This invention relates to froth flotation cells, and in particular to the provision of a lid for froth flotation cells.

Froth flotation is a method of performing a separation that is used in various different industries. For example, froth flotation is used to separate different minerals in an ore, or for de- inking paper or for cleaning coal.

The present invention, and the background thereto, is discussed primarily with reference to the separation of minerals in an ore, but the invention is not limited to this particular use of froth flotation. The invention is applicable to all froth flotation processes.

Mineral froth flotation is a known industrial process used for extracting valuable mineral content from ore obtained for example through mining. It is a surface chemistry process used to separate solids, typically fine solids, by exploiting the variation in hydrophilicity between different materials.

A flotation cell or vessel, contains a pulp of matter such as ore from which the mineral is to be extracted mixed with liquid. Gas is flowed through the pulp and separation is achieved by the selective adherence of hydrophobic particles to gas bubbles whilst any hydrophilic particles remain in the liquid which flows between the gas bubbles in the vessel. When bubbles rise to the top of the vessel a froth is formed.

The froth extends from the pulp-froth interface to a bursting surface, which is typically above the overflow lip. A "froth depth" is defined as the distance between the pulp-froth interface and the overflow lip. A "froth height" is defined as the distance from the overflow lip to the bursting surface.

The froth can be arranged to overflow from the flotation vessel with both hydrophobic and hydrophilic particles comprised therein. Those particles can be extracted as a concentrate. Typically, in mineral froth flotation, it is the hydrophobic particles which are the desired product, and are intended to be recovered from the froth.

The remaining pulp in the flotation vessel is commonly referred to as the tailings. In some froth flotation processes (such as the de-inking of paper) it is the remaining pulp in the flotation vessel that is the desired product.

In practice a froth flotation plant will contain multiple cells, typically arranged in banks of similar type, where material is fed through the bank, cell by cell, and then on to the next bank. Cell types may differ between banks, the initial bank, for example, containing "roughers" which are used for initial crude separation of desired matter from undesired matter. Downstream, banks may include secondary roughers, also known as "scavengers", which perform additional separation on the pulp that remains in a rougher after froth has been overflown therefrom.

Downstream banks may also include "cleaners", which perform separation on froth which has been extracted from roughers or scavengers.

The performance quality of a flotation process can be measured with respect to two characteristics of the concentrate that is extracted from the flotation vessel - "grade" and

"recovery". When referring to mineral systems in which the desired product is recovered from the froth, grade indicates the fraction of desired solids in the concentrate as compared to the total solids in the concentrate. Recovery indicates the fraction of desired solids in the feed that is recovered in the concentrate. An industrial flotation process is manipulated in order to achieve an optimal balance between grade and recovery, with an ideal flotation process producing high recovery of high grade concentrate.

It is known that several controllable factors can affect the performance quality of a flotation process. These include the pH of the pulp, the concentration of various chemicals added to the flotation vessel, solids concentration and gas flow rate into the flotation vessel. However, the presence of so many variables makes quantitative control of froth flotation processes difficult.

A discussion of investigating froth flotation performance is provided in Barbian et al, "The Froth Stability Column - Measuring Froth Stability at an Industrial Scale", Minerals Engineering, 2006, Vol 19, No. 6-8, 713-718 in which correlations are identified between a froth stability factor, gas rate and froth depth in a single cell. The discussion concludes that metallurgical results indicate that changes in air flow rate result in variations in flotation performance that can be attributed to changes in froth stability, and that their study showed that high froth stability conditions occur at medium air flow rates which in turn result in improved flotation performance.

An earlier paper, "The froth stability column: linking froth stability and flotation performance", Minerals Engineering, 2005, Vol 18, 317-324, presents results that show that high froth stability conditions occur at lower air flow rates, and result in improved flotation performance.

