbon dioxide
is returned to the leaching circuit (4). The optimum
calcination temperature is in excess of 600C, that is,
above the decomposition temperature of the hydrated
magnesium carbonate and/or basic magnesium carbonate.
Subsequent heating so as to ensure that the product has a
suitably high bulk density for furnace brick manufacture
usually requires calcination in the range 1600C to 1800C,
with or without intermediate briquetting or pressing of the
magnesium oxide produced at 600C.
To recover part of the magnesium precipitated as the
complex magnesium-iron-aluminium compound on addition of the
aluminium sulphate, or other water-soluble aluminium salt or
solution of such a salt, and to reduce the amount of
aluminium salt that is required to lower the [Fe x 100/Mg]
concentration ratio to the desired level, the magnesium-
iron-aluminium compound separated from the pressure
filtration circuit (7) is passed to a reaction vessel
tl2). The magnesium-iron-aluminium compound is reacted with
sulphuric acid/ the amount of acid added being suf~icient to
produce a slurry pH in the range 3.5 - 4.5 under these
- 12 -
.
~20(~0~6
conditions the magnesium and aluminium components of the
magnesium-iron-aluminium compound dissolves. The iron
component does not dissolve and is removed by conventional
filtration means (13). The iron-free magnesium-aluminium
sulphate solution is then used, together with any necessary
aluminium sulphate, to precipitate the iron from fresh
iron-containing magnesium bicarbonate liquor in reaction
vessel (6) as described above.
Figure 2, which illustrates the second method of
ensuring that the final product has an iron content within
specification limits, is essentially the same as that
described above and illustrated by Figure 1, except that the
aluminium sulphate or other water-soluble aluminium salt is
added to the leaching circuit rather than to the clarified
pregnant liquor.
The second method, as illustrated by Figure 2, consists
of a crushing circuit (1), calcination circuit (2) and
grinding circuit (3) which are the same as those described
above and shown in Figure 1. Leaching of the crushed,
ground calcined crude magnesite is carried out as previously
described in a suitable reaction vessel with the exception
that the aluminium sulphate or other water-soluble aluminium
salt or a solution o~ aluminium sulphate or other salt is
also added to the leaching vessel (4). The leaching
conditions are identical to those described above. Removal
of unreacted solid, which in this case includes the
magnesium-iron-aluminium compound that precipitates, is
carried out by pressure filtration usiny carbon dioxide as
the pressurizing atmosphere (5). Recovery of hydrated
magnesium carbonate and/or basic magnesium carbonate by air
sparging and/or heating the pregnant magnesium bicarbonate
solution (6), recovery of the solid hydrated magnesium
carbonate and/or basic magnesium carbonate (7), calcination
of the latter to the product, magnesium oxide, in a suitable
kiln (8), recovery of carbon dioxide (9) and recycling of
mother liquor to the leachiny circuit are all carried out as
described above.
:
- 13 -
., .
lZ00~76
As with the first method, as illustrated by Fiyure 1,
part of the magnesium component of the precipitated
magnesium-iron-aluminium compound is recovered by
dissolution (10) of the preci2itate, which in this case
includes leach residue, in dilute sulphuric acid such that
the resultant slurry pH is in the range 3.5-4.5. The leach
residue plus the iron component of the magnesium-iron-
aluminium compound are removed by conventional filtration
means (11), the clarified aluminiurn-magnesium solution being
uesed to precipitate iron from fresh pregnant iron-
containing magnesium bicarbonate solution (4).
Thus the second embodiment of the invention, as
illustrated by Figure 2, has the advantage over the first
embodiment, as shown in Figure 1, in that the former
requires one less reaction vessel and one less solid/liquid
separation circuit - (6) and (7) in Figure 1. However, both
embodiments result in the formation of a pregnant magnesium
bicarbonate solution low in iron and from which high grade
magnesium oxide can be recovered. By judicious use of
leaching conditions and the amount o~ aluminium salt used
to precipitate any soluble iron, it is possible to ~orm a
range o~ magnesium oxide products with iron contents of
0.05% or even lower.
The following examples are provided to illustrate the
features of the invention. These examples in no way limit
the purpose of the invention.
EXAMPLE 1
.
