Field of the invention
The present invention relates to a process for the preparation
of silicon carbide from spent pot liners generated from aluminum smelter plants.
Carbon pot liners are used as cathode in electrolysis cell
for extracting aluminum by smelter plants and at the end of their service life these
carbon cathode blocks are rejected and new carbon electrodes are installed. These
used and rejected carbon liners are called Spent Pot Liners and they are highly
contaminated by fluoride, cyanide, alkali etc. Being contaminated with highly toxic
elements, their disposal poses a great problem for all aluminum smelter plants.
Accordingly, the present invention provides a means for utilization of these pot
liners and convert them to a useful industrial product, namely silicon-carbide.
Background of the invention
Attempts have been made earlier to remove contaminants
from spent pot liners. Reference may be made to (
U.S. Patent 6498282
, granted 24 December, 2002) wherein oxides of iron was added to spent
pot liner and melted in a high temperature furnace in order to remove the contaminants
in the form of slag. In yet another prior art (U.S
, granted September 14,m 1993) lime was added to crushed spent pot liner
in an aqueous medium and the fluoride precipitated as calcium fluoride by heating
to 140°C in presence of clay to fix soda as well. In yet another prior art
, granted September 24, 1996) Henning & Mollgard disclosed a process whereby
aluminum and fluoride in spent pot liner was recovered as AlF2OH by leaching
with dilute sulfuric acid and adjusting fluorine to aluminum ratio between 1.8-2.2.
In yet another similar prior art (U.S
, U.K. Patent Application
and U.S Patent
, granted in 1986), the spent pot liner after crushing was treated with
concentrated sulfuric acid and heated to elevated temperature (1200°C) whereby
fluoride values are reacted to and withdrawn as hydrofluoric acid. These pyrohydrolysis
processes exhibits inconvenience like corrosion to equipments etc and thus have
not been commercialized yet. In yet another prior art (U.S Patent
) the spent pot liner was treated with a mixture of hydrochloric acid and
sulfuric acid and the fluoride value was precipitated as cryolite. The process has
inherent drawback of contaminating the process liquor and precipitated end product
due to dissolution of iron and silica and coprecipitation of iron hydroxide. In
yet another prior art (
U.S. Patent 4889695
European Patent Application No. 117616
) spent pot liners were treated with alkali like sodium-hydroxide. Although
fluoride value was recovered by precipitating end product, alkali route takes a
long time to reach completion, results in foam formation and requires rather complicated
careful controlled conditions. Further alkali is not removed completely by such
treatment. In yet another prior art (
U.S. Patent 4508689
) removal of alkali in above process attempted by incomplete neutralization
method to make precipitated product (aluminum fluoride) sodium free. But incomplete
neutralization to pH below 5 results in significant loss of fluorine value. Further
attempts have been made to decontaminate and produce silicon-carbide from spent
pot liner in a closed electrothermic smelting furnace. Reference may be made to
a prior art disclosed by Lindkvist et al (
) crushed untreated spent pot liner was mixed with silica and melted at
a temperature of 1300 to 1750°C. The spent pot liner was oxidized in smelting
furnace after incorporating oxidizing agents like calcium oxide to form calcium-fluoride,
calcium aluminate or calcium aluminate silicate slag. However such processes are
simply not well suited to remove all impurities contained in the waste material
(spent pot liner). In a similar prior art (
U.S. Patent 6471931
granted October 29,2002) Brosnan disclosed a process where the spent pot
liner first crushed to less than 1 mm size, then mixed with appropriate amount of
silica and then heated in a electric resistance furnace under close control of reducing
atmosphere to a temperature between 1800 to 2200°C. Silica for such purpose
was derived from fumed silica, fly ash, clay or metal silicones. The product was
then separated gravimetrically. In such process starting material being untreated
highly contaminated carbon source, the final product purity is unknown. Further
emanation of hydrofluoric acid may corrode the reacting furnace and its accessories.
Energy consumed both in pulverization of spent pot liner and prolonged heating in
resistance furnace is also high. Other similar prior arts for making silicon carbide
from carbon include
U.S. Patents 492767
Objects of the invention
The main objective of the present invention is to provides
a process for making silicon carbide from spent pot liners (hereinafter referred
to as "SPL") generated by aluminum smelter plants which obviates the drawbacks of
the hitherto known prior art as detailed above.