Another paper, "Simple relationships for predicting the recovery of liquid from flowing foams and froths", Minerals Engineering, 2003, Vol 16, 1123-1130, is primarily directed to 2- phase systems and states that the amount of water collected is intimately related to the amount of gangue (undesired solids) collected, which in turn helps dictate the grade of the product obtained. This paper also teaches that the amount of water collected will be virtually

independent of foam depth and that there is no significant change in water recovery with foam height. WO 2009/044149 discloses a method of froth flotation control in which the flow rate of gas into the cell is varied in order to optimise the fraction of the input gas which is recovered in the froth overflowing the cell (as opposed to gas input into the cell which forms bubbles that subsequently burst and therefore escapes from the cell). Hence, WO 2009/044149 discloses how one variable (the gas flow rate) may be optimised for a froth flotation system. However, as mentioned above, there are many other variables which can affect the performance of a froth flotation system.

WO 2012/066348 identifies that the gas recovery in the froth is a relatively easily quantified value, and that optimising the gas recovery in the froth by varying the flowrates through the cell leads to froths of both higher grade and having higher mineral recovery.

In contrast to the prior art presented above, the present invention considers how the construction of the flotation cell can affect the cell output. It is known that introducing features such as 'crowders' into the froth, or drop-in launders, can affect the behaviour of the froth. However, those features take up space within the froth itself, reducing the total froth volume, which is operationally undesirable when the froth is the desired product.

US 2003/0070992 discloses a flotation cell in which a skimmer is positioned above the overflow level of the cell. The skimmer, driven by a motor, rotates about the axis of the cell to assist the froth in leaving the cell. In contrast to the lid of the present invention, as discussed below, a skimmer does not give the same technical effect of the present invention, namely to reduce the possibility for bubbles to burst and release gas to the atmosphere before the bubbles are recovered. This is because a skimmer has a long, thin profile which does not cover the operating surface area of the flotation cell by any substantial amount.

US 4,060,481 discloses a closed flotation cell with a sloping top formed integrally with the walls of the cell. In contrast, the lid according to the present invention is a covering that is separate from the tank and leaves at least part of the surface open to the surroundings and/or atmosphere. Therefore, the cell of US 4,060,481 cannot be considered to have a lid as in the present invention, because the top of the cell is formed integrally with the cell.

According to a first aspect of the present invention there is provided a froth flotation cell comprising: an overflow point at which, in use, froth generated in the froth flotation cell overflows to leave the cell, the overflow point defining an overflow level of the froth flotation cell, and a lid at or above the overflow level of the froth flotation cell, and extending over a portion of the flotation cell so that, in use, the lid comes into contact with the froth without substantially reducing the volume of froth within the cell.

According to this aspect of the invention, the gas recovery in the froth can be increased, because the presence of the lid reduces the possibility for bubbles to burst and release gas to the atmosphere before the bubbles are recovered in the launder. As a result, improvements in grade and product recovery can be achieved, even in existing systems, by introducing the presence of the lid. The lid does not reduce the volume of froth within the cell (although it may reduce the height of the froth above the overflow point in the region of the lid), and so allows the froth to develop within the cell as normal (contrary, for example, to a crowder) but then prevents bubbles in the froth from reaching the bursting surface if they rise too far from the overflow point.

The lid can have a lower surface substantially level with the overflow level of the cell. Alternatively, the lower surface can be above the overflow point, as long as it is still in contact with the froth.

Side surfaces of the lid can be substantially perpendicular to a/the lower surface of the lid or slope inwardly, with respect to the lid, from the lower surface of the lid. This prevents collection of froth on the upper surface of the lid.

In some examples, the perimeter of the lid is in register with the overflow point.

However, the lid does not necessarily extend over all the operational surface area of the flotation cell. In particular, if the lid is at the same level as the overflow level, it is preferable to leave some of the surface area free in the region of the overflow point. That is, the lid does not extend to the overflow point. Preferably, the lid extends over a point in the flotation cell that is the furthest point from the overflow point, because bubbles rising at the point would otherwise have the longest exposure time to the atmosphere as they travel to the overflow point (and thus the greatest chance of bursting before reaching the launder). The lid can extend across 90% or less of the operating surface area of the froth flotation cell, optionally across 70% or less of the operating surface area, further optionally across 50% or less of the operating surface area, still further optionally across 30% or less of the operating surface area and still further optionally across 10% or less of the operating surface area. In particularly preferred embodiments, the lid covers 8 to 20% of the operating surface area.

