FIELD OF THE INVENTION

The present invention relates to a method of controlling gas circula- tion in mineral flotation for separation of minerals from ore and concentrates, in particularly in separating molybdenum from copper containing sulfide minerals.

BACKGROUND OF THE INVENTION

For froth flotation, the ground ore is mixed with water to form a slurry and the desired mineral is rendered hydrophobic by the addition of a surfac- tant or a collector chemical, such as a depressant, although some mineral surfaces are naturally hydrophobic requiring little or no addition of collector. The particular chemical depends on the nature of the mineral to be recovered and, perhaps, the natures of those that are not wanted. As an example, sodium hy- drosulfide (NaHS) may be added as a depressant in the selective flotation of molybdenum to separate it from copper. This slurry of hydrophobic particles and hydrophilic particles is then introduced to tanks known as flotation cells that are aerated to produce bubbles. The hydrophobic particles attach to the gas bubbles, which rise to the surface, forming a froth. The froth is removed from the cell, producing a concentrate of the target mineral.

The minerals that do not float into the froth are referred to as the flotation tailings or flotation tails. These tailings may also be subjected to further stages of flotation to recover the valuable particles that did not float the first time. This is known as scavenging. The final tailings after scavenging are normally pumped for disposal as mine fill or to tailings disposal facilities for long- term storage.

Flotation is normally undertaken in several stages to maximize the recovery of the target mineral or minerals and the concentration of those minerals in the concentrate, while minimizing the energy input.

Addition of flotation reagents is adjusted based on the pH and redox potential measured from the slurry. High content of oxygen in a flotation cell is known to increase the consumption of the flotation reagent and increase flotation costs. Therefore several methods for controlling the oxygen content of the utilized process gas, i.e. flotation gas, have been developed.

WO2004/080599 discloses a method for separating minerals from a slurry containing valuable minerals, wherein gases fed in different process steps, including flotation, are recirculated in an essentially closed gas circulation created around the equipment used. According to the publication, in flotation, the recirculation of the flotation gases allows more efficient optimization of the froth structure. In accordance with the method disclosed in the publication grinding, flotation, precipitation and filtering should all be performed in a fully sealed, controlled recirculating gas atmosphere.

Replacing air with a non-oxidizing inert gas in mineral separation has been proposed, for instance in US6032805, US6036025, US6041941 and US6044978.

The existing methods do not allow utilization of a closed system with controlled point of release for expulsed flotation system without a need for expensive and large gas buffer tank.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a method and an apparatus for implementing the method so as to so as to alleviate the above disadvantages. The objects of the invention are achieved by a method and an arrangement, which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the idea of achieving partial recirculation of flotation gas in a closed circuit forced gas flotation system. Flotation gas is recirculated from all the flotation cells in the system, except at least one. Gas may be extracted from the system continuously though the said cell, herein referred to as a flushing cell unit. The flushing cell unit is characterized in that is operated atmospherically and allows direct balancing of the flotation gas with ambient air.

The method and the arrangement of the present invention allow constant controlled flushing of the recirculating flotation gas with provided process gas supplied to the mineral flotation process. The flushing cell unit also provides a controlled point of release for expulsed flotation gas without a need for a large and expensive gas buffer tank. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

Figure 1 shows a first embodiment of the arrangement of the present invention for circulation of gases in a mineral flotation process;

Figure 2 shows a second embodiment of the arrangement of the present invention for circulation of gases in a mineral flotation process; and

Figure 3 shows a third embodiment of the arrangement of the pre- sent invention for circulation of gases in a mineral flotation process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for controlling gas circulation in mineral flotation with an apparatus comprising a gas recirculation loop, at least one flushing flotation cell unit and one or more sealed flotation cell units, comprising

supplying flotation gas from the gas recirculation loop to each of the flushing flotation cell units and to each of the sealed flotation cell units;

allowing flotation gas to expulse from the flushing flotation cell unit(s), wherein

when the pressure of the flushing flotation cell unit(s) is less than the pressure of the sealed flotation cell units and greater than or equal to the atmospheric pressure at least part of the flotation gas is expulsed to the atmosphere,

