AU2020465091A1

AU2020465091A1 - A method and an arrangement for improving a mineral flotation process

Info

Publication number: AU2020465091A1
Application number: AU2020465091A
Authority: AU
Inventors: Kaj Jansson; Eija Saari
Original assignee: Metso Outotec Finland Oy
Current assignee: Metso Finland Oy
Priority date: 2020-08-28
Filing date: 2020-08-28
Publication date: 2023-04-06
Also published as: BR112023003695A2; CN114100862A; WO2022043601A1; CN216396702U; EP4204364A1; CA3192185A1

Abstract

The specification relates to a method for improving a mineral flotation process. The method comprises adding coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to an aqueous stream (100, 200a, 300a, 400b) of a minerals processing plant in order to facilitate formation of flocs including colloidal particles comprising electrochemically active metal(s), and in order to form a treated aqueous stream (101, 201, 301, 401), subjecting the treated aqueous stream (101, 201, 301, 401) to cleaning flotation in order to separate at least some of the colloidal particles comprising electrochemically active metal(s) and at least some of the microbes contained by the treated aqueous stream (101, 201, 301, 401) as a cleaning flotation overflow (103, 203, 303, 403) and in order to form a cleaning flotation underflow (104, 204, 304, 404), removing the cleaning flotation overflow (103, 203, 303, 403), and utilizing at least part of the cleaning flotation underflow (104, 204, 304, 404) as process water in minerals processing. The cleaning flotation comprises gas bubbles, at least 90 % of the gas bubbles having a diameter of from 0,2 to 250 µm.

Description

A method and an arrangement for improving a mineral flotation process

Technical field

This specification relates to a method and an arrangement for improving a mineral flotation process. Particularly, the specification relates to removing microbes and colloidal particles comprising electrochemically active metal(s) from an aqueous stream of a minerals processing plant and utilizing the water thus formed in minerals processing.

Background

Water from the tailings handling operations as well as other feed water streams, such as water from waste rock area or open pit, may have significant concentrations of dissolved substances, such as metals. As the different water streams get mixed and enter the flotation process having a high pH, solubility of various chemical compounds changes. This may cause various problems that may eventually result in loss of valuable metals as well as reducing the recovery and quality of the final products.

Summary

Aim of this specification is to provide a method and an arrangement for improving a mineral flotation process. Particularly, aim is to utilize water wherefrom at least some of microbes and colloidal particles comprising electrochemically active metal(s) are removed, thus improving the recovery and quality of the recovered product and consequently improving the overall plant performance.

According to an embodiment, a method for improving a mineral flotation process is provided. The method comprises adding coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to an aqueous stream of a minerals processing plant in order to facilitate formation of flocs including colloidal particles comprising electrochemically active metal(s), and in order to form a treated aqueous stream, subjecting the treated aqueous stream to cleaning flotation in order to separate at least some of the colloidal particles comprising electrochemically active metal(s) and at least some of the microbes contained by the treated aqueous stream as a cleaning flotation overflow and in order to form a cleaning flotation underflow, removing the cleaning flotation overflow, and utilizing at least part of the cleaning flotation underflow as process water in minerals processing. The cleaning flotation comprises gas bubbles, at least 90 % of the gas bubbles having a diameter of from 0,2 to 250 pm.

According to an embodiment, an arrangement for improving a mineral flotation process is provided. The arrangement comprises a mixing system arranged to provide an aqueous stream of a minerals processing plant with coagulant(s) and/or flocculant(s) and/or flotation chemical(s), a cleaning flotation unit arranged to separate at least some of colloidal particles comprising electrochemically active metal(s) and at least some of microbes from the aqueous stream as a cleaning flotation overflow and to form a residual process water as a cleaning flotation underflow, and means for directing at least part of the cleaning flotation underflow for use as process water in minerals processing.