The height of the lid can be adjustable, to account for changes in operation of the froth flotation cell.

The froth flotation cell can comprise a mixer for mixing the contents of the froth flotation cell and a shaft of the mixer can pass through the lid.

The lid can be made of metal, optionally steel or aluminium as these are materials commonly used for flotation tanks, and thus will not introduce any further surface chemistries into the process. The lid may be constructed of any other suitable material that will not introduce any further surface chemistries into the process.

Preferably, the froth flotation cell is adapted for processing metal ore. The side surfaces of the lid can extend above, in use, the froth bursting surface. Also, the lid can be positioned, in use, within a height of the froth above the overflow point. That is the lower surface of the lid can project into the froth height, whilst the upper surface is above the froth height. This avoids collection of froth on the top of the lid.

According to another aspect of the invention, there is provided a method of retro-fitting a froth flotation cell with a lid, comprising: determining an overflow point at which, in use, froth generated in the froth flotation cell overflows to leave the cell, the overflow point defining an overflow level of the froth flotation cell, and positioning a lid at or above the overflow level of the froth flotation cell, and extending over a portion of the flotation cell so that, in use, the lid comes into contact with the froth without substantially reducing the volume of froth within the cell.

According to this aspect, a pre-existing froth flotation cell (such as a stirred tank or column) can be subsequently modified to include a lid, to obtain the benefit of the present invention. As such, the invention provides the opportunity to improve gas recovery of existing apparatuses, already in situ, and thus avoids the expense of completely replacing such preexisting equipment. The various modifications and limitations discussed with respect to the first aspect of the invention also apply to this aspect.

According to another aspect of the invention, there is provided a method of operating a froth flotation cell having an overflow point defining an overflow level for the froth, comprising: providing a lid at or above the overflow level of the froth flotation cell, extending over a portion of the flotation cell; generating a froth in the froth flotation cell, which comes into contact with the lid; wherein the volume of froth within the cell is not substantially reduced by the presence of the lid. The various modifications and limitations discussed with respect to the first aspect of the invention also apply to this aspect.

The method of this aspect can be applied to obtain a substance from a liquid containing two or more substances, by adding said liquid to a froth flotation cell, operating the cell according to the method of this aspect, and obtaining the substance from the froth which overflows the cell during operation. For example, this could be used to obtain a refined ore.

The method of this aspect can be applied to obtain a substance from a liquid containing two or more substances by adding said liquid to a froth flotation cell, operating the cell according to the method of this aspect, and obtaining the substance from the matter which remains in the froth flotation cell. For example, this could be used to obtain a refined ore.

The method of this aspect can be applied to obtain a substance recovered from froth which overflows from a froth flotation cell, or liquid retained in the cell, wherein said froth flotation cell is operated according to the method of this aspect. The invention will now be described, by way of example only, with reference to the exemplary Figures, in which:

Fig. 1 shows a schematic view of a flotation circuit; and

Fig. 2 shows a schematic view of a flotation cell according to the invention.

The present invention identifies that a change in construction of a froth flotation cell can increase the gas recovery in the froth, which in turn can increase both the product (e.g. mineral) grade and recovery.

Referring to Fig. 1 , a typical froth flotation circuit is shown, having a number of banks or sub-banks, each including a plurality of froth flotation cells 100. It will be appreciated that the particular layout of the flotation circuit, the numbers of cells 100 that comprise each bank or sub- bank and the flow configuration of the various streams can vary widely. Each bank or sub-bank of cells may include any number or arrangement of cells 100, dependent on the practical conditions to be achieved. The cells 100 are connected to one another by any known means so that at least some of the contents of one cell 100 can be channelled into another cell 100. The practice of froth flotation and the design of such operations is known to the skilled person and is described in detail in, for example, Wills' Mineral Processing Technology, 7th edition (Wills, B.A. and Napier-Munn, T.).

