News 2008

Munich, January 9, 2008

Trouble with compressed air under a hot sun

New field of application for high pressure compressors in the extraction of nickel ore

High pressure compressors compress air from 90 bar to 500 bar. In the field of ore mining in New Caledonia, highpressure air supplements the processing in an acid autoclave. The special plant for nickel processing impresses with its container type construction which is a match for the extreme climatic conditions.

In the Australian New Caledonia, which possesses one of the largest ore deposits of lateritic nickel worldwide, a complete, huge industrial complex for extracting the metal is being built near Goro, comprising production facilities, power supply, harbour facilities, streets and work for about 800 persons. Over 50 million tons of mineral rock are embedded there, the open cast mining of which will take place over many years. The operating company of this ore mining project in the region around Goro is Brazilian - Companhia Vale do Rio Doce - which already has considerable global success in the field of nickel extraction. A yearly yield of about 60 000 t nickel is expected and, as a byproduct, up to 5000 t cobalt will be produced. For the purchase of the mine, the construction of the production facilities and for meeting the obligations for environmental protection, very large costs were involved.

The nickel ore extracted by lateritic mining is treated in a metallurgical process by the use of acid, water and compressed air within an autoclave. Thereby, a nickel sulphate solution is formed from which nickel sulphide precipitates and dissolves under pressure in sulphuric acid. By means of further processes, pure nickel is eventually obtained.

Modern method

This procedure (high pressure acid leaching), which is not yet used frequently, is especially energy-saving compared to the classical smelting in a rotary furnace. The basic material mixed with water (nickel ore concentrate = slurry) is fed by diaphragm pumps to the acid autoclave with a temperature of about 200 – 250 °C, and sulphuric acid and compressed air at about 60 - 80 bar is admitted. The materials thus introduced are thoroughly mixed by stirring devices. The autoclave has a diameter of about 5 m and a length of about 30 m.

The compressed air injected with 60 - 80 bar serves primarily to support the process in the autoclave. In addition, the internal pressure has to be maintained at a constant level in case of a power drop, in order to protect the seals against excessive wear. Moreover, compressed air has to be available for the autoclaves when they are taken out of operation for cleaning purposes.

ASelection of the compressed air station

In order to protect the compressors against the dusty atmosphere and the intense insolation, feed and return air vents and sand traps are installed in the large enclosing containers. For minimizing noise, the containers are constructed with noise damping insulation and reach a sound pressure of about 73 dB(A), measured at a distance of 1 metre according to DIN 45635.

For supplying the three autoclaves with compressed air, three independently operating container compressor stations (20 ft. each) were planned. Due to a well controlled feed and return of the cooling airflow inside the containers and as a result of a very sophisticated compressor technology, the high pressure compressors operate optimally under these difficult environmental conditions.

To cover the total air requirement of about 300 m3/h for the three autoclaves, the operating company opted for three serial compressor plants of type I 25.0-45 with a throughput of 114 m3/h each and an adjustable final pressure of 90–350 bar. This model is an air-cooled and pressure oil lubricated, four-stage compressor which has proven itself ideally suited to continuous use round-the-clock in countries with hot climates where even sandstorms are not unusual.

Sophisticated concept

The containers protect the compressors against dust and intense insolation.

Each plant is individually controlled and equipped with a fully automatic compressor control - BAUER B-CONTROL. It monitors all operational data of the compressor and the motor. In case of a malfunction of the monitoring data, the compressor is automatically shut down and the error message is transmitted to and monitored at a superordinate control station.

The compressed air has to be available for different, usage-oriented tasks within the autoclave. In order to avoid the running time of the compressor lasting only a few minutes, it makes sense to compress the air to about 300 bar and to store it in batteries of storage cylinders, each battery having a water volume of 12 x 80 l. Thus it can be guaranteed that the compressor is not switched on and off more than 4-5 times per hour. When extracting the compressed air from the storage station, it is fed to the pressure-reducing station.

After the storage station, the air compressed to 300 bar is adjusted to the respective overpressure required in the autoclave, monitored and secured by a pressurereducing station. The system pressure required for the principal procedure is between 60 and 80 bar. In order to avoid that the decrease of pressure from 300 to 60 bar overpressure leads to icing of the regulator, the compressed air has to be dried after the last stage of the compressor. This procedure is carried out by a high pressure refrigeration drier with a maximum allowable working pressure of over 300 bar, the air being adjusted to a pressure dew point of about + 3 °C. In addition, a downstream coalescence filter removes the aerosols generated during the compression.

Author:
L. Kühlwein, graduate engineer, project engineer in the industrial sector