FIELD OF THE INVENTION

The invention relates to a method for gas handling in a mineral flotation circuit that is maintained under an essentially closed recirculating gas atmosphere. The invention also relates to a system for gas handling in such a mineral flotation circuit.

BACKGROUND OF THE INVENTION

When separating molybdenum from copper containing sulfide minerals, a collective concentrate is first produced in a bulk flotation stage. In a subse- quent flotation stage, molybdenum is separated from copper by adding chemicals that further alter the copper-bearing mineral surface. For example, sodium hy- drosulfide is a powerful depressant for copper and iron sulfide minerals that have been floated with xan- thate in the previous bulk flotation step. Addition of chemicals is adjusted based on the pH and redox poten ^¬ tial measured from the slurry. High content of oxygen in a flotation cell is known to increase the consumption of depressant and increase operating costs.

US 7789332 discloses a method for separating valuable minerals from an ore, wherein grinding, flo ^¬ tation, precipitation and filtering are carried out under an essentially closed recirculating gas atmos ^¬ phere. The composition of the recirculating gas atmos- phere is controlled depending on the measured electro ^¬ chemical potential of the slurry or the content of ox ^¬ ygen measured in the recirculating gas atmosphere. The method as described is primarily a laboratory method which has not been done at full scale in practice. Grinding, flotation, precipitation and filtering should all be performed in a fully sealed, controlled recirculating gas atmosphere. The economics of combin ^¬ ing all of these mineral processing steps under a sealed controlled atmosphere is questionable at full scale, as it would be very difficult, if not impossi- ble, to actually achieve such a mineral processing plant configuration in practice and make it operate economically. It is doubtful whether the system can return the inert gas escaping via the froth launders back to the flotation cells. Make up for the gas es- caping along with the froth concentrate may be up to 10% of the total amount of flotation gas. In US 7789332 it is stated that the recirculating gas atmos ^¬ phere may contain an oxidizing gas, presumably air. In that case the oxygen content in the recirculation gas atmosphere is assumed to be 12% or more.

Replacing air with a non-oxidizing inert gas in mineral separation has been proposed, for instance, in US 6032805, US 6036025, US 6041941 and US 6044978. US 6032805 teaches adding a non-oxidizing gas to the mineral separation circuit prior to and/or simultane ^¬ ously with the addition of sulfoxy radical containing reagent in a quantity sufficient to achieve a chemical environment conducive to flotation separation of min ^¬ erals. US 6036025 teaches conditioning a milled slurry or a slurry of flotation concentrate with an inert/non-oxidizing gas and/or reducing/deoxifying agent to achieve a controlled dissolved oxygen content or electrochemical reduction potential conducive to the separation of the valuable sulfidic mineral from the non-sulfidic gangue material. It is preferable to achieve a controlled dissolved oxygen content of less than 1 ppm in the slurry or an electrochemical potential of between zero and minus 700 mV which is condu ^¬ cive to the flotation of sulfide minerals in prefer- ence to gangue minerals. It is doubtful whether this process as described is actually economic in a full scale mineral processing plant as the nitrogen gas would require a separate manufacturing plant to be constructed alongside the concentrator to produce the nitrogen gas for the process. US 6041941 discloses a method for reducing both reagent consumption and scale formation in a mineral separation circuit employing sulfoxy compounds as reagents. The method comprises introducing into the mineral separation circuit a non- oxidizing gas in a quantity sufficient to reduce the degree of oxidation of the sulfoxy radical. US 6044978 discloses a process where slurry is conditioned with a non-oxidizing gas and the final flotation treatment is carried out with a gas having a higher oxygen content than the non-oxidizing gas.

The inventors have recognized the need for an improved method for maintaining a closed recirculating gas atmosphere in a mineral flotation process.

The object of the present invention is to substantially ameliorate or overcome some or all the problems of the prior art.

