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Coal flotation cells

26th October 2015

Posted By Paul Boughton


Maximising flotation equipment performance is key to improved CHPP operations
Outotec has conducted pilot flotation work and sampling of existing mechanical cells within Australian CHPPs

How to ensure you get the most from your CHPP flotation cells – presenting some simple, cost-effective solutions

Froth flotation is used in many Australian coal handling and preparation plants (CHPP) for upgrading fine and ultrafine coal. Although coal and sulphide ore flotation are very different, there are also some useful similarities. Here, we discuss some simple, cost-effective solutions that can be adopted in CHPPs to maximise flotation equipment performance.

There are some similarities between coal flotation and the cleaning stages in a sulphide concentrator – and the nature of concentrate (or product) removal is also similar, with most of the mass in coal flotation generally recovered in the first 1-2 cells.

Given these similarities between coal flotation and sulphide cleaners, there should also be benefits in sharing ideas to those in charge of monitoring and optimising these operations.

Sulphide concentrators and CHPPs have the following common objectives: highest possible yield (recovery) at the required product ash (concentrate grade); high equipment availability with minimal maintenance requirements; ability to handle high variability in loading and feed types (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 findings from optimising traditional sulphide flotation circuits?

CHPP product yield trends

A critical factor in coal flotation is froth management. Over the past four years, Outotec has undertaken pilot flotation 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.

The typical yield profile can be looked upon as two flotation extremes occurring in a single bank of cells: high yield on first cells, with rapid kinetics; yield falls off rapidly down the bank – and substantial froth crowding is required to maintain the product ash content.

The majority of coal comes off the first cell due to the rapid coal flotation kinetics. After this there is a large drop off in mass yield.  The first cell in the bank recovers around 60-70% of the mass in the feed, and then falls off significantly in the remaining cells. Though they are all in the same bank, the individual cells have to operate under very different duties in terms of froth handling and also solids concentration in pulp. This calls for a tailored approach to the cell design.

CHPP product ash management

Control of product ash in mechanical flotation cells is another important aspect in CHPP operation, with product ash generally increasing down a flotation bank. The primary reason for the increase in ash content down the bank is probably due to the lack of attention to froth stability in coal flotation cell design. In a typical CHPP flotation bank where most of the mass is recovered in the first 1-2 cells, there will be significantly less material available to float 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 flotation stages, with very little or no froth depth. This tends to lead to higher product ash.

CHPP froth management

Flotation cells will work most efficiently if the launders and froth crowding are tailored to the specific needs of the coal. To determine the correct launder and crowding configuration on a mechanical flotation cell, the correct froth carry rate (FCR) and lip loading (LL) ensures that the tank and launder configuration selected will be suitable for the duty.

In an operating plant, there are five main ways to manage froth in CHPPs: froth washing; froth crowding and launder configuration; split feeding; froth cameras; and reagent addition.

Froth washing is the addition of water to the froth phase to wash non-floating ash particles back into the slurry phase. It is typically used to combat high product ash levels early in a CHPP flotation bank.

Froth crowding and launder configuration affect the froth surface area at the top of a flotation cell. Optimising froth crowding not only improves ash control but generally improves yield. This works by allowing all cells in a bank to operate with a deeper froth, which increases froth drainage and leads to lower ash in the product.

When it comes to froth crowding there are two basic configurations to consider. The first has a larger froth surface area, more lip length and little or no crowding – it is ideally suited to the first cell in a coal flotation bank to recover a high mass. Conversely the second configuration has less froth surface area, less lip length and more crowding – it is better suited to the middle and end of a typical CHPP flotation bank for recovering a lower mass of product. It is possible to retrofit new froth crowders to existing flotation cells to improve froth recovery and cell performance.

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 first cell and maximising yield at the required ash value.

With regard to froth cameras, froth imaging technology is becoming smaller, lighter and more cost effective. Froth cameras are installed over the cell lip to measure the velocity, colour and stability of froth. The froth velocity correlates with the mass pull from the flotation process and is used to control the operating parameters of the flotation cell; ie, air flow rate and froth depth. Benefits of controlling froth velocity include maintaining a high product yield, minimisation of product ash, and reduced operator input.

When it comes to reagent addition, as with most flotation concentrators, reagents are added to assist in the recovery of coal. Too much can result in a very stable and frothy product, which does not readily collapse. This is analogous to a sulphide concentrator cleaning circuit when frother has been overdosed and the concentrate thickener overflow clarity increases. In this regard, both staged and dosing in the correct locations can assist in optimising the amount of reagents added whilst also avoiding over-dosing.

Many characteristics of fine coal flotation are analogous to sulphide cleaner flotation where the emphasis is on handling high mass recoveries and rapid kinetics. Froth management is one of the key considerations in obtaining an optimal float circuit design. Flotation cells with the correct crowding and lip configuration will maximise yield while minimising the ash reporting to the product.

Additionally, there are a number of options in retrofitting or modifying an existing CHPP float circuit. Strategies such as retrofitting of new froth crowders, improved circuit control through using froth cameras, and split feeding can all help improve froth recovery and the flotation cell performance, with minimal operator input even when encountering feed variations. 









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