The copper concentrates are received by rail and off loaded at the receiving bay. During the preparation of a charge for the reverberatory or ausmelt furnaces the necessary concentrates, fluxes and fuels are blended for introduction into the furnaces.
Copper Reverberatory Furnace
The reverberatory furnace was traditionally the main furnace, before the ausmelt was commissioned for smelting copper concentrates received at the smelter. Concentrates are blended with crushed reverts and various fluxes to produce a furnace charge material. The charge is fed into the furnace which is fuelled by pulverised coal. The charge mix is smelted at approximately 1200 ºC to form a slag and a matte (metal sulfides) phase. The molten slag, which has a lower density and floats on top of the matte, is removed from the furnace by skimming and granulated in water. The slag is further processed at the slag mill if the copper content is more than 0.6%. The matte phase is tapped from the reverb furnace into ladles and transported to the converters for further processing.
Hot off-gases pass through a waste heat boiler, which recovers heat from the off-gases for steam generation. The steam is used at the power plant to generate electricity while the hot air from the air heaters is introduced to the Reverb furnace to aide in combustion. The off-gas is further cooled in a spray cooling or conditioning tower to 120oC before the cooled gas enters the baghouse where solids (baghouse dust) and gas are separated when passing through the glass fibre bags. Final cleaned off-gases are released to the atmosphere via a high stack. Dust from the baghouse is either processed at the arsenic plant or dumped at the waste disposal site which is under construction.
Reverberatory Furnace
Ausmelt Furnace
The original lead smelting Ausmelt furnace has been refurbished to smelt copper concentrates and was re-commissioned in 2008. The Ausmelt is a Top Submerged Lance ("TSL") smelting furnace charged with copper concentrates and various fluxes and fuelled with locally produced charcoal and heavy furnace oil. The iron and sulphur in the feed is utilised as the main energy source for smelting the concentrates. Oxygen enriched air is used for combustion. With the first oxygen plant commissioned and subsequent addition of more oxygen into the furnace there is significant potential capacity of the ausmelt furnace. The molten material from the Ausmelt is tapped from tap holes and introduced to the reverberatory furnace –for holding and slag cleaning or alternatively slag is granulated while Matte from the TSL can go directly to the Converters. The granulated slag goes to the Slag Milling Plant for processing.
Off-gases from the Ausmelt pass through a spray cooling system to cool it down to 120ºC before entering the baghouse where dust is separated from gas before the cleaned gas is released to the atmosphere via a second stack. Dust recovered at the baghouse is taken to the arsenic plant for processing.
Ausmelt Furnace
Pierce Smith Converter Furnace
Molten matte tapped from the reverberatory furnace is transferred to the Pierce Smith converters for the production of blister copper. Air is blown into the matte and the oxygen reacts with sulfur, iron, lead and zinc. The sulfur from the metal sulfides provides the energy to complete the conversion of matte to blister copper. The blister copper (98.5 % Cu) is cast into 1.62 t bars for shipment to refineries.
Slag is formed when the oxidised iron reacts with silica that is added during the slag blowing stage. This slag is skimmed into ladles and granulated in water for treatment at the slag mill or returned hot to the reverb furnace. Off-gases from the converter are cooled in a water spray cooling tower to 120ºC and pass through the converter baghouse before being released to the atmosphere via the same stack used for the reverberatory furnace. Dust collected in the baghouse is treated at the arsenic plant or dumped into the waste disposal site.
Pierce Smith Converter Furnace
Arsenic Plant
Concentrates and other secondary material processed at NCS are traditionally relatively high in arsenic. The arsenic passes through the smelter and is captured from the off-gases in the baghouses. Baghouse dusts with high arsenic levels are used as feedstock into the arsenic plant. Baghouse dusts that cannot be processed in the arsenic plant will be disposed of in the waste disposal site.
The arsenic plant is used to produce arsenic trioxide from dusts recovered during the smelting process. Four single hearth Godfrey roasters are used to convert the material into arsenic trioxide, which is sold for the manufacture of pesticides and wood treatment.
Slag Mill
Slag skimmed from the various furnaces is granulated in water and passed to the slag mill for screening, milling and concentration of copper by conventional flotation. The over sized material is crushed and returned to the milling circuit for processing. The concentrates produced are re-incorporated into the smelting process. Slag mill tailings are placed on the old tailings dam to the west of the smelter. Water from the tailings dam is used in the smelter for slag granulation and dust suppression.
Power Plant
Electrical power for some of the smelters operations are supplied by an on-site power plant. The plant has the capacity to produce 2.5 MW of power. The power plant uses the steam produced from the hot off-gases from the reverb furnace in two waste heat boilers. The steam is used to drive turbines in the power plant and generate electricity. Additional electrical power is drawn from the national grid as required.
Mill Tailings Dam
Historically under Tsumeb Corporation Ltd. operations, the tailings originating from the slag mill were deposited north of the slag mill adjacent to the access road to the smelter. The tailings however still contain pay metals including copper and it is likely that the dump will be reprocessed in order to recover these metals.
A new tailings dam to the east of the smelter was also used to deposit Slag mill tailings – this unit will undergo rehabilitation.