Froth Flotation – Summary For a complete explanation of froth flotation, see the later Froth Flotation section. Froth flotation was the key process in extracting the valuable minerals from the waste rock. The basic principal is to blow air through a mixture of ground ore/water/chemical reagents. The valuable minerals stick to the bubbles which rise to the surface to be collected.

Froth flotation grinding mill

Froth flotation grinding mill

When the ore is ready for froth flotation, it is a pulp, i.e. a mixture of ore, waste rock and water. This is fed into long troughs, called froth flotation tanks (or cells). The pulp was mixed with a chemical ‘frothing agent’ – at Britannia, steam-distilled pine oil was the favoured agent – and air was blown into the bottom of the cells, frothing it into bubbles. Another chemical reagent, known as a ‘collector’ was also added. This chemical acted as a waterproofing agent, chosen specifically for its properties to waterproof the ore minerals to be collected. When a mineral particle was waterproof, it stuck to a bubble as it rose to the surface. When the mineral-laden bubbles reached the surface they were skimmed off the top. The waste rock particles that hadn’t been waterproofed stayed at the bottom of the tanks. They were said to have been ‘depressed’. They could be removed and disposed of.

The process of ‘bulk flotation’ was pioneered at Britannia. In this, the first flotation was done to separate all the sulphide minerals from the waste rock. The collector agent used was generally potassium xanthate. This process left a ‘concentrate‘ that contained a mixture of sulphide minerals (e.g. pyrite, chalcopyrite, sphalerite). The waste rock, known as ‘tailings’, were dumped in the Howe Sound, ultimately creating some of the land that is built upon today. It is estimated that 40 million tones of tailings were dumped offshore.

‘Selective’ or ‘differential’ flotation came after bulk flotation. Britannia was the first copper producer in North America to use selective flotation, increasing their recovery from 60% to over 90%. For selective flotation, the concentrate was reground in ball mills to produce a finer powder. Flotation was repeated but to ‘depress’ all the sulphide minerals with the exception of a particular metal sulphide. For example, pyrite and sphalerite were depressed in order to collect chalcopyrite.

Lime was an important part of the froth flotation process, also. Lime depressed pyrite and was added at the regrinding stage (just before selective flotation).

There were different types of froth flotation machines on the market. Britannia used these 3 , but they also developed their own. Known as the ‘Britannia Deep Cell’ system, they were used for bulk flotation. Normally, flotation tanks were 2 – 3 feet deep, whereas the Britannia Deep Cells were 8 feet deep. These proved to be highly effective for the Britannia ore, leading to a recovery rate of 95%. However, it seems that although bulk flotation became a commonly used method elsewhere (and is still used in mining today), the Deep Cell system was unique to Mill #3.

Dewatering

The final step in the Mill was to remove the water from the pulp – to leave the valuable ore that could then be shipped to the smelter. Removing as much water as possible had a number of benefits. It reduced the weight of the ore, reducing shipping costs, and also made the ore more stable in transit (dry products move/slosh around less than a slurry). It also allowed the chemicals and water that were used in flotation to be recycled back into the system. It is essentially a two–stage process – thickening and filtration. Thickening removes the majority of the water, and filtration using a vacuum pump removes additional moisture, producing a concentrate that is only about 10% water. It was then ready for shipping.

Thickening – At Britannia, dewatering happened in and around large tanks on the ground floor (now the three large wooden platforms). There were between two and five thickening tanks in the Mill, varying from year to year. Dorr Thickeners were the favoured tanks at Britannia. They consist of a large cylindrical tank, with a concave base. As the concentrate/water mix is fed into the tank, the solids settle by gravity to the bottom. A large rake/paddle rotates slowly, dragging the settled solids to a discharge pipe in the centre of the tank base. The weight of the overlying layer of solids naturally squeezes out most of the water from the solids near the rake at the base. A (diaphragm) pump removes the dewatered concentrate through the discharge pipe.

Filtration – There were two types of vacuum filtration, using either drum or disc filters. At Britannia, disc filters were favoured. These consisted of a series of circular discs mounted vertically on a heavy hollow shaft, mounted over a tank. Each disc was covered by a filter bag (e.g. of cotton), and was grooved to channel away the water as it was filtered. The discs rotate on the shaft, dipping into the thickened concentrate, then rotating into the air. Vacuum is applied through the hollow shaft, sucking the water through the filter bag and into the drainage channels. The resulting dry cake was left stuck to the filter bag until it was removed from the discs before they rotate fully back into the concentrate in the tank. The vacuum was created in the Roots Blower Shed outside the Mill.

Mill Trivia

Production

Although designed to process 2000 – 2500 tons daily, by only 1924 it was determined that it could process up to 2800 tons daily. During World War II, and after many upgrades, production hit a high of 7200 tons daily; this was due to high copper prices. By 1952, production had dropped to around 3000 tons daily, when 5 grinding mills were taken out of service.

Technology

  • Manganese steel was used in the crushing and grinding equipment as manganese increased the steel’s strength. This steel was manufactured on the site foundry
  • Tramp iron (i.e. stray iron objects from mining operations) that has been inadvertently incorporated with the ore, was removed from the Mill system by magnets

Experimentation

There was an almost continual process of experimentation to improve recovery rates and lower milling costs. There was a well equipped lab on site for technical analysis, including a small flotation machine and small rod or ball mill. Hundreds of tests were run to find better oils and reagents. A new mixture or product was never tried in the plant until it gave good results in the lab. Considerable work was done on the theory of flotation.

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One Response to “ Froth flotation grinding mill ”

  1. Demand:Copper ore crusher | Materials: copper ore | Customer from Peru Says:

    Please provide the details regarding the set of Machines required for the concentration copper ore(Oxide & Sulphate) Thanks & Best Regard

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