A liquid containing two or more substances can be added to a froth flotation cell or cells

100 for separation, either wherein a desired substance is extracted from the froth which overflows the cell or wherein the froth includes undesired substances, so that a desired substance can be extracted from the pulp which remains in the cell after operation. In the context of the minerals industry, the substances are metal-containing minerals in an ore containing the minerals and gangue.

In the embodiment shown in Fig. 1 the flotation circuit includes a bank of rougher cells 104 into which a liquid feed typically water containing particles of ore, is introduced.

Downstream from the rougher bank 104 there is provided a secondary rougher or "scavenger" bank 108 and a cleaner bank 110. Optionally, the circuit may include more than one rougher 104, scavenger 108 or cleaner 110 bank or sub-bank. In addition, both cleaners 110 and re- cleaners may be included. According to the embodiment as shown, both the cleaner 110 and the scavenger 108 include feedback channels for re-introducing material into the rougher 104 for additional processing.

In operation, ore from which a desired metal-containing mineral is to be separated and then extracted is crushed using any appropriate means. The crushed matter is then fed into a mill to be further broken down into a fine particle size, for example powder. The required particle size in any given situation will be dependent on a range of factors, including mineralogy, etc and can readily be determined. After milling, the particles are chemically treated in order to induce the appropriate wettability characteristics of the desired mineral which is to be separated and then extracted using the flotation process. According to a preferred embodiment, the particles are treated so that the surface of desired mineral is both hydrophobic and aerophilic. This ensures that the mineral will be strongly attracted to a gas interface such as a gas bubble and that air or other flotation gas will readily displace water at the surface of the desired mineral.

All undesired matter is preferably chemically treated so as to be hydrophilic. The methods for chemical treatment of the particles are well known and so are not discussed further herein.

In order to carry out a froth flotation process and separate and extract the desired mineral, the chemically treated particles are introduced into a cell 100 with water or other liquid. Bubbles of air or other gas are then introduced into the liquid (also referred to as a "slurry", due to the presence of the solid particles) at a controlled rate via one or more gas inlets (not shown).

Typically, the gas is supplied to the gas inlet or inlets of the cell 100 via a blower or other suitable apparatus, as in a self-aerated cell for example. During this operation of the cell 100, the slurry at least partially separates so that at least some of the hydrophobic particles of desired mineral adhere to the gas bubbles whilst hydrophilic particles of undesired material and, dependent on conditions in the cell, some of the hydrophobic particles, will remain in the liquid.

The difference in density between the gas bubbles and the liquid dictates that the bubbles rise to the upper surface of the slurry in the cell 100, to create a froth thereon. The froth contains both bubbles and liquid which flows in the channels formed between the bubbles. The froth therefore contains both desired particles and undesired particles. In order for the desired particles to be extracted, conditions in the cell 100 are controlled so that at least some of the froth overflows from the cell 100. The froth that overflows or is removed from the cell 100 is either introduced into a further flotation cell 100 and/or forms a concentrate which includes the desired mineral to be recovered therefrom. Methods of concentrate recovery from froth and methods of extraction of valuable materials from such a concentrate will be well known such that further discussion of these is not provided.

In the embodiment shown in Fig. 1, once feed has been introduced into the rougher 104, the rougher 104 perfonns a froth flotation process as described above. The froth produced by the rougher 104 during that process is channelled into the cleaner 110 whilst the tailings from the rougher 104 are introduced into the scavenger 108. Both the scavenger 108 and the cleaner 110 then perform a froth flotation process as described. The froth produced by the scavenger 108 and the tailings produced by the cleaner are reintroduced into the rougher 104 for further processing. The tailings from the scavenger 108 are then discarded whilst the froth output from the cleaner 110 is harvested for extraction of the final concentrate as described above.