when the pressure of the flushing flotation cell unit(s) is greater than the pressure of the sealed flotation cell units then at least part of the flotation gas is expulsed from the flushing flotation cell unit(s) to the sealed flotation cell units; and/or

when the pressure of the flushing flotation cell unit(s) is less than atmospheric pressure then gas is drawn into the flushing flotation cell unit(s);

and

collecting flotation gas from the headspace(s) of the sealed flotation cell unit(s) and recirculating the collected flotation gas back to the flotation cell units via the gas recirculation loop. In accordance with the present invention the flotation gas is provided to the system initially as pressurized process gas, fed to flotation cells, and recirculated from all of the flotation cells in the system, except at least one. The flotation gas is preferably oxygen-deficient, i.e. it is either free of oxygen gas or contains a volume fraction of oxygen gas that is lower than the volume fraction of oxygen gas in ambient air. The flotation gas is preferably an inert gas that is essentially free of oxygen gas or has only a very low oxygen gas content. In a preferred embodiment, the flotation gas consists essentially of inert gas or has a very high content of inert gas, for example, nitrogen, argon, helium and/or carbon dioxide, with nitrogen gas being particularly preferred as the inert gas. However, initially the flotation gas may be ambient air, which during the process is depleted of oxygen as it is consumed by reactions between the flotation reagent and the feed. The flotation gas preferably comprises at least 85 volume percent of inert gas, which may be a mixture of multiple inert gas compo- nents, more preferably at least 90 volume percent, even more preferably at least 95 volume percent, and most preferably the flotation gas consists essentially of only inert gas. When the flotation gas includes some oxygen gas, it should be only a small amount, as noted. Preferably, the flotation gas comprises no more than 15 volume percent oxygen gas, more preferably no more than 10 volume percent oxygen gas and even more preferably no more than 5 volume percent oxygen gas.

Process gas is typically added to the gas recirculation system in an amount that is required to maintain the amount of flotation gas at desirable level. The added process gas may be any of the gases discussed above, pref- erably inert gas such as nitrogen. Process gas can be introduced into the gas recirculation loop either to the suction side of the recirculating compressor or to the pressure side of the recirculating compressor. When liquid ring compressor is utilized as the recirculating compressor, process gas is preferably introduced into the gas recirculation loop in the pressure side of the recirculating com- pressor.

The flushing flotation cell unit serves to adjust pressure of the flotation gas in the flotation system. This is achieved by allowing the flotation gas to freely balance between the flushing flotation cell unit(s) and atmosphere, and optionally the preceding and/or successive sealed flotation units. When the pressure of the flushing flotation cell unit(s) is less than the pressure of the sealed flotation cell units and greater than or equal to the atmospheric pres- sure at least part of the flotation gas is expulsed to the atmosphere, when the pressure of the flushing flotation cell unit(s) is greater than the pressure of the sealed flotation cell units then at least part of the flotation gas is expulsed from the flushing flotation cell unit(s) to the sealed flotation cell units; and/or when the pressure of the flushing flotation cell unit(s) is less than atmospheric pressure then gas, present in the stack, is drawn into the flushing flotation cell unit(s). Typically the gas present in the stack is ambient air, however it may also be scrubbed flotation gas expulsed from the flotation cell units, or any mixture thereof. Expulsion of the flotation gas and/or withdraw of ambient air is achieved through a cell breathing conduit. The cell breathing conduit may be provided with a scrubber, e.g. a packed bed scrubber, for cleaning the expulsed flotation gas before it is released to the atmosphere, typically though a stack.

