RESEARCH IMPROVED SLAG FUMING

The significant improvement to the slag fuming process is based on a theory developed at The Centre for Metallurgical Process Engineering at the University of British Columbia The centre was established in 1985 to conduct research on processes in which metals are extracted from their ores and converted into useful products It is now the largest research facility of its kind at a North American university

Much of the research at the centre is conducted in close co-operation with Canadian companies such as Cominco, Stelco, Noranda Inc and Inco Ltd , as well as with many foreign companies All activities are concerned with technology transfer in the field of metallurgical processes The thrust of the research is computer- aided analysis (based on knowledge derived from plant and laboratory measurements) for the improvement of metallurgical processes

Zinc slag fuming is a metallurgical process for the recovery of zinc from slag through treatment of the slag with a mixture of coal and air The slag, a waste product from the smelting of lead sinter in the blast furnace, contains economically recoverable quantities of zinc

A mathematical model, or computer simulation, was developed at the centre to help understand the basic mechanism of the process and was used as a preliminary step in the design of the zinc slag fuming process The model development also involved a parallel experimental program conducted on an operating zinc fuming furnace

The modelling exercise is generally simple and straightforward compared with the experimental work, which rarely is easy and usually is expensive The strength of the mathematical model is that it links the operating process to fundamental principles and the laboratory, providing a basic understanding of the process Such knowledge is necessary if expensive trial-and-error procedures in process development are to be avoided

The role of mathematical modelling in the optimization of the zinc slag fuming process at Cominco’s Trail smelter shows the importance of combining mathematical modelling with experimental work in the plant In the slag fuming process a 45- to 55-tonne batch of liquid slag is treated with coal and air to recover dissolved zinc oxide This is done in a furnace with a rectangular cross-section (typically 3 m wide by 8 m long) made of individual water jackets Two sets of submerged tuyeres run down the length of each side of the furnace just above the bottom Coal is conveyed to the tuyere with primary air and enters the tuyere at an angle from above The main blast, or secondary air, enters the tuyere behind the coal stream

Within the bath, the coal/air mixture reduces the zinc oxide from the slag, producing metallic zinc vapor In the process, the carbon is oxidized to carbon monoxide (co) and carbon dioxide (co2) Above the bath, the zinc, co and any unburned coal are combusted with leakage or “tertiary” air The zinc oxide fume produced this way is captured in a baghouse for subsequent recovery by leaching

The process has operated virtually unchanged since 1930; however, there has been historical debate over the fundamental mechanisms of the process One argument has been that fuming takes place on entrained coal particles in the slag, which would suggest that kinetic factors such as injection phenomena are important

Following this line of reasoning, a major evaluation of the process was undertaken at the centre by G G Richards and J K Brimacombe It began with a series of measurements on several operating furnaces A picture emerged that divided the process into two main reaction zones: reduction in the slag by entrained coal particles and oxidation of coal and ferrous iron in the slag by oxygen in the column of gas discharging from the tuyeres

Richards and Brimacombe found that two routes were available for improving the process: increase the amount of reductant (coal) entrained in the slag or decrease the rate of ferrous oxide oxidation The former factor is the most easily varied and, therefore, a model of the slag fuming process was developed which accounts for the direct reduction of slag by entrained coal particles and oxidation phenomena in the tuyere gas streams

A series of full-scale plant tests were conducted at Cominco’s Trail smelter In these tests a high-pressure coal injection unit was installed and operated to increase the fraction of coal entrained in the slag High-pressure coal injection tests were conducted over 40 to 50 minutes of a slag fuming cycle that normally runs 200 minutes During the time of the tests, zinc fuming rates in units of percentage zinc per minutes were improved by 70% to 80% This result translates into an expected reduction in the normal slag fuming cycle of 15% to 20% This means that 15% to 20% more zinc may be fumed in the same time and with the same coal as in normal slag fuming

The mathematical modelling analysis of the zinc slag fuming process yields a clear insight into the kinetics of the process This new understanding of the process was verified in a number of plant tests suggested by the model and it led to significant improvements in fuming rates and coal utilization efficiency REFERENCES Brimacombe, J K , Role of Mathematical Modelling in Metallurgical Engineering International Conference on “Progress in Metallurgical Research: Fundamental and Applied Aspects,” Feb 11-15, 1985, Kanpur, India Richards, G G , Dreisinger, D , Peters, E and Brimacombe, J K Mathematical Modelling of Zinc Processes The Centre for Metallurgical Process Engineering Joyce Musial is a Toronto-based geologist and freelance writer

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