Brief description of the drawings

Fig. 1 illustrates, by way of an example, a schematic process flow chart according to an embodiment,

Fig. 2 illustrates, by way of an example, a schematic process flow chart according to an embodiment,

Fig. 3 illustrates, by way of an example, a schematic process flow chart according to an embodiment, and

Fig. 4 illustrates, by way of an example, a schematic process flow chart according to an embodiment.

The figures are schematic. The figures are not in any particular scale.

Detailed description The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.

In this description and claims, the term “comprising” may be used as an open term, but it also comprises the closed term “consisting of’.

Following reference numbers are used in this specification:

100 aqueous stream

101 treated aqueous stream

102 cleaning flotation unit

103 cleaning flotation overflow

104 cleaning flotation underflow

105 dewatering equipment

200a aqueous stream

201 treated aqueous stream

202 cleaning flotation unit

203 cleaning flotation overflow

204 cleaning flotation underflow

205a tailings thickener

211 mineral flotation circuit

212 slurry

213 underflow of a mineral flotation circuit

214 overflow of a mineral flotation circuit

215 tailings thickener underflow

216 flotation arrangement

300a aqueous stream

301 treated aqueous stream

302 cleaning flotation unit

303 cleaning flotation overflow

304 cleaning flotation underflow

305a tailings thickener

311a first mineral flotation circuit

311 b second mineral flotation circuit

312 slurry

313a underflow of a first mineral flotation circuit

313b underflow of a second mineral flotation circuit 314a overflow of a first mineral flotation circuit

314b overflow of a second mineral flotation circuit

315 tailings thickener underflow

316 flotation arrangement

400b aqueous stream

401 treated aqueous stream

402 cleaning flotation unit

403 cleaning flotation overflow

404 cleaning flotation underflow

405b concentrate thickener

411 mineral flotation circuit

412 slurry

413 underflow of a mineral flotation circuit

414 overflow of a mineral flotation circuit

416 flotation arrangement

425 concentrate thickener underflow

In mining industry, beneficiation refers to a process that improves the economic value of the ore by removing gangue minerals, the process resulting in a higher grade product (concentrate) and a waste stream, i.e. tailings. Examples of beneficiation processes include e.g. froth flotation and gravity separation. Term “gangue” refers to commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit.

Beneficiation by froth flotation is typically used for the recovery and upgrading of sulfide ores. Froth flotation is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and gangue are increased through the use of surfactants and wetting agents. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement.

For froth flotation ground ore is mixed with water to form a slurry and the desired mineral is rendered hydrophobic by the addition of a surfactant or a collector chemical. The particular chemical depends on the nature of the mineral to be recovered, and most often on the natures of those that are not wanted. The slurry comprising hydrophobic particles and hydrophilic particles is introduced to tanks known as flotation cells that are aerated in order to produce bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed from the cell, producing a concentrate of the target mineral. Froth flotation is typically undertaken in several stages to maximize the recovery of the target mineral or minerals and the concentration of those minerals in the concentrate.

Typically, the gangue removed in the beneficiation process is sent to a tailings dam where the long resident time, typically 20-40 days, is expected to sediment and separate the solids as well as decompose residual flotation chemicals from the collected and reusable process water. The collected process water may then be recirculated back into the beneficiation process. The quality of the recirculated process water plays a role in obtaining target recoveries and qualities of the final product.

Today, water shortage, ecological demands placed by legislation and public pressure, costs and extensive space requirements of the conventional tailings methods for process water treatment increasingly put pressure to recirculate process waters as main processes in beneficiation become at least partially closed-loop systems in terms of water usage.

In closed-loop systems, metals may get enriched into the process water. In a case process water is taken from the tailings area, the pH of the water taken is low, as the pH of the tailings water is getting lower in time. Metals contained by the tailings get more easily dissolved into the water of low pH. For the flotation process, the pH of the slurry has to be raised, which causes the once dissolved metals to form colloids or even to precipitate. Metal(s)-containing colloids and precipitates, for one, cause problems for the flotation process. The metals contained by the water used may cause lowering of the quality of the mineral surface already in grinding.

