The Ausmelt furnace at Chanderiya produces Lead bullion using Lead concentrate and is designed to handle 3.9% Zinc and 5% Graphitic Carbon. Higher Zinc in this concentrate results in higher Zinc percentage in slag and higher Graphitic Carbon results in poor separation of slag from Lead bullion. It also lowers the life of the lance.
Since September 2016, Ausmelt plant was getting Lead concentrate with 6-9% of Zinc and 5-16% of Graphitic Carbon which was more than the optimal percentage of Zinc and Graphitic Carbon.
A team led by Mr. Pranabesh Roy - SBU Head and comprising Mr. T.V.S.S. Nagaraju - AM, Process, Mr. Vinay Kanthalia - AM, Process, Mr. Shyam Goswami - Jr. Executive, Process, Mr. Vivek Menaria - Jr. Executive - Process, Mr. Sandeep Paliwal - AM, Mechanical and Mr. Anil Inani - AM, Instrumentation as team members took the task to overcome this challenge.
It was studied and observed that when the ratio between Iron to Silica (Fe/SiO2) is more than 0.9 and Zinc is above 20-22% it results in formation of severe build-up inside the furnace. Also, to control bath temperature due to high Graphitic Carbon, Ausmelt furnace needs at least 25-30 MT/day of extra secondary material. The only in-house secondary material available was ISF dross which contains 25-30% Zinc and 55-60% Lead.
The team studied the effect of Zinc and other impurities like Iron & Silica in slag. To maintain the ration of Iron to Silica (Fe/SiO2) below 0.9, Silica was added as flux which brought down the ratio to 0.6 to 0.7 and thus was able to handle 32-35% Zinc in slag.
The team was successful to use high Zinc and Graphitic Carbon concentrate and was able to consume 800-1000 MT per month of ISF dross inventory which otherwise was increasing every month. Optimization of this slag chemistry also helped to get highest lance life ever (6 batches per lance) for the month of December 2016 with record of 16 batches for a single lance. This increased the average life of lance by 3 times.