Another objective of this process is to remove the corrosive
constituents like fluoride, for example, in the SPL before reacting it at high temperature
in a furnace in order to avoid corrosion to the furnace lining and its accessory
Summary of the invention
According to the present invention, there is provided a
process for the preparation of silicon-carbide from spent pot liners generated from
aluminum smelter plants, the said process being characterized by comprising the
- a. crushing the SPL to a size range of half-inch to dust,
- b. treating said crushed material with concentrated chromic acid,
- c. washing the treated material with water until the filtrate is neutralized,
- d. drying the washed material
- e. subjecting the dried material to a thermal shock to obtain carbon powder,
- f. mixing said carbon powder with silica powder in a molar ratio of about 1:3
to 1:5 and reacting the resultant mixture in a high temperature furnace to obtain
The process according to the present invention provides
a unique acid treatment process by which the majority of contaminants in the SPL
are removed before the reaction of recovered carbon powder with silica powder in
a furnace to produce silicon-carbide. The process reacts the said recovered carbon
powder from SPL along with commercially available silica powder in a high temperature
furnace, preferably an argon plasma furnace, thereby reducing formation time of
the end product silicon-carbide to few minutes only and provides a tool for handling
and processing of large volume of raw material in a shorter time to manufacture
silicon-carbide. SPL processed by the said process removes the majority of all contaminants,
generates a carbon powder clean enough not to produce toxic fumes of the contaminants
generally occurring in SPL and thus furnace linings are not affected.
In an embodiment of the process according to the present invention the starting
SPL obtained from aluminium smelter plant has fluorides, alkalies and free aluminium
in the range of 8-9%, 7-8% and 2-3%, respectively.
In another embodiment the free flowing carbon powder obtained
in step (e) has an average particle size of about 20 micrometer. The process of
the present invention provides finer particles of carbon particles from the SPL
by chemical reaction for preferred and complete reaction between silica and the
recovered carbon from SPL in the high temperature reaction step. The carbon powder
obtained after the drying step may be free flowing.
The carbon powder recovered from SPL is heated along with
silica in a special furnace such as, for example, a plasma furnace, so that the
reaction time can be reduced considerably and thereby enhance the rate of production
of silicon carbide on a commercial scale.
In yet another embodiment the yield of the carbon powder
obtained in step (b) is in the range of 30-35% of the starting weight of SPL.
In still another embodiment the percentage conversion to
silicon carbide obtained is in the range of 70 to 85%.
In step (a) the SPL is crushed so as to have a particle
size of from half-inch to dust. Preferably, about 70% of the crushed material is
constituted by the half-inch fraction.
The crushed SPL may be treated with chromic acid for a
period of 10 to 30 minutes, more preferably for a period of 15 to 25 minutes. The
reaction is exothermic and preferably, the temperature of reaction is maintained
below about 90°C.
Preferably, the material after treatment with chromic acid
may be washed with distilled water.
After washing, the treated material may be dried at a relatively
low temperature such as about 100°C, for example.
The thermal shock treatment of step (e) may be carried
out within a temperature range of 800 to 1000°C, however, a temperature in
the region of 900°C is preferred.
The thermal shock step may be carried out for a period
of 1 to 2 minutes.
The silica powder may have a particle size of less than
-100 BS mesh or, more preferably, a particle size of less than -200 BS mesh.
The reaction time in the high temperature furnace, for
example, an argon plasma arc furnace, may be in the range from 1 to 4 minutes, for
example, about 2 minutes.
Detailed description of the invention
The present invention relates to a process for use of SPL
by means of comminution of SPL; by a chemical reaction; followed by mixing it with
appropriate proportion of commercially available quartz powder (high purity silica
powder preferably having a particle size of -100 BS mesh); and, further followed
by heating it in an argon plasma furnace for about 3 minutes. The product of this
process is a granular powder of silicon-carbide as determined by XRD analysis. Silicon
carbide is a hard refractory material having high thermal conductivity value. It
possesses high strength even at elevated temperature and is a useful commodity in
commerce with many useful applications like abrasive material, high temperature
resistant refractory block, structural ceramics, material for making hard dies,
crucibles in metallurgical industries etc. Other uses are expected along the lines
of commercially manufactured silicon carbide.
In the present invention the SPL collected from aluminum
smelter plants is first broken down by jaw crusher/hammer mill to have a size from
half-inch to dust size with half-inch particles constituting about 70% of the bulk.
In laboratory experiments these broken pieces of SPL were placed in a glass vessel
and then slowly, freshly prepared chromic acid was added with constant stirring.
The reaction is exothermic and addition rate of the acid needs to be controlled
to keep the reaction temperature within about 90°C. The total amount of acid
needs to be added just to make the mixture a thick paste. The mixture is then allowed
to stand for about 15-25 minutes. End of reaction is indicated by no more emanation
of gas bubbles from the mixture. The mixture is allowed to cool down to about room
temperature and then equal volume of distilled water added with stirring followed
by filtration through a regular filter paper placed in a funnel. The residue is
repeatedly washed with distilled water until the filtrate is free of acid. The fluoride
escapes as hydrofluoric acid and can be trapped by bubbling the emanated gas through
a dilute solution of sodium hydroxide. The filtrate containing chromium (III) ion
can be precipitated by addition of sodium hydroxide solution. The washed carbon
powder is practically free from all contaminants as can be seen in Tables 1 and
2 below. The carbon powder thus derived from SPL is then dried in an oven at around
100°C for two hours and then subjected to a thermal shock in a preheated furnace
(preheated to about 900°C for about a minute. This results in a free flowing
carbon powder having average particle size of 20 microns as determined by Malvern
Particle Size Analyzer.
