Tunnel Kilns for Mineral Processing
Tunnel kilns are an alternative to rotary kilns, but they are typically ignored by the mineral processing industry despite their better performance, lower fuel consumption and reduced installed capital cost.
To keep capital cost down, rotary kilns are kept short and the heat transfer is intensive. Shorter kilns also have high exhaust temperatures and consume more fuel. To maintain structural integrity, the shell must be kept cool and therefore surface heat loss is high. Dust generation by the tumbling action in the rotary kiln is high and with high gas velocities this dust is carried out of the kiln. This typically results in increases in capital costs of 50% with the additional of dust recovery and gas cleaning equipment. Rotary kilns are also elevated, often assembled onsite and require refractory installation after equipment installation.
By contrast, tunnel kilns can have heat introduced anywhere along its length and gas temperature can be controlled to within 5°C anywhere in the kiln, thereby avoiding product damage. In addition, thermal efficiency is high with all exhausts being just above dew point and equipment surface temperatures safe to touch. The solids are stationary on the cars and the gas velocities are generally insufficient to generate dust. Dust removal from the off-gas is not necessary despite processing ore containing clay up to 13% (<5 µm). The kiln can be shipped ready for assembly including refractory and can be installed at ground level, thereby minimising installed cost.
This paper presents pilot plant results for a 180t/h tunnel kiln operating to 975°C under forced and natural convection operating conditions, roasting briquetted feed. Data presented includes pressure drops, heat transfer coefficients, solids heating rates, and off-gas evolution.
The pilot results are used to size and specify an industrial sized tunnel kiln. A comparison is made between industrial rotary and tunnel kilns based on process guaranteed vendor quotes, installation and operating costs to process 180 t/h of roaster feed.