This example shows the effect of the calcination
conditions on the rate at which the crude magnesium oxide
reacts with carbon dioxide and the percentage dissolution of
the crude magnesium oxide in a given time.
A samæle of crude magnesite was crushed and the
-1/4" ~ 7 mesh (BSS) fraction collected. This fraction
contained 23.6% ~g, 3.34~ Ca, 1.84% Fe, 67.5% C03 and 3.31%
acid insoluble. ~ineralogical analysis indicated that the
sample contained abou-t 75% ma~nesite, 15% dolomite, ~%
siderite, the balance being made up on calci~e, talc and
quartz. The siderite was shown to be in solid solution with
the magnesite, by means of electron microprobe analysis of
individual~grains. - 14 -
....... ...... , ., , . . , ...... ~, .. . , .. . . .. . . . , ... , . , .. ,, . ,, , .. . ~ . . . ... ...
` lZ00076
250 g samples of the crushed crude ma~nesite wereheated for specified times in a muffle furnace at a
particular ternperature to determine tne calcination
behaviour of the crude maynesite. The weight loss on
ignition (LOI), percentage decomposition, surface area and
magnesium, calcium and iron contents of the products were
determined. The results of these tests are shown in
Table 1.
30 g of each calcine, or crude magnesium oxide, was dry
ground to 100~ passing through a 150 micron mesh and then
slurried with one litre of magnesium- and iron-free water in
an autoclave. After n.5 h, the slurry was subjected to a
carbon dioxide partial pressure of 700 kPa and this pressure
was maintained throughout the leaching test. ~he autoclave
agitator was rotated at 1200 rpm to maintain adequate
dispersion of the carbon dioxide throughout the slurry.
These leaching conditions do not necessarily represent
optimum leaching conditions but they do allow comparisons to
be made, and in particular show the effect of calcination
conditions on leaching behaviour. During leaching, samples
of the reacting slurry were collected at regular intervals
and after filtration, the solutions analyzed for their
soluble magnesium and iron contents. The results of these
tests are given in Tables 2 and 3.
The test data in Tables 2 and 3 clearly indicate the
effect of calcination temperature, and to a lesser extent
the calcination time, on the rate and percentage of
magnesium dissolved as magnesium bicarbonate from the crude
magnesium oxide calcines. For this particular sample of
crude magnesite, the optimum calcination conditions are
clearly 700C for one hour when considering both the rate
and percentage of magnesium dissolved. It is to be noted,
however, that these conditions lead to the maximum amount of
iron dissolution, which in turn highlights the need to
develop a satisfactory iron removal method to produce high
grade or pure magnesium oxide.
- 15 -
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- 18 -
~200 [)76
E~A~lPLE 2
This example illustrates the ef~ect of the temQerature
of the crude magnesium oxide slurry and the time between the
formation of the slurry and when it is contacted with the
carbon dioxide on the amount of iron dissolved during
leachiny. The temperature and time are referred to as the
slake temperature and slake time respectively.
The calcine used in this example was derived from the
crude rnagnesite described in Example 1. The calcine, which
contained 39.1~ Mg, was formed at 700C and was dry ground
to 100% passing through a 150 micron mesh. Leaching was
carried out in an autoclave at a temperature of 15.5C, with
a carbon dioxide partial pressure of 700 kPa, and agitator
speed oE 1200 rpm and using 30 g of calcine in one litre of
magnesium- and iron-free water. The results of these tests
are given in Table 4.
Comparison of the sets of test run numbers 11,12 and
13, and 12 and 1~ clearly indicates that an increase in
slake time and slake temperature does not affect the amount
or rate of magnesium dissolution but that it causes an
increase in the amount of iron dissolved, and hence in the
[Fe x 100/~lg] concentration ratio of the pregnant liquor.
The data in Table ~ clearly indicate that the
temperature at wl-ich the crude magnesium oxide slurry is
prepared and how long before it is contacted with carbon
dioxide should both be kept to a minimum. In particular,
the data indicate that dry grinding of the crude magnesium
oxide calcine, rather than wet grinding, should be used to
reduce the size of the crude magnesium oxide calcine to a
size suitable for leaching.