SUMMARY OF THE INVENTION

The method and the system according to the present invention are characterized by what is pre ^¬ sented in independent claims 1 and 12, respectively.

In the method according to the present inven ^¬ tion, pressurized process gas is fed as a flotation gas into one of more sealed flotation cells which op ^¬ erate under a small overpressure at the top of the cell. Process gas is collected from the tops of the cells and circulated to a blower that pressurizes the process gas and feeds it back to the flotation cells. The pressure in the suction side of the blower is bal ^¬ anced with the help of a level controlled gas buffer tank. Process gas is bled from the gas recirculation loop when necessary. The oxygen content of the process gas is measured and nitrogen gas is added to the gas recirculation loop in case the oxygen content of the process gas rises above a predetermined upper limit.

In one embodiment of the present invention, process gas is collected from the tops of the cells via a closed launder system which has been provided for froth collection. The process gas can be directed to the blower from a launder channel or from a pump sump that follows the launder channel.

The upper limit of the oxygen content of the process gas can be 10%, more preferably 5%, most pref ^¬ erably 1%.

In one embodiment of the present invention, the pressure level in the pressure side of the blower can be adjusted by directing a part of the pressurized process gas via a bypass line to the suction side of the blower. The bypass line is provided with a control valve .

The tops of the flotation cells are prefera ^¬ bly maintained under an overpressure of 2-10 mbar, more preferably 5-8 mbar.

The gas pressure in the pressure side of the blower can be maintained in the range of 0.2-1.0 bar, more preferably 0.3-0.8 bar, most preferably 0.4-0.6 bar .

In one embodiment of the present invention, the process gas contains over 85%, preferably over 90% nitrogen .

The gas recirculation loop can be provided with a scrubber for cleaning the process gas before it enters the blower. This internal scrubber can be used to remove solid particles and/or ¾S from the recircu ^¬ lating process gas.

The bleeding line can also be provided a scrubber for cleaning the cleaning the bleed gas be- fore it is released to the atmosphere.

In one embodiment of the present invention, the process comprises the steps of utilizing in the mineral flotation circuit a reducing agent selected from a group comprising sodium sulfide, sodium hy- drosulfide and hydrogen sulfide; measuring the pH and redox potential of the mineral slurry; and controlling the addition of the reducing agent based on the meas ^¬ ured values of pH and/or redox potential.

The process can further comprise the steps of measuring the H ₂ S content of the process gas and ad ^¬ justing the pH of the slurry when the ¾S content of the process gas rises above a predetermined upper lim ^¬ it .

Alternatively, the process can comprise the steps of measuring the H ₂ S content of the process gas and bleeding a part of the circulating process gas when the H ₂ S content of the process gas rises above a predetermined upper limit.

In both cases the predetermined upper limit of the H ₂ S content can be 1%.

The system for gas handling in a mineral flo- tation circuit comprises: at least one blower for feeding pressurized process gas into one or more sealed flotation cells which operate under a small overpressure; a closed launder system for collecting process gas and froth concentrate from the tops of the flotation cells; a pipework for recirculating collected process gas back to the blower; a level controlled gas buffer tank for balancing the pressure in the suction side of the blower; means for bleeding process gas from gas recirculation loop when the level in the gas buffer tank rises above a predetermined upper lev ^¬ el: means for measuring the oxygen content of the pro ^¬ cess gas; and means for adding ₂ gas to the gas re ^¬ circulation loop when the oxygen content of the pro ^¬ cess gas rises above a predetermined upper limit. The present invention provides a method for gas han ^¬ dling in a mineral flotation circuit maintained under an essentially closed recirculating gas atmosphere. The method comprises feeding pressurized process gas as a flotation gas into one or more sealed flotation cells which operate under a small overpressure, col ^¬ lecting flotation gas from the upper parts of the flotation cells and passing the collected gas to a blow ^¬ er, pressurizing the process gas and feeding it back to the one or more flotation cells, balancing the pressure in the suction side of the blower with the help of a gas buffer tank, adding nitrogen to the gas recirculation loop in case the pressure in the suction side of the blower decreases below a lower limit, and bleeding process gas from the gas recirculation loop in case the pressure in the suction side of the blower rises above an upper limit.