A range of variables and operational boundary conditions in the froth flotation cells 100 can be monitored and controlled in an attempt to achieve good recovery and good grade of the extracted concentrate.

The skilled person will appreciate that flotation froths are stabilised by the hydrophobic particles. The amount of particles which become loaded onto the bubbles is an important factor in the stability of the froth and will depend on the input gas flow rate.

Fig. 2 depicts an example of a froth flotation cell according to the present invention. The cell 200 can be part of any of the previously discussed rougher, scavenger or cleaner banks 104, 108, 110. Alternatively, the cell 200 could be operated in isolation.

The cell 200 comprises a tank 201, into which the pulp is fed through inlet 202. The tank 201 is also supplied with gas through a gas feed 203. In the arrangement of Fig. 2, the gas enters the tank 201 through a sparger 204, in order to breakup the gas as it enters into the pulp.

However, the tank 201 is also provided with a mechanical mixer 205, which stirs the pulp and breaks up the gas entering into the pulp, to encourage the formation of a froth. As such, the presence of the sparger 204 is not essential, and alternative arrangements can be used. For example, the gas feed 203 might be integrated with the mixer 205. Various types and designs mixers 205 are available, and the invention is not limited to any particular arrangement of mixer. As the bubbles rise in the cell, a froth is formed on the surface of the pulp. As such, the cell 200 can be considered to have a lower "pulp zone" 206, in which the pulp is present, and an upper "froth zone" 207 in which the froth is present.

The arrangement of Fig. 2, utilising a motor-driven mechanical mixer 205, is commonly called a stirred-tank. Another common arrangement in flotation cells utilises a columnar arrangement, which may or may not include a packed bed and may or may not include an active mechanical mixer.

Returning to Fig. 2, the froth builds up above the pulp and overflows at the overflow lip 209. From there, it is collected in the launders 210, as shown by arrows in the Fig. 2. Whilst Fig.2 shows the use of external launders 210, internal launders 210 can also be used, positioned within the froth. If an internal launder 210 is used, the overflow lip 200 will be the lip of the launder 210, which may not be at the same level as the lip of the tank 201. The lip 209 of an internal launder will be at or below the top of the tank 201. However, in each scenario, the overflow lip 209 acts as an overflow point, and defines an overflow level for the tank. It is noted that whilst present invention is discussed using the term "overflow point" for convenience, this should not be interpreted as meaning that the froth overflows in one, localised, position. The skilled person will understand that the froth will overflow from a "length" rather than a "point". For example, when using a continuous external launder 210, as shown in Fig. 2, the froth will overflow from the entire perimeter of the tank 201, and the entire perimeter is to be regarded as the "overflow point". If an internal launder is being used, the length of the lip of the internal launder over which froth is collected will be the "overflow point". If multiple, discrete, launders are provided, the "overflow point" will be combined lengths available for the froth to overflow. The overflow point is preferably at least 10% of the entire perimeter of the tank 201, optionally at least 30% of the entire perimeter of the tank 201, further optionally at least 50% of the entire perimeter of the tank 201, still further optionally at least 70% of the entire perimeter of the tank 201 and still further optionally at least 90% of the entire perimeter of the tank 201.

The tailings from the pulp zone 206 are removed from the tank 201 through an outlet 208. In some industries, the tailings may contain the product of interest. In practice, the flow rates through the pulp inlet 201, gas feed 203 and tailings outlet 208 can be controlled in order to operate the tank in a steady state, keeping the froth depth (i.e. thickness of the froth between the pulp/froth interface and the overflow lip 209) the same.