The arrangement may comprise one or more flushing flotation cell units, but typically only one flushing flotation cell or cell unit is required. The flushing flotation cell (or the flushing cell unit(s)) may be located at any desired location in the sequence of the flotation cell units. Preferably the flushing flotation cell unit is the flotation cell unit where most harmful or undesired byproduct gas, e.g. H ₂ S, resulting from the reaction between the flotation reagent and the feed is produced, as this avoids accumulation of the byproduct gas in the recirculating flotation gas and advantageously also the necessity to include an internal scrubber to the gas recirculation loop for removing the byproduct gas from the recirculating flotation gas. In an embodiment of the present invention at least one flushing flotation cell unit is arranged as the first flotation cell unit in the sequence of the flotation cell units. In another embodiment of the present invention at least one flushing flotation cell unit is arranged as the last flotation cell unit in the sequence of the flotation cell units. In still another embodiment of the present invention at least one flushing flotation cell unit is arranged in the middle of the sequence of the flotation cell units.

The "term flotation cell unit" as used herein as such or in context of

"flushing flotation cell unit" or "sealed flotation cell unit" as used herein refers to a single individual flotation cell or to a bank of flotation cells, i.e. a serial arrangement of flotation cells where tailings from the first cell move on as the feed to the second cell and so on and the tailings from the last cell form the final tailings of the bank. The number of cells in a bank varies depending on cell size, application and plant circuit configuration. Flotation gas is collected from the headspaces of the sealed flotation cells and recirculated back to the flotation cells through a gas recirculation loop. The recirculated flotation gas is pressurized in a compressor, preferably a liquid ring compressor, before it is fed back to the flotation cells. Process gas is added to the gas recirculation loop if necessary. It is to be understood that the recirculation loop is provided with the necessary structures, such as pipelines, lids, seals, vents, blowers etc. required for ensuring recovery and recirculation of gas and its maintenance in the system as well as pressure balancing. If desired the recirculating flotation gas may be scrubbed by a scrubber included in the gas recirculation loop to remove solid particles and/or other harmful or un- desired substances, e.g. H ₂ S, from the gas before it is reintroduced into the flotation cells.

The flushing flotation cell unit is connected to the directly preceding and/or directly following sealed flotation cell unit by a gas conduit that allows transfer of flotation gas from flushing flotation cell unit to the sealed flotation cell unit and vice versa. Thus the gas conduit allows equalization of gas pressure between the cell units. The gas conduit preferably comprises a water lock which permits direct control of the gas pressure and operation of the sealed cells in slight overpressure or underpressure while resisting the interchange of gases between the flushing cell unit and the sealed cell units by the pressure resistance caused by the water lock liquid column. When the gas conduit does not comprise a water lock the headspaces of the connected sealed cell units are close to atmospheric pressure. When the gas conduits connecting the headspaces of the sealed flotation cells to the flushing flotation cell unit(s) are equipped with water locks, the said headspaces can be maintained under an underpressure, preferably 5 to 8 mbar.

The gas recirculation loop may also comprise a bleeding line for bleeding recirculating flotation gas from the loop when necessary. The bleeding line may also be provided with a scrubber, e.g. a packed bed scrubber, for cleaning the bleed gas before it is released to the atmosphere. Typically the bleed gas is released to the atmosphere through the same stack as the flotation gas expulsed from the flushing flotation cell unit.

Figure 1 illustrates as a first embodiment of the invention an arrangement in which the first flotation cell is utilized as the flushing flotation cell.

With reference to Figure 1 , an arrangement for gas circulation comprises a gas recirculation loop comprising a recirculating compressor 40 for pressurising recirculating flotation gas and a gas feed manifold 21 , 22, 23, 24, 25 for providing the pressurized recirculating flotation gas into a flushing flotation cell unit 1 1 and into four sealed flotation cell units 12, 13, 14, 15, means 20 for providing process gas into the gas recirculation loop, and a gas suction conduit 26 for collecting flotation gas from the headspaces of the sealed flotation cells and transferring it to the recirculating compressor;

a cell breathing conduit 28 for connecting the flushing flotation cell unit 1 1 to atmosphere via a gas scrubber 60 and stack 70 for allowing expul- sion of flotation gas from the flushing flotation cell unit(s);

a gas suction conduit 26 , and optionally conduit 27, for collecting flotation gas from the headspace of the sealed flotation cell units 12, 15, and optionally the head space of a second concentrate sump 32, respectively, and transferring it to the recirculating compressor 40; and

a gas conduit 50 arranged between the flushing flotation cell unit 1 1 and the directly following sealed flotation cell unit 12 for equalizing the gas pressure between the said flotation cell units.

