The Role of the Rotary Kiln in Non-Ferrous Metallurgy
In non-ferrous metallurgy, concentrates, fluxes, solid fuels and the various slags produced by the process frequently have to be dried before they can be charged to a furnace, and the rotary kiln is the principal drying equipment for that duty. The kiln also serves as the main process vessel in several smelting routes: precipitation rotary kilns used to separate copper oxide ore in precipitation processes that are difficult to treat by other means, kilns in which zinc-bearing lead and copper blast furnace slag together with zinc calcine leach residue are reduced and volatilised to recover zinc and lead oxides, and kilns used for many roasting operations.
Because these duties differ so widely, kiln parameters vary greatly from plant to plant. Temperature, atmosphere, pressure, flue gas volume, fuel type and fuel consumption all change the demands placed on the refractory lining, and a single lining specification cannot serve every kiln in a works.
Lining Practice: From Small Brick to Large Brick
Historically the working lining of a rotary kiln was built from brick of 230 mm or 250 mm length. In recent years the industry has moved to much larger brick formats, and kiln campaign life has improved significantly as a result, because fewer joints mean fewer weak planes through which gas, liquid slag and process material can penetrate the lining.
In high-temperature kilns, where the lining has to resist corrosion by high-temperature materials as well as scouring and abrasion, a stronger class of material is required. Magnesia-chrome bricks, magnesia-spinel bricks and high alumina refractory bricks are the common choices, and the working layer is backed by an insulating layer to limit heat loss through the shell.
| Kiln duty | Working lining material | Primary wear mechanism |
|---|---|---|
| Drying and moderate-temperature service | High alumina bricks, clay bricks | Abrasion and thermal cycling |
| High-temperature reduction and volatilisation | Magnesia-chrome bricks | Chemical attack by molten material and scouring |
| High-temperature zones with strong cycling | Magnesia-spinel bricks | Thermal shock combined with corrosion |
| Backup and insulating layer | Insulating bricks and insulating castables | Heat loss control and interface spalling |
The Two-Layer Lining Problem
Dense basic bricks such as magnesia-chrome and magnesia-spinel have relatively high thermal conductivity, so heat loss through the kiln shell is considerable. To save energy and extend lining life, insulating bricks are built outside the working layer. The difficulty is that the two layers form separate, unlinked rings: the working brick and the insulating brick cannot be tied into one monolithic masonry, so the working layer is not restrained by the shell.
In service the inner layer frequently fails not by wear or corrosion but by temperature fluctuation. The brick body rises, falls and expands unevenly around the circumference, and once the working layer separates from the insulating layer behind it, individual bricks can loosen and fall out. In severe cases the failure propagates around an entire ring, the lining collapses, and kiln lining life is cut short. This mechanism, rather than the nominal refractoriness of the brick, is often the true limit on campaign length.
How to Improve Kiln Lining Reliability
Lining life is improved by treating the working layer, the insulating layer and kiln operation as one system rather than three independent choices.
Match the working brick to the actual duty of each zone instead of using one grade over the full kiln length.
Control kiln operation so that the amplitude of temperature swings is limited, particularly during start-up, shutdown and feed interruptions.
Design the interface between the two layers with expansion allowance and shims, so that the working layer can move without separating from the backup layer.
Select brick with stable reheat linear change, so that the working layer does not shrink away from the shell during service.
Monitor shell temperature and lining profile regularly, so that a localised failure is repaired before it develops into a full ring failure.
Lining quality is verified through the same family of tests applied to other dense shaped refractories: reheat linear change to ASTM C113/C113M, cold crushing strength and modulus of rupture to ASTM C133, abrasion resistance to ASTM C704, and classification of the dense shaped product under ISO 10081. Consistency between the delivered brick and the qualified sample matters as much as the nominal grade name, because a small change in raw material or pressing practice shifts the thermal expansion and reheat behaviour of the whole ring.
Frequently Asked Questions
Q: What is a rotary kiln used for in non-ferrous metallurgy?
A rotary kiln dries concentrates, fluxes, fuels and slag before charging, and also acts as the main smelting or roasting vessel in several reduction and volatilisation processes.
Q: Why have kiln linings moved from 230 mm brick to larger formats?
Larger brick reduces the number of joints in the lining, and fewer joints mean fewer weak planes for gas and liquid attack, so campaign life improves.
Q: Which refractory materials are used in high-temperature rotary kilns?
Magnesia-chrome bricks, magnesia-spinel bricks and high alumina refractory bricks are used where the lining must resist corrosion, scouring and abrasion.
Q: Why is an insulating layer needed outside the working lining?
Dense basic bricks conduct heat readily, so an insulating layer outside the working layer reduces shell heat loss and helps to protect the kiln shell.
Q: Why does the working lining fail by thermal cycling rather than wear?
Because the two layers form separate rings, temperature fluctuations make the working layer expand and contract unevenly and it can separate and fall out.
Q: How can ring collapse be prevented?
By matching materials to each zone, limiting temperature swings, and designing the layer interface so that expansion of the working layer is accommodated rather than restrained.
