Chemistry: electrical and wave energy – Processes and products
Patent
1983-06-30
1984-05-29
Williams, Howard S.
Chemistry: electrical and wave energy
Processes and products
204245, C25C 306, C25C 314
Patent
active
044513372
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of heat recovery from a furnace for manufacturing aluminium by melting electrolysis of alumina, the furnace gases being passed through a bed of alumina while the bed is being fluidized.
The melting electrolysis takes place in furnaces which have been developed specifically for that purpose and are formed as troughs which usually consist of steel and have a brick lining. Normally, the cathode is located in the bottom of the furnace and is made of carbon while the anode can be e.g. of prebaked type or Soderberg type and usually is fed from above to the electrolysis bath. It is consumed during the process and must be replaced continuously.
The alumina and certain solid additives are supplied to the electrolyte through or laterally of the anode, and the produced aluminium is drawn off batchwise by siphon or by suction.
The gas produced in the electrolysis, the furnace gas, consists of carbon monoxide, carbon dioxide, and a mixture of hydrocarbons and fluorhydrocarbons, etc., and normally is evacuated mixed with ventilation air either directly to the surrounding air in the space where the furnace is located, or to some collection means, e.g. metal sheet hoods, for the recovery of solid particles consisting of fluor salts which are entrained in the furnace gases. However, the escaping furnace gases represent a substantial amount of energy which in a plant for production of 80,000 tons raw aluminium per year is of the order of 20 MW.
For the recovery of this amount of energy and for the reduction of the energy losses in the manufacture of aluminium while the fluor salts entrained in the furnace gases are at the same time separated and recovered, it has been proposed to pass the furnace gases, when they have passed through the bed of alumina, through a heat exchanger for heat exchange between the furnace gases and an external fluid (U.S. Pat. No. 3,664,935). In that case the heat exchanger is rapidly clogged, however, due to deposition of dust from the furnace gas, such that it is necessary to clean frequently the heat exchanger.
In order to overcome this drawback it is proposed according to the invention to fluidize the bed of alumina around one or more tube coil or coils for heat exchange between the furnace gases and an external fluid passing through the tube coil or coils, respectively.
It is achieved by this procedure not only that the tube coil or coils, respectively, are continuously blown off by the particles in the fluidized bed but also that the tube coil or coils, respectively, can be made with a smaller surface and thus can be made smaller and cheaper for the transfer of a predetermined amount of heat energy, because the heat transfer factor will be several times larger due to the fluidization.
If several tube coils are provided they can be connected in series.
It is particularly advantageous to let the furnace gases pass the cathode arranged for the melting electrolysis before they are supplied to the fluidizing bed and the coil or coils, respectively. It is common that the cathode is cooled by free convection. In that case it is neccessary in the winter time when the air is cool, to compensate for the more considerable heat loss be increased supply of electric energy for the electrolysis. Due to the fact that the cooling of the cathode is obtained by means of the furnace gases there is obtained a controlled heat transport from the cathode, the energy consumption for the electrolysis thus being reduced.
In order to explain the invention more clearly it will be described in more detail below, reference being made to the accompanying drawing which discloses in a diagrammatic vertical sectional view a plant for working the method.
In the drawing, there is shown a furnace 10 for melting electrolysis of alumina comprising anodes 11 and a cathode 12. An inlet 13 for the supply of alumina to the furnace is arranged at the top. The furnace space is closed and is connected through a conduit 14 with a cooling jacket 15 arranged around the cathode 12, which is connected
REFERENCES:
patent: 3664935 (1972-05-01), Johnson
patent: 4108968 (1978-08-01), Jacobs et al.
patent: 4274478 (1981-06-01), Stendahl
Frilund Eyvind
Holmberg Per
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