A slurry 1 comprising valuable minerals and produced in preceding process steps is introduced into the first flotation cell unit which in this embod- iment is the flushing flotation cell unit 1 1 . The slurry progresses though the following sealed flotation cell units 12, 15 and is finally removed as tailings 9 from the last sealed flotation cell unit 15. Froth 2 from the flushing flotation cell unit 1 1 is collected from the top of the cell and guided to a first concentrate sump 31 . Further froth 3, 6 from the sealed flotation cell units 12, 15 is collected from the top of each sealed flotation cell unit via a launder system 30 and guided to a second concentrate sump 32, which also receives froth from the first concentrate sump 31 . The headspaces of the first concentrate sump 31 and the second concentrate sump 32 are not allowed to balance. The froth concentrate from the first concentrate sump 31 is extracted from below the froth surface, in order to create an isolation between the sump 31 and sump 32 headspaces. The combined froth concentrate 8 can be treated in further flotation steps before it is subjected for further processing.

Pressurized flotation gas is supplied into each flotation cell unit 1 1 , 15 via a gas feed manifold 21 , 25. Each supply line is equipped with a individually adjustable control valve that regulates the distribution of the flotation gas to each of the flotation cell units. The supply of the flotation gas is con- trolled so that desired gas flow rate is reached in each of the flotation cell units as discussed above.

The flushing flotation cell unit 1 1 is operated atmospherically and the gases present in the headspace of the said cell, and optionally the head- space of the first concentrate sump 31 , are allowed to freely balance between the flushing flotation cell unit 1 1 and atmosphere via cell breathing conduit 28 and conduit 29, respectively. The expulsed flotation gas is forced to flow to a scrubber 60, either by a sucking action of a high pressure venturi scrubber or by a blowing action caused by an optional blower or compressor at the inlet of the scrubber. The expulsed gas is then preferably scrubbed in the scrubber 60 before releasing to the atmosphere though stack 70. The system typically comprises a conduit 61 , which allows to replace the volume of expulsed gas, which is forced to flow through the scrubber 60 towards the stack 70, with scrubbed gas from the outlet side of the scrubber 60, from the stack 70 and finally from ambient air.

The headspaces of the flushing flotation cell unit 1 1 and the immediately following first sealed flotation unit 12 are connected by a gas conduit 50 comprising a water lock 52. The gas conduit allows equalization of the gas pressure between the said flotation cell units while the water lock prevents in- terchange of the gases between said flotation cell units unless the pressure difference between the said cell units exceeds the required pressure for the gases to breach the water lock, thus permitting controlling the gas pressure and operation of the sealed cells in slight under- or overpressure. The pressure difference for breaching the water lock is determined by water lock depth. The depth is preferably 2 to 10 cm water column, resulting in 2 to 10 mbar pressure resistance. Thus water 51 is utilized to adjust the desired pressure.

Flotation gas from the sealed flotation cell units 12, 15, and optionally from the headspace of the second concentrate sump 32, is collected from the headspace of said sealed flotation cell units and sump and guided into the gas recirculation loop via a gas suction conduits 26 and 27, respectively. The gas recirculation loop comprises at least one recirculating compressor 40, which may be of any suitable type, such as conventional fan blower or, preferably, a liquid ring compressor. The gas recirculation loop may also comprise a bypass conduit 41 which allows further control of the volume of the flo- tation gas feed to the flotation cells. The gas recirculation loop also comprises means 20 for introducing process gas, typically nitrogen, into the recirculating loop. Process gas may be added either before or after the recirculating compressor 40. Introduction of process gas through means 20 causes an increase of total flotation gas vol- ume, which is then compensated by expulsion of flotation gas from the flushing cell unit(s), driven by the pressure differences caused. When liquid ring compressor is utilized as the recirculating compressor the means 20 for introducing process gas into the gas recirculation loop are preferably arranged after the recirculating compressor 40. Process gas is added to the mineral flotation pro- cess in order to control the amount of flotation gas in the system and/or the oxygen level in the flotation gas and/or the electrochemical potential of the slurry and/or to expulse flotation gas with undesired byproduct gas, e.g. H ₂ S, which might be forming in the flushing cell unit(s). Utilization of an inert gas as the flotation gas reduces the consumption of flotation chemicals, such as NaHS, in the mineral flotation.