Colloids formed for example as a result of the pH change may cause various problems that may eventually result in loss of valuable metals as well as reducing the recovery and quality of the final products. Problems may arise for example through adsorption of colloids to mineral surfaces, thus preventing collector adsorption on mineral surfaces. Adsorption of collectors into colloids prevents collectors from adsorbing on mineral surfaces, which is essential in order to promote flotation, thus causing decrease in flotation kinetics. Further, valuable metals may get trapped inside gel matrix, thus causing decrease in metal recovery. Further, gels and colloids may cause thickening problems (non-settleable colloids, their circulation back to the flotation process) or even concentrate filtration problems (higher cake moisture due to an aqueous gel).

Important concern is posed by the presence of colloidal particles comprising electrochemically active metals, such as Fe, Cu, Ni, Mn, Pb and Zn, as said metals may be responsible for redox-reactions taking place on the mineral surfaces. Oxidization of the mineral surfaces affects chemistry of the froth flotation, thus reducing the recovery yield. Frothing properties of the oxidized mineral surface differ from those of the unoxidized one.

Further, colloidal particle surfaces in a water system provide a good platform for microbial growth. Microbes, such as bacteria, archaea and fungi may get adsorbed onto the mineral surfaces. Microbes adsorbed onto the mineral surfaces may prevent the adsorption of collector chemicals, therefore negatively influencing the mineral flotation process and its outcome.

This specification aims to provide a method and an arrangement for improving a mineral flotation process. This may be achieved by improving water quality of a minerals processing plant and thus for example enhancing the yield of the froth flotation. Particularly, the aim of the method and the arrangement is to remove microbes and colloidal particles comprising electrochemically active metal(s) from an aqueous stream of a minerals processing plant and to utilize the water thus formed in minerals processing.

Within context of this specification, a colloidal suspension or a colloid is a mixture in which a substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance. In order to qualify as a colloid, the insoluble or soluble particles do not settle or would take a very long time to settle appreciably. Herein, the colloidal suspension comprises colloidal particles comprising electrochemically active metal(s) and may also be referred to as an electrochemically active metal(s)-containing colloid or an electrochemically active metal colloid. Within context of this specification, “electrochemically active metal” refers to a metal that is in a state capable of taking part into redox-reactions. Particularly, the electrochemically active metal refers to a metal that is capable of causing oxidization of mineral surface. The electrochemically active metal may be Fe, Cu, Ni, Mn, Pb or Zn. In an example, the electrochemically active metal is Cu, Ni, Mn, Pb or Zn. The electrochemically active metal may be for example in form of an oxide, oxyhydroxide or hydroxide.

Within context of this specification, “removal” or “removing” of microbes and colloidal particles comprising electrochemically active metal(s) may refer to a process of entirely eliminating said substances or to a process wherein the amount of said substances is reduced, i.e. the amount of microbes and colloidal particles comprising electrochemically active metal(s) in the aqueous stream to be treated is higher than the amount of said substances in the stream obtained after performing the method disclosed herein. Colloidal particle comprising electrochemically active metal(s) may contain one or more metal species within one particle.

In a method according to an embodiment and as illustrated in Fig. 1 , coagulant(s) and/or flocculant(s) and/or flotation chemical(s) is/are added to an aqueous stream 100 of a minerals processing plant. Thus, colloidal particles and suspended solids may be destabilized and combined into even larger aggregates that can be separated from the aqueous solution. A treated aqueous stream 101 is formed. The treated aqueous stream 101 comprises flocs including colloidal particles comprising electrochemically active metal(s). The colloidal particles comprising electrochemically active metal(s) may have a diameter of from 10 nm to 10 pm.

