Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1999-12-17
2003-05-20
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C204S243100, C075S010620, C075S673000, C423S561100
Reexamination Certificate
active
06565733
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the production of primary aluminium from alumina ore and to an apparatus for performing the method.
BACKGROUND OF THE INVENTION
The most commonly used method for the production of liquid aluminium from alumina ore is the Hall-Heroult process in which, through electrolysis alumina is decomposed. In this process alumina is continuously fed and dissolved in a bath comprising liquid aluminium and molten cryolite, a mineral comprising fluorides of sodium, aluminium and calcium. A carbon anode is supplied in the bath of molten cryolite and aluminium. The cell containing the bath, is internally coated with lining of a conductive layer facing the inside and acting as a cathode. Liquid aluminium is formed at the cathode and collected on the bottom of the cell. From there it is periodically removed.
The Hall-Heroult process has a number of drawbacks. One drawback is the high, electrical, energy consumption. Another drawback is the emission of fluorides such as CF
4
and C
2
F
6
which are considered notorious green house gases, the emission of CO and of heavy metals. Also, the used lining of the cell, known as spent pot lining, is an environmentally unfriendly residu of the Hall-Heroult process. The Hall-Heroult process requires, for a practical production capacity, a large number of cells which together occupy a large area.
Already more than one hundred years ago an alternative process using aluminiumsulfide has been investigated. The investigation has been taken up again in the 1980s, however without success and the process was not pursued further at the time.
U.S. Pat. No. 4,265,716 discloses an embodiment of the aluminiumsulfide based process. In this document it is proposed to react, at a temperature between 1300 K and 1500 K alumina, carbon and sulfur-containing gas to form aluminiumsulfide and carbonmonoxyde. The sulfur containing gas may include sulfur in one or more of several forms, including S
2
, S
6
and CS
2
. Further it is mentioned that when the gas includes carbon as in CS
2
, it can replace at least a portion of the carbon otherwise introduced as coke or other solids. U.S. Pat. No. 4,265,716 aims at obtaining molten aluminiumsulfide. The molten aluminiumsulfide is then heated to a temperature of about 1600 K to 1800 K and held there for a sufficient time, about 15 to 60 minutes to cause decomposition to molten AlS and sulfur gas. The molten AlS is then cooled to a temperature sufficient to cause its disproportionation to aluminium and motlen aluminium. This disproportionation is performed in the temperature range of 1200-1370 K.
SUMMARY OF THE INVENTION
Although the invention U.S. Pat. No. 4,265,716 was published in 1981, the disclosed process has not been put into practice despite the potential advantages of the aluminiumsulfide process. Obviously, in the field of the production of aluminium from alumina, it had turned out that the proposed process was not feasible. Indeed, tester performed by the present inventors have confirmed this. Surprisingly, it was found that the aluminiumsulfide process wherein use is made of Al
2
S
3
can be performed in a new and inventive manner that is practicable and with which all the advantages as pursued in U.S. Pat. No. 4,265,716 are obtained. Besides, additional advantages over U.S. Pat. No. 4,265,716 are obtained.
These advantages are obtained with a method wherein the conversion of alumina into aluminiumsulfide is performed by reacting alumina with CS
2
containing gas at a temperature T
2l
whereby the alumina is mainly &ggr;-alumina like the current alumina used for the Hall Heroult process.
It was found that, different from what was proposed in the prior art the reaction rate of &ggr;-Al
2
O
3
and CS
2
is very high compared to the reaction rate between &agr;-Al
2
O
3
and CS
2
. Therefore, the method of our invention has the possibility to yield practical quantities of Al
2
S
3
, within a practical time frame. The method of the invention is based on the following net reaction:
2Al
2
O
3
+6CS
2
→2Al
2
S
3
+6CO+3S
2
(1)
COS can be formed as a by-product.
Tests have shown that the transformation from &ggr;-Al
2
O
3
to &agr;-Al
2
O
3
takes place in a rather wide temperature range. Therefore, the reaction temperature at which Al
2
O
3
is converted into Al
2
S
3
can be chosen within a wide range depending on other parameters.
In practice it is preferred that the temperature T
al
is lower than 1100° C., preferably lower than 1025° C. more preferably lower than 1000° C.
At temperatures above 1100° C. all Al
2
O
3
transforms quickly into &agr;-Al
2
O
3
. The reaction rate of &agr;-Al
2
O
3
into Al
2
S
3
by means of CS
2
proved to be very slow in our experience. Therefore, it is preferred to perform the method according to the invention at a temperature T
al
lower than 1025° C., more preferably lower than 1000° C. In particular above about 1000° C. the transformation from &ggr;-Al
2
O
3
to &agr;-Al
2
O
3
progresses very fast. In practice, this means that a substantial portion of the &ggr;-Al
2
O
3
has transformed to &agr;-Al
2
O
3
before sufficient Al
2
S
3
has been formed. By operating at a temperature below 1000° C., a substantial quantity of Al
2
S
3
can be formed before an imparing quantity of &agr;-Al
2
O
3
has developed. The conversion of Al
2
O
3
to Al
2
S
3
by means of reaction with CS
2
is also called sulfidation.
Because the method of the invention is performed at substantial lower temperatures than known in the prior art, a considerable reduction in energy consumption can be achieved. Furthermore, in the method of our invention solid Al
2
S
3
is formed, whereas in the prior art method molten Al
2
S
3
is formed. Therefore, also our method consumes less energy since the melting heat of the formed Al
2
S
3
is saved.
Preferably the temperature T
al
is higher than 700° C., preferably higher than 750° C. Under 700° C. the reaction rate is too low for industrial application.
It is preferred that the conversion of alumina into aluminiumsulfide is performed at a conversion pressure higher than 1 Bar absolute pressure.
Tests have shown that the reaction according to equation (1) progresses faster when it is performed at a conversion pressure higher than 1 Bar, which equals about 1 atmosphere. The reaction rate can further be increased with a further embodiment of the invention which is characterised in that the conversion pressure is higher than 5 Bar, preferably higher than 15 Bar. It has shown that, within practical limits, the reaction rate increases with increasing conversion pressure. Therefore, the conversion pressure is selected at a practical optimum, taking into account such parameters as availibility and costs of construction materials for reaction vessels, yield per unit of time and costs and efforts of safety measures.
As mentioned before in the method of our invention, solid Al
2
S
3
is formed. In practice, it may show that the shape of the particles of the bulk Al
2
S
3
is not very suitable for further processing. Therefore, another embodiment of the method of the invention is characterised in that the aluminiumsulfide is at least partly heated to a temperature over its melting temperature. Although the advantage of less energy consumption is reduced, this embodiment has the advantage that Al
2
S
3
is available in a reproducible form, suitable for further processing as will be discussed later.
In the event that molten Al
2
S
3
is not needed or wanted at that stage, a further embodiment of the invention is characterised in that the molten aluminiumsulfide is cooled so as to form small-sized grains, having average grain size smaller than the average grain size of the aluminiumsulfide prior to being heated. This embodiment yields Al
2
S
3
in a form that is easy to handle. Further, this embodiment does not reduce the advantage of the energy consumption essentially since solidification heat can easily be regained. In one possible embodiment the Al
2
S
3
particles formed in the sulfidation are slightly ra
Sportel Heiko
Verstraten Cornelis Wilhelmus Franciscus
Corus Aluminium Walzprodukte GmbH
Stevens Davis Miller & Mosher LLP
Valentine Donald R.
LandOfFree
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