E~A~PLE 3
This exarnple illustrates the effect of the weight of
crude magnesium oxide calcine per litre of water used or
slurrying and leaching purposes, that is, the effect of the
pulp density, on the amount of iron dissolved and hence on
the [Fe x 100/~lg] concentration ratio o~ the solution
produced during leaching. The calcine used in these tests,
-- 19 --
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~L20(~0~6
the results of which are given in Table 5, is the sarne as
that used and described in Example 2. Leaching was carried
out at 15.5C with a carbon dioxide partial pressure of
700 kPa, and agitator speed of 1200 rpm and a slake time and
slake temperature of 0.5 h and 15.5C respectively.
The results of these tests clearly indicate that the
amount of iron dissolved increases as the pu12 density
lncreases.
EXAMPLE 4
This example illustrates the effect of the leachiny
temperature on the amount of iron and magnesium dissolved
from the crude magnesium oxide calcine described in Example
2. A slake time and temperature of 0.5 h and the leaching
tem~erature respectively and an agitator rate of 1200 rpm
were used. Carbon dioxide partial pressures of 175 kPa and
700 kPa were used at pulp densities of 5% and 3% solids
respectively.
The results of these tests, reported in Table 6,
indicate that the amount of iron dissolved decreases as the
leaching teInperature increases. Moreover, at the higher
pulp density and the lower carbon dioxide partial pressure,
the solubility of magnesium bicarbonate is exceeded at the
higher leaching temperature t30C), so that only about 30
of the available magnesium is present in solution.
EXA~IPLE 5
This example illustrates the fact that provided the
leaching temperature and pulp density are such that the
solubility of magnesi~m bicarbonate is not exceeded, an
increase in the c:arbon dioxide partial pressure results in a
small increase in the amount of iron dissolved. The calcine
used is that described in Example 2; leaching was carried
out with a 0.5 h slake time at the leaching temperature of
15.5C using a 700 kPa carbon dioxide partial pressure, an
agitation rate of 1200 rpm and a pulp density of 3%
solids. The results of these tests are given in Table 7.
The data in Table 7 show that provided the solubility
of magnesium bicarbonate is not exceeded, then the operating
_ 22 -
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-- 24 --
~20V0~6
carbon dioxide partial pressure should be as low as
possible. In this way the leaching equipment can be
substantially simplified (unit ~ in Figures 1 and 2). In
addition, the lower carbon dioxide partial pressure means
that the load on the carbon dioxide purification and
compression circuit (unit 11 in Figure 1 and unit 9 in
Figure 2) is considerably reduced.
~XAMPLE 6
The data listed in Table 8 illustrate the efEect of the
agitation rate on the amount and rate of iron and magnesium
dissolved. The crude magnesiurn oxide calcine described in
Example 2 was leached at 15.5C aEter being slaked at 15.5C
for 0.5 h. A 3~ pulp density and a carbon dioxide partial
pressure of 700 kPa were used.
The agitation rate afEects the rate of magnesium and
iron dissolution as well as the amount of iron dissolved -
the higher the rate of agitation the higher the rate of
magnesium and iron dissolution and the higher the amount of
iron dissolved. It miyht be considered advantageous to use
a relatively low ayitation rate, say 900 rpm, such that the
amount of iron that is dissolved is reduced. However, the
rate of magnesium dissolution is substantially reduced at
the same time so that the throughput of crude magnesium
oxide caJcine per unit time is also reduced.
EXAMPLE 7
This example, the results of which are given in Tahle
9, illustrates the effect oE the composition of the recycle
liquor used to slurry fresh crude maynesium oxide calcine on
the dissolution of the magnesium and iron from the
calcine. Using the crude magnesium oxide calcine described
in Example 2, leachiny was carried out under the following
conditions: 0.5 h slake at 15.5C, leaching at 15.5C, 700
kPa carbon dioxide, 3~ solids and agitation at 1200 rpm.
These results clearly indicate the advantage of keeping
the magnesium content of the recycle liquor at a minimum so
as to ensure that the amount of iron dissolved is also kept
to a minimum.
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.. . . .. . ..
12()(~6
E.YA~lPLE ~
This example shows the eEfectiveness oE addition of
aluminium sulphate to clarified pregnan-t iron-containing
magnesium bicarbonate solution to remove the dissolved iron,
that is, the method of iron rernoval indicated in Fiyure 1.