The treated aqueous stream 101 is subjected to cleaning flotation in a cleaning flotation unit 102. The cleaning flotation comprises gas bubbles, wherein at least 90 % of the gas bubbles have a diameter of from 0,2 to 250 pm. At least some of the colloidal particles comprising electrochemically active metal(s) and at least some of the microbes contained by the treated aqueous stream 101 are arranged to be separated as a cleaning flotation overflow 103. A cleaning flotation underflow 104 comprises residual process water. The cleaning flotation overflow 103 comprising at least some of the colloidal particles comprising electrochemically active metal(s) and microbes may be removed as tailings. The method comprises utilizing at least part of the cleaning flotation underflow 104 as process water in minerals processing. This means that at least part of the cleaning flotation underflow 104 is used for example in grinding and/or in mineral flotation.

Use of the treated process water wherefrom at least part of the microbes and colloidal particles comprising electrochemically active metal(s) have been removed, i.e. the cleaning flotation underflow 104, enables the circular economy approach in terms of water re-use without compromising the yield and quality of the recovered product.

The aqueous stream 100 wherefrom the microbes and colloidal particles comprising electrochemically active metal(s) are to be removed may comprise water from a dewatering equipment 105. The dewatering equipment 105 may comprise a sedimentation device or a filter. The sedimentation device may be for example a thickener or a clarifier. The aqueous stream 100 to be treated may comprise at least a part of a stream obtained from the dewatering equipment 105. Alternatively or additionally, the aqueous stream 100 may comprise mine drainage water or water collected from a tailings dam.

According to an embodiment and as illustrated in Fig. 2, the aqueous stream 200a obtained from the dewatering equipment, herein a tailings thickener 205a, originates from a flotation arrangement 216 comprising a mineral flotation circuit 211 arranged to treat ore particles suspended in a slurry 212 by flotation for recovery of ore.

The mineral flotation circuit 211 is arranged to separate the slurry 212 into an underflow of the mineral flotation circuit 213 and an overflow of the mineral flotation circuit 214. The overflow of the mineral flotation circuit 214 comprises recovered material.

The flotation arrangement 216 may be arranged to recover at least one of: Fe, Cu, Ni, Mn, Pb, Zn. According to an embodiment illustrated in Fig. 3, the flotation arrangement 316 may comprise a first mineral flotation circuit 311 a arranged to recover first metal, such as Cu, and a second mineral flotation circuit 311 b arranged to recover second metal, such as Ni. The first and the second mineral flotation circuits may have common or separate water circuits. According to an embodiment, the dewatering equipment comprises a sedimentation device, which is a thickener. The thickener is configured to operate as a solid-liquid separator in order to separate a sediment, i.e. the thickener underflow, from a supernatant, i.e. the thickener overflow. The thickener underflow comprises particles having a density higher than the one of the liquid, and thus ending up in the sediment.

The thickener may be a so-called concentrate thickener 405b, as illustrated in Fig. 4. The overflow of the mineral flotation circuit 414 may be led into the concentrate thickener 405b. In the concentrate thickener 405b water absorbed by the particles and increasing the density of the recovered ore may be removed in order to enable the concentrate to be transported easily and to allow further processing thereof. In the concentrate thickener 405b the overflow of the mineral flotation circuit 414 is dewatered in order to produce a concentrate thickener overflow 400b and a concentrate thickener underflow 425. The concentrate thickener underflow 425 comprises the recovered ore, i.e. the concentrate, and is taken from the concentrate thickener 405b to further processing. The concentrate thickener overflow 400b may be treated and thereafter utilized according to a method disclosed herein.

Alternatively, the thickener may be a so-called tailings thickener 205a, 305a, as illustrated in Figs. 2 and 3. The underflow of the mineral flotation circuit 213, 313b may be led into the tailings thickener 205a, 305a. In the tailings thickener 205a, 305a the underflow of the mineral flotation circuit 213, 313b is dewatered in order to produce a tailings thickener overflow 200a, 300a and a tailings thickener underflow 215, 315. The tailings thickener overflow 200a, 300a may be treated and utilized according to a method disclosed herein. The tailings thickener underflow 215, 315 is removed from the tailings thickener 205a, 305a. The tailings thickener underflow 215, 315 is typically removed from the thickener as tailings. The solids content of the tailings thickener underflow 215, 315 may be at least 80 wt.%.