The method of the present invention is par ^¬ ticularly suitable for the separation of molybdenite from copper sulfide minerals and pyrite in selective flotation. The invention could also be used in separa ^¬ tion of minerals where oxidation of the regulating re ^¬ agent, such as a ₂ S or NaHS, or oxidation of the min ^¬ eral surface should be controlled. Such processes com- prise, for instance, nickel or free gold flotation, particularly in the grinding circuit where the mineral surfaces are fresh and can be quickly oxidized on con ^¬ tact with air. BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing shows a flow chart illustration of one embodiment of a process according to the present invention. DETAILED DESCRIPTION OF THE INVENTION In the process according to the present in ^¬ vention, a mineral flotation step is carried out in a closed recirculating gas atmosphere consisting of non- oxidizing inert gas. That is to say, the flotation gas supplied into the flotation cells in order to cause mineral separation is recirculated in an essentially closed gas recirculation loop which has been created around the flotation equipment. The use of a closed loop of oxygen-free or low-oxygen content process gas helps in avoiding excessive oxidation of minerals or flotation reagents in connection with flotation.

Providing a closed gas recirculation loop requires that the process equipment is provided with the necessary structures, such as lids, seals and blowers, in order to ensure the recovery of gas and maintenance of a small overpressure in the system. In addition, the process equipment comprises pipelines needed for gas recirculation as well as pressure balancing means.

A molybdenum flotation circuit comprising a gas handling system according to the present invention is shown in the accompanied drawing. The molybdenum flotation circuit comprises a distribution box 9 and a bank of flotation cells la, lb, lc, Id, le. Collective concentrate produced in preceding process steps is fed via the distribution box 9 to the first flotation cell la of the cell bank. Copper concentrate is removed from the last flotation cell le of the cell bank. Mo ^¬ lybdenum rich froth concentrate is collected from each flotation cell la,...,le via a launder system 2 and guided to a pump sump 3. The Mo concentrate can be pumped to a further cleaning flotation stage which is also connected to the same gas handling system. Chemi ^¬ cals can be added to the slurry at any stage of the flotation process. Reduction-oxidation potential and pH of the slurry can be measured at any stage of the process. NaHS is preferably used as a depressant, which induces separation of copper and iron sulfides from molybdenite. Instead of NaHS, the reducing agent added can be a ₂ S or H ₂ S. The pH of the slurry can be adjusted by addition of alkali, acid or another pH regulator, for instance CO ₂ .

Pressurized process gas is supplied into each flotation cell la,...,le via a flotation gas manifold 8. Each flotation cell la,...,le is provided with a sealed lid. The inlet pipes to the flotation cells la,...,le are provided with individually adjustable con- trol valves 18. The flotation gas supply is controlled so that a small overpressure can be maintained in the top of each flotation cell la,...,le. Preferably, the overpressure in the top of the cell is in the range of 2-10 mbar, more preferably 5-8 mbar .

The flotation cell bank is provided with a closed launder system 2 for removing froth concentrate from the tops of the cells. The froth removed from the flotation cells la,...,le is collected in a pump sump 3, where gas is separated from the concentrate. The mo- lybdenum concentrate is pumped to further refining steps, which may comprise one or more new flotation steps which are also connected to the gas handling system. The process gas is conveyed from the pump sump 3, or from the closed launder channel of the launder system 2, via a pipe system 4a, 4b, 4c to a blower 7, which pressurizes the process gas before it is fed via the flotation gas manifold 8 back to the flotation cells la,...,le. Also gas collected from the distribu ^¬ tion box 9 can be guided to the gas recirculation loop via a pipeline 17.