The present invention provides a lid 211 for the froth flotation cell 200. The lid 211 is preferably separate and/or separable from the froth flotation cell 200. In other words, the lid 211 is not integrally formed with the froth flotation cell 200 and preferably leaves part of the operating surface open to the surroundings and/or atmosphere. The lid is fixed in position with respect to the cell. That is, the lid is preferably stationary when the froth flotation cell is in operation. As shown in Fig. 2, the lid 211 is preferably positioned at or above the overflow level of the froth flotation cell 200. The lid 211 can slightly project into the froth height. As such, the lid 211 does not substantially reduce the volume of the froth within the tank 201 (i.e. the lid 211 does not intrude into the "froth depth", although it may locally affect the "froth height" above the overflow point). The lid 211 is positioned so that the lower surface of the lid is below the bursting surface of the froth, when the froth flotation cell is operating.

As such, the lid 211 comes into contact with the froth in the froth zone 207.

In the absence of the lid 211, a bubble rising and reaching the bursting surface in the centre of the tank 201 will only be recovered if it travels to the overflow lip before it bursts. The length of time before the bubble bursts (the "bubble lifetime") will depend on the process and process conditions, but in all cases the bubble will not be recovered if its lifetime is shorter than the time required to move to the overflow point. When the lid 211 is present, as in Fig. 2, bubbles in the froth that come into contact with the lid 211 are prevented from reaching the "bursting surface" with the surrounding atmosphere. As a result, gas that would otherwise be lost from the flotation cell 200 due to bubbles bursting on the froth surface, before they are collected by the launder 210, is retained. The bubbles can travel under the lid, towards the launder 210. When the bubbles emerge from the edge of the lid 211, they can then rise to the bursting surface. However, as the bubble reaches the bursting surface at a point closer to the overflow point, there is a greater chance that the bubble will reach the launder 210 before it bursts.

That is, the lid 211 prevents bubbles that rise to the top of the froth, at positions far from the overflow point, from bursting and increases the opportunity for those bubbles to overflow by reducing the distance they must travel whilst exposed to the atmosphere. As such, greater gas recovery is possible.

In Fig. 2, the lid 211 only partially extends over the surface area of the tank 201. When the lid 211 is positioned at the same level as the overflow level, some area is left free around the overflow point 209, to allow overflow of the froth to occur. On the other hand, if the lid 211 is positioned slightly above the overflow level (but still below the level of the froth bursting surface), the lid could extend to cover the entire surface area of the tank 201. That is, the lid 211 could cover the entire operating surface area, the operating surface area being the open surface area of the tank 201 at the overflow point 209.

When the lid 211 does not cover over the entire internal surface area of the tank 201, it is preferable that the lid is positioned to cover an area most distant from the overflow point. That is, it is preferable that the lid 211 covers the point within the tank 201 at which a rising bubble has the longest lateral journey to the launder 210. The lid can extend across 90% or less of the operating surface area of the froth flotation cell, optionally across 70% or less of the operating surface area, further optionally across 50% or less of the operating surface area, still further optionally across 30% or less of the operating surface area and still further optionally across 10% or less of the operating surface area. In particularly preferred embodiments, the lid covers 8 to 20% of the operating surface area. In other embodiments, it is preferable that the lid 211 extends across at least 10% of the operating surface area, optionally across at least 30% of the operating surface area, further optionally across at least 50% of the operating surface, still further optionally across at least 70% of the operating surface area and still further optionally across at least 90%) of the operating surface area.

In a preferred embodiment, the profile of the lid 211 corresponds with the profile of the lip of the cell. That is, when viewed from above, the perimeter of the lid corresponds to the perimeter of the overflow point. For example, if the cell is round, the lid is round, having an outside diameter being the same as the inside diameter of the cell mouth, and the lid is located concentrically with the cell mouth. In the same way, if the cell mouth is rectangular, the lid is the same size rectangle, positioned in register with the cell mouth. In general, flotation cells are cylindrical or square/rectangular prisms.

By positioning the lid at the furthest point from the overflow point, those bubbles which would otherwise rise to the top of the froth and meet the atmosphere at the longest distance from the overflow point, are prevented from reaching the atmosphere so early, and thus experience a reduced exposure time whilst travelling to the overflow point.