In a preferred embodiment the principal steps of this invention
- a) Breaking the SPL to a size range of half inch to dust (half inch fraction
constituting at least 70% of the bulk).
- b) Reacting the said crushed SPL with concentrated chromic acid with constant
stirring maintaining reaction temperature to about 90°C.
- c) Washing the reacted product with distilled water until neutral and drying
the recovered carbon powder in an oven at 100°C for about 1 hour.
- d) Subjecting the said recovered carbon powder to thermal shock for about a
minute in a preheated (preheated to about 900°C) furnace to obtain a free flowing
- e) Mixing the said carbon powder with stoichiometric proportion of commercially
available silica powder and reacting in an argon plasma furnace for about 3 minutes
to obtain silicon-carbide in granular form.
In another preferred embodiment of the present invention
a process for making silicon carbide from SPL generated by aluminum smelter plants
is provided, which comprises process steps involving crushing the SPL to a size
range of half inch to dust (half inch fraction constituting about 70% of the mass),
reacting the powdered spent pot liner with concentrated chromic acid for about 15
minutes, washing the reacted mass with distilled water until the filtrate is neutral,
drying the recovered carbon powder in an oven at about 100°C for about an hour
and thereafter subjecting the said carbon powder to thermal shock in a 900°C
preheated furnace for about a minute to transform it to a free flowing carbon powder,
thereafter mixing the said carbon powder with about a stoichiometric proportion
of commercially available silica powder of less than 200 BS mesh and reacting the
mixture in an argon plasma furnace for about 3 minutes to obtain silicon-carbide
in granular form.
In Example 1 of the invention, the said carbon powder derived
from SPL was mixed (3:1 molar ratio) silica powder (commercially available quartz
powder having particle sizes less than 100 BS mesh) and reacted in the high temperature
of an argon plasma furnace. The powder is carried by argon gas into the plasma furnace
and starts reacting as soon as it enters the high temperature zone of the furnace.
Total reaction time is about 3 minutes. The product silicon-carbide was collected
in a graphite crucible placed at the bottom of the hearth of the plasma furnace.
Supply of argon gas into the furnace was continued for about another 1 minute after
the reaction was over.
The collected sample of silicon-carbide was analyzed by XRD apparatus and found
to contain substantial amount of silicon-carbide in the powder product.
Carbon powder derived from SPL was mixed in molar ratio
of 4:1 (stoichiometric ratio being 3:1) with silica powder (commercial grade quartz
powder having a particle size less than 100 BS mesh) and the experiment repeated
exactly same as above for Example 1. The granular product indicated a substantial
amount of silicon carbide in the product by XRD analysis.
Carbon powder derived from SPL was mixed in molar ratio
5:1 with silica powder (commercial grade quartz powder having particle size less
than 100 BS mesh) and the experiment repeated in the argon plasma furnace exactly
in the same manner as described for Examples 1 and 2 above. XRD analysis of the
granular product indicates presence of substantial amount of silicon-carbide.
The following data is given by way of illustration and
therefore should not be construed to limit scope of the present invention:
Results showing chromic
acid treatment of two different size SPL
S inch to dust
the carbon powder derived from SPL
SPL Sieve size
Average Particle Size
S inch to dust
Molar ratio of Silica:Carbon*
Type of plasma% used
Conversion to SiC
Above results shown in Table 3 indicate decontaminated
SPL treated by the process according to the present invention can be substantially
converted into silicon carbide using the said process steps. Further, the treatment
time being small it will be easier to process large volume of said raw materials
in an actual commercial scale unit.
Advantages of the invention are:
- 1. The process provides a means for utilizing waste material such as SPL of
aluminum smelter plants.
- 2. The said SPL need not be powdered to very fine size and thus saving both
energy and time in processing the starting material.
- 3. Since the SPL is cleaned of its contaminants before reacting in high temperature,
the furnace and its ancillaries are not corroded by the said process.
- 4. The process is simple and can be carried out in shortest possible time.
- 5. The process does not require stringent control of reaction parameters as
required in solution precipitation techniques as mentioned in prior art for deriving
the final product.
- 6. The process is easy to scale up.
- 7. The process generates a product which has high commercial value.