Smelter upgrade – addressing improved air quality concerns

Smelter upgrade – addressing improved air quality concernsSmelter upgrade – addressing improved air quality concerns

In line with Implats' air quality management programme and its aims of reducing and limiting the quantity of airborne pollutants and emissions, Impala Platinum recently undertook a R850 million environmental upgrade and expansion of its smelting facilities near Rustenburg. Key to this smelter expansion project was the reduction in air pollution, particularly with respect to SO2 emissions, to bring these in line with future environmental permit requirements and to meet legislated ambient air quality standards. The planned smelter upgrade would improve air quality in the vicinity of the Impala Rustenburg operation, the occupational health of employees and the visual perception of the public and NGOs. It would also lead to reduced consumption of resources such as water and coal.

In tandem with this, and to accommodate Implats' overall growth strategy, this project allowed for smelter capacity to increase from around 2.1 million ounces of platinum per annum to 2.65 million ounces. The smelter expansion project involved providing for three full-time furnaces with concomitant increases in dryer, converter, acid plant and Sulfacid™ plant capacity. The off-gas equipment upgrade included (for the first time at Impala), the capture and treatment of fugitive off gas from the furnaces and converters, as well as the scrubbing of the tail gas from both the acid plant and the Sulfacid™ plant.

In particular, from an environmental point of view, the smelter expansion and upgrade included the following:

  • A new flash dryer whose advantages include reductions in:
    • the consumption of coal - 70% less coal is required to dry filter cake in a flash dryer rather than a spray dryer (before the upgrade Impala only operated spray dryers);
    • SO2 emissions - 70% less SO2 is emitted from the flash dryer as a result of the lower coal consumption;
    • quantity of water consumed - previously filter cake had to be repulped and this consumed 9Ml of water per month. No water is required for the operation of the flash dryer.
  • A new 7 500-tonne silo, which increases storage capacity of dried concentrate by 120%. The new silo provides buffer capacity to allow the mining and concentrating of ore to continue when off gas equipment is unavailable (during these periods no converting is allowed to take place). As soon as the off gas equipment is available again, the new silo can be emptied, as excess smelting and converting capacity is available.
  • A new Peirce Smith Converter which brings the total number of large converters at Impala Rustenburg to three. In addition to providing increased converter capacity and smelter redundancy, its benefits include improved fugitive gas capture through a secondary hood installation. Following optimisation of the secondary hood, it will also be installed onto each of the other two large converters.
  • A new acid plant mist eliminator which effectively removes acid mist (caused by SO3 particulates in the acid plant outlet during upset conditions) from the acid plant tail gas before the tail gas enters a new tail gas scrubber. The visual impact of the acid plant during an upset condition is therefore eliminated while sulphur capture is improved.
  • The new fugitive gas capture and treatment equipment which plays a key role in improving air quality (occupational hygiene) and reducing pollution. The equipment consists of fugitive hooding for the furnace tapholes, matte ladles and converters, all the required ducting and suction fans to transport the fugitive gas into a new fugitive gas scrubber. At the scrubber, the SO2 is captured into gypsum, a saleable product. Fugitive gas capture and treatment results in:
    • Reduced occupational exposure for employees,
    • Increased capture of SO2,
    • Decline in the fall out of SO2 at ground level,
    • Reduced visual impact.
  • The new primary furnace off-gas equipment which incorporates a new four-field electrostatic precipitator that reduces dust concentration in the primary furnace off gas from all three furnaces to below the required permit limit. The off gas then passes through an upgraded quench scrubber before entering the Sulfacid™ plant.
  • Expansion of the Sulfacid™ plant, which was originally commissioned in 2002. Four new Sulfacid™ pots were added to the original eight pots, enabling the plant to handle the off gas from three operating furnaces. New larger ID (induced draught) fans were installed to cater for the increased volume of furnace off gas. The Sulfacid™ plant captures up to 80% of the gaseous SO2 produced in the furnaces through an activated carbon and water spray system, converting SO2 into weak sulphuric acid.
  • The sulphuric acid plant, which employs a single-contact process, was upgraded to increase the flow of converter off gas through the plant from 12 Nm3/s to 15 Nm3/sec. The acid plant converts the SO2 in the converter off gas to strong (98 %) sulphuric acid, which is sold to a fertiliser manufacturer. The enhanced acid plant enables improved primary suction from the Peirce Smith converters which in turn has lead to reduced fugitive emissions.
  • A new tail gas scrubber, which treats the tail gas from both the Sulfacid™ and the sulphuric acid plants. Up to 85 % of the SO2 in the tail gas is captured into a saleable gypsum by-product. The high quality of gypsum produced is suitable for wallboard manufacturing.

The construction of the smelter expansion project began in June 2007 and was completed in March 2009, at a cost of R849 million, within the original planned budget. The total amount spent on environmental improvements totalled R484 million. A focus on safety was an integral aspect of the project which reported an excellent safety performance - an LTIFR of 0 was recorded. During the more than 1.9 million work hours spent in total on the project, only one, very minor, lost-time injury was reported.

Implats Sustainable Development Report 2009