According to an embodiment, the aqueous stream 100, 200a, 300a, 400b wherefrom the microbes and the colloidal particles comprising electrochemically active metal(s) are to be removed comprises thickener overflow. The thickener overflow may originate from a concentrate thickener 405b and/or a tailings thickener 205a, 305a. The thickener overflow comprises process water, microbes and colloidal particles comprising electrochemically active metal(s). The thickener overflow may further comprise other undesired, detrimental or unrecovered material or compounds such as fine particles and larger particles, starch-based depressants etc., suspended and/or dissolved in process water.

Prior to subjecting the thickener overflow to the cleaning flotation, it may be led to a thickener overflow tank in order to stabilize the thickener overflow. The coagulant(s) and/or flocculant(s) and/or flotation chemical(s) may be added to the aqueous stream 100, 200a, 300a, 400b in any suitable manner, as long as proper mixing of the coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to the aqueous stream 100, 200a, 300a, 400b is ensured. For example, the coagulant(s) and/or flocculant(s) and/or flotation chemical(s) may be added in a mixing unit.

According to an embodiment, the cleaning flotation is dissolved air flotation (DAF). DAF is a flotation process which is used in various applications in water or effluent clarification. Solid particles are separated from liquid by using small flotation gas bubbles, which may be called microbubbles. The microbubbles may be for example generated by dissolving air or other flotation gas into the liquid under pressure. The bubbles are formed in a pressure drop when dispersion is released. The particles of solid form attach to the bubbles and rise to the surface. A formed, floating sludge may be removed from the liquid surface with sludge rollers as DAF overflow.

According to an embodiment, the cleaning flotation utilizes laminar flow. Laminar flow may be beneficial for maintaining the flocs including colloidal particles comprising electrochemically active metal(s). In some cases a coarse flotation may be responsible for too turbulent flow, thus causing fragile flocs to break.

According to an embodiment, the cleaning flotation underflow 104, 204, 304, 404 or at least part of it is recirculated back into the flotation process or into a process preceding flotation. The cleaning flotation underflow 104, 204, 304, 404 or at least part of it may be recirculated for example via grinding into the flotation. Figs 2, 3 and 4 illustrate an arrangement wherein the cleaning flotation underflow 204, 304, 404 or at least part of it is recirculated back into the flotation arrangement 216, 316, 416 for use in the mineral flotation.

The coagulant may be chosen from a group comprising: inorganic coagulants, aluminium salts, iron salts, organic coagulants. Preferably the coagulant is an aluminium salt or an iron salt. The coagulant is arranged to produce coagulation. Coagulation refers to a process through which colloidal particles and suspended solids are destabilized to form “microflocs” that can begin to agglomerate if the conditions are appropriate. Coagulation is a chemical process that involves neutralization of charge. Coagulation is effected by the type of coagulant used, its dose and mass; pH and initial turbidity of the water being treated; as well as properties of the unwanted matter present.

In a colloidal suspension, particles settle very slowly or not at all because the colloidal particles carry surface electrical charges that mutually repel each other. This surface charge may be evaluated in terms of zeta potential. In order to induce coagulation, a coagulant with opposite charge is added to the water to overcome the repulsive charge and to destabilize the suspension. Once the repulsive charges have been neutralized, van der Waals forces will cause the particles to agglomerate and form floc(s).

The flocculant may be a synthetic polymer or a natural polymer or their derivative. Flocculants are agents that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculation refers to a process wherein destabilized particles are actually combined into even larger aggregates known as flocs so that they can be separated from water by sedimentation or flotation. Many flocculants comprise multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged molecules may interact with negatively charged particles and molecules in order to reduce the barriers to aggregation. Some flocculants may react with water and form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into a larger floc. Natural polymer or its derivative may include for example starch or modified starch, or polysaccharides. Examples of synthetic polymers include for example high molecular weight (over 500 000) flocculants such as polyacrylamides (negatively or positively charged, or neutral), or Mannich products (positively charged); and low molecular weight (under 500 000) flocculants such as polyamines (positively charged), polyepiamine (positively charged), polyDADMAC (positively charged), poly(ethylene) imines (positively charged), or polyethylene oxide (neutral).