The tests were carried out by leaching separate samples of
the crude magnesium oxide calcine described in Example 2
under the Eollowing conditions: 0.5 h slake at the leaching
temperature of 15.5C, 700 kPa carbon dioxide, 3% solids and
agitation at 1200 rpm. After 2.5 hours the unreacted
residue was removed and the clarified pregnant iron-
containing magnesium bicarbonate solution was reacted with a
known amount of aluminium sulphate, ~12(SO4)3.16H20, under a
carbon dioxide partial pressure of 700 kPa, for one hour.
The carbon dioxide partial pressure was used to prevent the
precipitation of hydrated magnesium carbonate. The
temperature and agitation rate were maintained at 15.5C and
1200 rpm respectively. After the one hour interval, the
magnesium and lron contents of the solution were determined.
To clearly show the effectiveness oE the addition of
aluminium sulphate to the clarified pregnant iron-containing
magnesium bicarbonate solution in removing the soluble iron,
the liquors obtained after filtering off the precipitated
magnesium-iron-aluminium compound were sparged with air at 5
lpm for 2 hours using a solution temperature of 45C.
Nesquehonite, MgCO3.3H20, precipitated rapidly. The
nesquehonite was collected and air dried at ambien-t
temperature for several days. The nesquehonite samples were
analyzed for their magnesium and iron contents, as were
samples of magnesium oxide derived from the nesquehonite
samples by calcination at 1000C for 6 hours.
Table 10 lists the magnesium and iron contents and the
[Fe x 100/Mg] concentration ratio of the pregnant liquor
before and after addition of the aluminium sulphate, as well
as the magnesium and iron contents of the air dried
nesquehonite and magnesium oxide products. The data clearly
show that magnesium oxide with a very low iron content can
120~0~6
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~20~ 7~
be produced by additioll of aluminium sulphate to prec~nant
magnesium bicarbonate solutions followed by recovery of
nesquehonite and -the calcination of the latter.
It will be appreciated that addition of aluminium sulphate
to other magnesium bicarbona-te solutions formed under different
leaching conditions and having lower initial iron contents
will, in the end, result in magnesium oxide products with con-
siderably lower iron conten-ts, that is, lower than 0.1%.
While this example illustrates the use of aluminium
sulphate hydrate, any other water-soluble alurninium saIt or
solution is also within the scope of the invention.
EXAI~IPLE 9
In this example, details are given of the removal of
soluble iron while leaching the crude magnesium o~ide
calcine by addition of aluminium sulphate to the calcine
prior to leaching. The crude magnesium oxide calcine used
is that described in Exarnple 2. The required amount of
aluminium sulphate was added to the crude maynesium oxide
calcine and the mixture leached under the Eollowing
conditions: 0.5 h slake at the leaching temperature of
15.5C, 700 kPa carbon dioxide, 30 g calcine per litre and
agitation at 1200 rpm. The soluble magnesium and iron
contents of the reaction slurry were determined at regular
intervals. After leaching for 2.5 h the pregnant magnesium
bicarbonate solution was separated from unreacted residue
and the p~ecipitated magnesium-iron-aluminium compound. Nes-
quehonite and magnesium oxide were recovered from the pregnant
magnesium bicarbonate solutions as described in Example 8~
The results o these tests are given in Table 11. It
can be seen that by increasing the amount oE aluminium
sulphate added the iron content of the pregnant magnesium
bicarbonate solution, and hence of the nesquehonite and
magnesium oxide derived therefrom, decreases quite
significantly. It is also to be noted that for the same
aluminium sulphate addition, the resulting iron content of
the pregnant magnesium bicarbonate solution,nesquehonite and
magnesium oxide respectively produced ~7hen the iron is
removed during leaching (Example 9, Table 11~ is less than
that when the iron is removed from clarified pregnant
magnesium bicarbonate solution (Example 8, Table 10).
- 30 -
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As in the previous example, maynesium oxide products with
even lower iron contents can be prepared by usiny leachiny con-
ditions particularly leaching temperature, which reduces the
amount of iron dissolved.
The aluminium sulphate could also be fed in as a
solution to the leaching vesseL. This example encompasses
the availability in the leachiny vessel of a soluble
aluminium salt during leaching.