In the example of Fig. 2, it is the lower surface of the lid 211 that is level with the overflow level. As such, the lid 211 may be solid in cross section (as shown), or have a hollow on the upper side, without affecting the manner in which the lid 211 interacts with the froth at the lower surface. However, it is desirable for all the side surfaces of the lid 211 to extend at least above the bursting surface of the froth, to avoid collecting froth on the upper surface of the lid 211. Similarly, to avoid the lid 211 undesirably projecting into the froth depth, in one arrangement it is preferable that the lid 211 has a flat lower surface (i.e. so that the lower surface is parallel with the overflow level). It is also preferable for the side surfaces of the lid 211 to be perpendicular to the lower surface. In other arrangements, the bottom surface of the lid can be curved to be convex when viewed from the froth side (i.e. lower in the middle than at the edges when viewed from the side).

Preferably the lid 211 extends across ninety percent or less of the operating surface area of the froth flotation cell (i.e. in Fig.2 the inner surface area bounded by the walls of the tank 201, but in general the inner surface area of the cell 200 bounded by any walls and any overflow points), optionally across seventy percent or less of the surface area, further optionally across fifty percent or less of the surface, still further optionally across thirty percent or less of the surface area, and still further optionally across ten percent or less of the surface area. It is also preferable that the lid 211 does not extend to the overflow point or points, especially when the lid 211 is provided at the level of the overflow level. As mentioned before, when the lid 211 is above the level of the overflow level, it is possible for the lid 211 to extend over and beyond the overflow point or points.

In some embodiments, especially if the froth flotation cell is reconfigurable for processing different pulps under different conditions, the position of the lid 211 can be adjustable. In particular, the height of the lid 211 with respect to the froth flotation tank 201 can be changed, to ensure that the lid 211 comes into contact with the froth. For example, the tank 201 might be operated with drop-in launders below the lip 209 of the tank201, and the lid 211 would therefore be positioned below the lip 209 of the tank 201. As shown in Fig. 2, if the active mixer 205 incorporates a mixing shaft that extends upwards, the lid 211 can be constructed to pass around the shaft of the mixer 205. That is, the shaft of the mixer 205 can pass through the lid 211.

The materials used to construct the lid 211 may depend upon the properties of the froth into which the lid 211 is intended to come into contact. More generally, however, the lid 211 will be made of similar materials to that of the froth flotation tank 201. That is, the lid 211 is preferably made of metal, optionally steel or aluminium.

In the arrangement of Fig. 2, in operation pulp and gas are fed to the tank 201 via pulp inlet 202 and gas feed 203. Mixer 205 introduces turbulence into the pulp zone 206 of the tank 201, encouraging the formation of a froth. The froth collects in the froth zone 207 of the tank 201, and comes into contact with the lid 211. Bubbles come into contact with lid 211 instead of reaching a bursting surface with the surrounding atmosphere. The bubbles can travel under the lower surface of the lid 211 and emerge at a point closer to the launders 210. The bubbles then have a shorter exposure time to the atmosphere, as they travel to the launders 210, therefore making it less likely that the bubbles will burst before reaching the launders.

One particular advantage of the present invention relates to the possibility of retro-fitting existing froth flotation cells. The lid 211 of the invention is relatively straight-forward to fit to an existing cell, especially if the cell is operated under standard conditions such that the overflow point is constant. As such, a relatively easy modification can change the functioning of existing equipment to increase the gas recovery. That is, the invention avoids the need to replace existing flotation cell equipment, because it offers advantages that can be achieved by modifying an existing apparatus.

The preceding description has been directed mainly to extracting mineral from ore however it will be appreciated that the control and calibration methods can be used in any froth flotation process. Examples include de-inking of paper, wherein it is the undesired ink which is removed via the froth, and the desired paper that remains in the pulp in the flotation cell. The present method can also be applied to froth flotation cells for protein separation, molecular separation and waste separation for example.