Figure 2 illustrates as a second embodiment of the invention an arrangement in which the third flotation cell is utilized as the flushing flotation cell. In Figure 2, like components are designated by the same reference numerals as used in Figure 1 .

In embodiment presented in Figure 2, the sequence of the flotation cell units is arranged such the third flotation cell is utilized as the flushing flotation cell unit. Thus referring to Figure 2 and in accordance with the invention, a slurry 1 comprising valuable minerals and produced in preceding process steps is introduced into the first flotation cell unit which in this embodiment is a sealed flotation cell unit 12. The slurry progresses though the following sealed flotation cell unit 12, the flushing flotation cell until 1 1 and the remaining sealed flotation cell units 14, 15 and is finally removed as tailings 9 from the last sealed flotation cell unit 15.

The flushing flotation cell unit 1 1 now arranged in middle of the se- quence of the flotation cell units is operated atmospherically and the gases present in the headspace of the said cell, and optionally the headspace of the first concentrate sump 31 , are allowed to freely balance between the flushing flotation cell unit 1 1 and atmosphere via cell breathing conduit 28 and conduit 29, respectively, as discussed with reference to Figure 1 . The headspaces of the flushing flotation cell unit 1 1 and the immediately following third sealed flotation unit 14 are connected by a gas conduit 50 comprising a water lock 52. The headspaces of the first concentrate sump 31 and the second concentrate sump 32 are not allowed to balance. The froth concentrate from the first concentrate sump 31 is extracted from below the froth surface, in order to create an isolation between the said sump headspaces.

Figure 3 illustrates as a third embodiment of the invention an arrangement in which the last flotation cell is utilized as the flushing flotation cell. In Figure 2, like components are designated by the same reference numerals as used in Figure 1 and 2.

In embodiment presented in Figure 3, the sequence of the flotation cell units is arranged such the last flotation cell is utilized as the flushing flotation cell unit. Thus referring to Figure 3 and in accordance with the invention, a slurry 1 comprising valuable minerals and produced in preceding process steps is introduced into the first flotation cell unit which in this embodiment is a sealed flotation cell unit 12. The slurry progresses though the following sealed flotation cell units 12, 15, and the flushing flotation cell until 1 1 and is finally removed as tailings 9 from the flushing flotation cell unit 1 1 .

The flushing flotation cell unit 1 1 now arranged last in the sequence of the flotation cell units and is operated atmospherically. The gases present in the headspace of the said cell, and optionally the headspace of the first con- centrate sump 31 , are allowed to freely balance between the flushing flotation cell unit 1 1 and atmosphere via cell breathing conduit 28 and conduit 29, respectively, as discussed with reference to Figure 1 . The headspaces of the flushing flotation cell unit 1 1 and the immediately preceding fourth (i.e. last) sealed floatation unit 15 are connected by a gas conduit 50 comprising a water lock 52.

Similarly to embodiment of Figure 1 , in the embodiments of Figures 2 and 3, froth 2 from the flushing flotation cell unit 1 1 is collected from the top of the cell and guided to a first concentrate sump 31 and froth 3, 6 from the sealed flotation cell units 12, 15 is collected from the top of each sealed flota- tion cell unit via a launder system 30 and guided to a second concentrate sump 32, which also receives froth from the first concentrate sump 31 . Similarly to embodiment of Figures 1 and 2, the said sump headspaces are isolated from each other. Flotation gas from the sealed flotation cell units 12, 15, and optionally from the headspace of the second concentrate sump 32, is collected from the headspace of said sealed flotation cell units and sump and guided into the gas recirculation loop via a gas suction conduits 26 and 27, respectively. It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.