EXA~IPLE 10
In this example, the recovery of part of the
precipitated magnesium-iron-aluminium co~pound formed as
illustrated by Figure 1 is described. That is, the
magnesium-iron-aluminium compound was formed by addition of
aluminium sulphate to a clarified magnesium bicarbonate
solution as described in Example 8, test number 3i.
The magnesium-iron-aluminium compound contained 15.2
MgO, 25.4%, ~1203, 0.75% Fe203 and 16.5~ CO2. This was
dissolved in the minimum volume of dilute sulphuric acid so
that at the completion of reaction the resultant p~ was
3.8. After clarification by vaccuum filtration the liquor
was made up to 1 litre and contained 2.74 gpl magnesium.
This liquor was used to slurry a fresh sample of crude magnes-
ium oxide which was processed as described in Example 2, test
number 12. The results of this experiment are shown in Table 12.
The data clearly show that the use oE the magnesium-
aluminium sulphate solution derived from t~ne magnesium-iron-
aluminium compound not only results in complete
precipitation of the soluble iron from the pregnant
magnesium bicarbonate liquor derived from ~resh crude
magnesium oxide calcine but also tha~ 73% of the magnesium
component of the magnesium-iron-aluminium colnpound is
dissolved as soluble magnesium bicarbonate. It is thus
clear that it is possible to recycle a substantial portion
of the aluminium sulphate used to remove the soluble iron
from the preynant magnesium bicarbonate solution.
~X.~.IPLE 11
In this example, the flow sheet according to Figure 2
is followed with respect to the dissolution o~ the
magnesium-iron-aluminium compound mixed with leach
residue. The solidl obtained as described in Example 9,
3~
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-- 34 --
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test nulr~ r 35, contain~-l 3'1.5'~, MCJ(), '1.5()'~ o~, ~.23~
Fe~03 ancl ~5.5% C02. ~ sample o~ the solid was treated with
dilute sulphuric acid to yield a ~inaL slurry pll of 3.~.
The resultant solution after fiLtration containecl ~.19 gpL
magnesium and this corresponds to the dissolution of 67.1
of the magnesium contained in the solid.
A sample of fresil crude magllesiulll oxide calcin~ was
slurried with one litre of the above magnesium-aluminium
sulphate solution and then processed as descri~ed in Example
2, test number 12. The results of this experiment are given
in Table 13.
The data clearly show that the use of the clarified
magnesium-aluminium sulphate solution derived from the
dissolution of the magnesium-iron-aluminium compound plus
leach residue in dilute sulphuric acid is effective in
reducing the iron content of the pregnant magnesium
bicarbonate solution formed when used to slurry fresh
magnesium oxide calcine. In addition it is possible to
recover 84% o~ the maynesium content of the magnesium-iron-
aluminium compound plus leach residue that was dissolved by
sulphuric acid. In a continuous process it is obvious that
the amount o~ aluminium sulphate necessary to reduce the
iron content oE the pregnant magnesium bicarbonate liquor to
the required level will thus be reduced.
EX~MPLE 12
This example describes the precipitation of hydrated
magnesium carbonate (nesquehonite, MgC03.3H20~ and/or basic
magnesium carbonate (hydromagnesite, Mg5(C03)4~0H)2.4H20)
from pregnant iron-containing magnesium bicarbonate
solutions. The solutions were obtained as described in
Example 2, test number 12. The clarified pregnant iron-
contàining magnesium bicarbonate solutions were air sparged
and heated as indicated in Table 14. The soluble magnesium
and iron contents of the slurries so produced were
determined at suitable time intervals. Also listecl in Table
1~ is an estimate of the bulk density of the precipitated
product that had been collected and air dried at ambient
termperature for several days.
.
~ 35 --
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1200()~6
It can be seen that both the temperature and degree of
air sparging both affect the rates of precipitation of
magnesium and iron as impure hydrated magnesium carbonate
and/or basic magnesium carbonate. The rate of iron and
magnesium precipitation is significantly greater at the
higher precipitation temperatures. Since it has been
previously shown that it is essential to have a recycle
liquor, that is, the filtrate recovered after the separation
of the hydrated macJnesium carbonate and/or basic ma~Jnesium
carbonate, which has a low magnesium content, preferably
less than 0.2 gpl, it is clear that precipitation should be
carried out at a moderate temperature, in the range 25C to
45~C with air sparging.
-37-