The gas recirculation loop also comprises a level controlled gas buffer tank 6, the purpose of which is to dampen fast disturbances in the gas pres ^¬ sure in the suction side of the blower 7. The gas buffer tank 6 is provided with a membrane for handling gas volume changes, permitting a stable internal pres ^¬ sure for the system. The gas recirculation loop comprises one or more blowers 7, which may be of any suitable type, such as conventional fan blowers, or liquid ring com ^¬ pressors. The gas recirculation loop also comprises a bypass line 14 for the flotation cells la,...,le. The pressure in the flotation gas manifold 8 feeding the flotation cells la,...,le can be controlled on one hand by the speed of rotation of the blower 7 and on the other hand by the amount of process gas directed to the bypass line 14. The gas pressure in the pressure side of the blower 7 can be for instance 0.2-1.0 bar, more preferably 0.3-0.8 bar, most preferably 0.4-0.6 bar. The gas pressure in each flotation cell la,...,le is adjusted by the control valves 18.

The gas recirculation loop can also comprise an optional gas scrubber 5 for washing the process gas before it is guided to the blower 7. The main purpose of the scrubber 5 is to remove solid particles from the process gas, but it can also be used for removing ¾S from the process gas. The scrubber 5 can also be applied for cooling of the circulation gas. The scrub ^¬ ber 5 can be, for instance, an ejector venturi scrubber .

The gas recirculation loop also comprises an oxygen content measuring device 19 and a ¾S content measuring device 20 arranged in the gas pipe 4c be ^¬ tween the scrubber 5 and the blower 7.

A bleeding line 15 is connected to the pres ^¬ sure side of the blower 7. The bleed gas is conveyed to one or more gas scrubbers 10, where H ₂ S is removed from the bleed gas. After scrubbing the bleed gas is conveyed via a release gas line 16 into a stack 13 where it released into the atmosphere. The release gas line 16 may be provided with a blower 12.

The gas recirculation loop is provided with a possibility to introduce 2 gas into the recirculating process gas. The ₂ gas can be added to the process gas either before or after the blower 7.

The main component of the circulating process gas is nitrogen, in addition to which the process gas can also contain small amounts of water vapor, carbon dioxide, hydrogen sulfide, and other inert gases. The amount of oxygen present in the recirculating gas is very low. According to the present invention, the oxygen content of the process gas can be less than 10%, more preferably less than 5%, most preferably less than 1%.

The oxygen content of the process gas is con ^¬ tinuously monitored by the measuring device 19. When the oxygen content of the process gas exceeds a prede- termined upper limit, nitrogen gas is added to the gas recirculation loop. The predetermined upper limit can be, for instance, 10% oxygen, more preferably 5% oxy ^¬ gen, most preferably 1% oxygen. N ₂ gas is added until the measured O ₂ content has dropped below the prede- termined upper limit. When nitrogen gas is introduced into the closed gas system, a level change is detected in the level controlled gas buffer tank 6. This induc ^¬ es the opening of a bleed valve 21 in the bleed line 15. Process gas is bled from the gas recirculation loop until the level controlled gas buffer tank 6 is in balance again. Addition of N ₂ gas and bleeding of process gas can be carried out simultaneously or at different times.

The hydrogen sulfide content of the process gas is continuously monitored by the H ₂ S measuring de ^¬ vice 20. The highest allowable ¾S content is prefera ^¬ bly 1%. When the ¾S content of the process gas ex ^¬ ceeds this predetermined upper limit, attempts are made to decrease the H ₂ S content. This can be done, for instance, by increasing the pH of the slurry in the flotation cells, or by bleeding process gas. In the latter case, ¾S must be removed from the bleed gas in an external gas scrubber 10 before the gas is released into the atmosphere.

Nitrogen gas is added to the process in order to control the oxygen level in the process gas and/or the electrochemical potential of the slurry. Using an inert recirculating process gas as a flotation gas re ^¬ duces the consumption of depressants, such as NaHS, in the mineral flotation.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.