Background:

Northam Platinum Zondereinde, located 100km From Rustenburg near Thabazimbi on the Northern part of the Western Limb of the bushveld complex, is one of the deepest platinum producing mines in the world. The main shafts operate at depths between 1200m - 2200m below surface, exploiting both the Merensky and UG2 reefs. Surface infrastructure comprises two concentrator plants (both for Merensky and UG2 ore), a smelter and base metals removal plant, where copper and Nickel is extracted from the high grade PGM concentrate. Northam Platinum employs approximately 8700 people, and production is bench-marked at 300000 oz annually. Northam pioneered the use of hydro-power in the mining industry – a now commonplace practice.

The Problem:

Supplying water underground for everything from drilling, cooling and cleaning requires a large volume of water, to the tune of 45 million liters of water per day! This water arrives at the Pre-cool towers at 25^oC and departs the surface at a temperature of between 4 – 6^oC, and this requires a large amount of cooling capacity. As with any industrial cooling unit, biological fouling is a big concern as it greatly reduces the cooling capacity of the system. Biological growth inhibits heat transfer mechanisms and leads to an inefficient process. This water also needs to be Class 1 drinking water quality, although this water is not intended for human consumption, the disinfection is a precaution so that should someone drink the water – they won’t be hospitalized through water-bourne viruses. Doing this requires large infrastructure and monitoring.

Northam’s previous water treatment consultants were unable to keep up with the disinfection demand required and as a result a large amount of biological growth was reported and sub-standard water was being delivered to the plant. Arch Chemicals was therefore called in and asked to draw up a plan to solve the disinfection requirements.

The Solution:

Arch Chemicals’ Granular Doser was identified as the ideal chemical dosing system for this application. With a hopper capacity of 175kg it is capable of treating up to 175 000 000 liters of water per day. The sophisticated electronic control mechanisms make use of a 4-20mA signal coupled to a variable speed drive, to ensure that the addition of concentrated HTH is done proportionately. 5 Granular Dosers were installed on site.

The HTH® Scientific Granular Dosers receive the 4-20mA Signal from the free chlorine monitors which are located on the water supply and return lines. Five of these units measure the Free Chlorine, pH and temperature 24hrs per day. Plans are in place to provide remote communication access to these units via a unique control system that will allow constant surveillance and reporting functionality.

Eight large Pre-Cool towers process the water returning from underground, at a rate of 45 000 000 liters per day. This water is measured for its free chlorine levels and HTH® Scientific is added automatically. As with any industrial cooling unit, biological fouling is a big concern as it greatly reduces the cooling capacity of the system. Biological growth inhibits heat transfer mechanisms and leads to an inefficient process.

This water is then further cooled by the Fridge plant systems, before it ventures underground. The fridge plant itself cools by means of a refrigerant. During the cooling process the refrigerant needs to be cooled in order for it to return to the liquid phase. This requires a further 8 cooling towers circulating 4 000 000 liters of water per day. This water is also measured and HTH® Scientific is automatically added to effectively disinfect the water and to kill all micro-organisms.

Finally, in the last phase of the circuit the water is measured before it departs the surface. This water is continuously supplied chlorinated at a Free Chlorine reading of 0.3ppm so that it may be fee of all pathogenic bacteria and have resistance to any further contamination.