SPECIALIST EQUIPMENT
(recirculating loads); operate with minimal supervision; and lowest possible reagent consumption and hence low OPEX. So, how can we maximise the return for CHPP based on fi ndings from optimising traditional sulphide fl otation circuits?
CHPP product yield trends A critical factor in coal fl otation is froth management. Over the past four years, Outotec has undertaken pilot fl otation work and sampling of existing mechanical cells within Australian CHPPs. One of the key observations was the large variation in the amount of froth being recovered down the bank. T e typical yield profi le can be looked upon as two fl otation extremes occurring in a single bank of cells: high yield on fi rst cells, with rapid kinetics; yield falls off rapidly down the bank – and substantial froth crowding is required to maintain the product ash content. T e majority of coal comes off the
fi rst cell due to the rapid coal fl otation kinetics. After this there is a large drop off in mass yield. T e fi rst cell in the bank recovers around 60-70% of the mass in the feed, and then falls off signifi cantly in the remaining cells. T ough they are all in the same bank, the individual cells have to operate under very diff erent duties in terms of froth handling and also solids concentration in pulp. T is calls for a tailored approach to the cell design. Control of product ash in mechanical fl otation cells is another important aspect in CHPP operation, with product ash generally increasing
down a fl otation bank. T e primary reason for the increase in ash content down the bank is probably due to the lack of attention to froth stability in coal fl otation cell design. In a typical CHPP fl otation bank where most of the mass is recovered in the fi rst 1-2 cells, there will be signifi cantly less material available to fl oat and help support the froth stability in subsequent cells. In this case of too low froth stability, operators will typically use level and air to try to pull concentrate from the latter fl otation stages, with very little or no froth depth. T is tends to lead to higher product ash.
CHPP froth management Flotation cells will work most effi ciently if the launders and froth crowding are tailored to the specifi c needs of the coal. To determine the correct launder and crowding confi guration on a mechanical fl otation cell, the correct froth carry rate (FCR) and lip loading (LL) ensures that the tank and launder confi guration selected will be suitable for the duty. In an operating plant, there are fi ve main ways to manage froth in CHPPs: froth washing; froth crowding and launder confi guration; split feeding; froth cameras; and reagent addition. Froth washing is the addition of water to the froth phase to wash non- fl oating ash particles back into the slurry phase. Froth crowding and launder
confi guration aff ect the froth surface area at the top of a fl otation cell. Optimising froth crowding not only
improves ash control but generally improves yield.
When it comes to froth crowding
there are two basic confi gurations to consider. T e fi rst has a larger froth surface area, more lip length and little or no crowding – it is ideally suited to the fi rst cell in a coal fl otation bank to recover a high mass. Conversely, the second confi guration has less froth surface area, less lip length and more crowding – it is better suited to the middle and end of a typical CHPP fl otation bank for recovering a lower mass of product. Split feeding is another option to
control the froth carry rate (FCR). By splitting the feed to both cells, the froth volume and mass yield is split (averaged) over both cells, thereby unloading the fi rst cell and maximising yield at the required ash value. With regard to froth cameras,
these are installed over the cell lip to measure the velocity, colour and stability of froth. Benefi ts of controlling froth velocity include maintaining a high product yield, minimisation of product ash, and reduced operator input. Finally, reagents can be added to assist in the recovery of coal. Many characteristics of fi ne coal
fl otation are analogous to sulphide cleaner fl otation where the emphasis is on handling high mass recoveries and rapid kinetics. Froth management is one of the key considerations in obtaining an optimal fl oat circuit design. Flotation cells with the correct crowding and lip confi guration will maximise yield while minimising the ash reporting to the product. ●
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