The flotation chemical(s) that may be added to the aqueous stream in order to facilitate formation of flocs including colloidal particles comprising electrochemically active metal(s), and in order to form a treated aqueous stream may comprise at least one of a group comprising: collectors, activators, depressants, frothers, modifiers. Collectors may comprise surface-active organic reagents such as thiol compounds, alkyl carboxylates, alkyl sulfates, alkyl sulfonates, alkyl phosphates, amines, chelating agents, and alkyl phosphonic acids. Activators may comprise for example metal hydroxo compounds or sodium sulfide. Depressants may comprise for example sodium sulfide or cyanide salts. Frothers may comprise for example alcohols, polyethers, ethylene oxide, and polyglycol ethers.

There may not be a need to adjust the pH of the (treated) aqueous stream prior to subjecting it to the cleaning flotation. Water obtained from the beneficiation, i.e. originating from the concentration thickener or from the tailings thickener, may have a pH of from 8 to 11 .5, which may be high enough in order to ensure that the electrochemically active metals are not in dissolved state and thus may be removed as being part of the colloidal particles. Water originating from the tailings area may have somewhat lower pH, for example between 6 and 7. In a case tailings area is the sole source of the water to be treated and thereafter utilized in the minerals processing, there may be a need to adjust the pH of the water to be above neutral, preferably at least 9-10.

The method described herein has the effect that the quality of the re-circulated water of the minerals processing plant is improved thus having positive impacts on mineral flotation. Particularly, the mineral flotation process is improved when compared to a one utilizing re-circulated water not being treated before re-use. Further, presence of microbial communities is decreased and thereby microbial growth in the water systems and on mineral surfaces may be prevented.

As a result of the method described, turbidity of the aqueous stream may be diminished. Turbidity refers to cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye. Turbidity is caused by the suspended solid matter of very small particles that settle only very slowly or not at all, or by the colloidal particles.

An arrangement for implementing the above described method comprises

- a mixing system arranged to provide an aqueous stream of a minerals processing plant with coagulant(s) and/or flocculant(s) and/or flotation chemical(s),

- a cleaning flotation unit arranged to separate at least some of colloidal particles comprising electrochemically active metal(s) and at least some of microbes from the aqueous stream as a cleaning flotation overflow and to form a residual process water as a cleaning flotation underflow, and

- means for directing at least part of the cleaning flotation underflow for use as process water in minerals processing.

The arrangement may further comprise a dewatering equipment arranged upstream of the mixing system. The dewatering equipment may be a source of the aqueous stream to be treated and utilized by the method. The dewatering equipment may comprise a sedimentation device or a filter. The sedimentation device may be a thickener.

Means for directing at least part of the cleaning flotation underflow for use as process water in minerals processing may be configured to recirculate at least part of the cleaning flotation underflow from the cleaning flotation unit back into the flotation arrangement or into a process preceding flotation, such as grinding. Means for directing at least part of the cleaning flotation underflow comprises any means necessary for transferring liquid from one location to another. For example, means for directing at least part of the cleaning flotation underflow may comprise any of the following: line, conduit, pipe, pump, valve, processor.

The arrangement may further comprise a flotation arrangement. The flotation arrangement comprises a mineral flotation circuit arranged to treat ore particles suspended in a slurry by flotation for recovery of ore. The flotation arrangement may be arranged to recover at least one of: Fe, Cu, Ni, Mn, Pb, Zn. The cleaning flotation unit may be a dissolved air flotation unit.

In an example, the aqueous stream to be treated was untreated process water originating from e.g. tailings area and/or (tailings) thickener, possibly further containing raw water. The untreated process water did not have any precipitated solids in the beginning. Turbidity of the water was not very high (8 FNU). With coagulant and flocculant addition, nice-floating fluffy flocs were formed. After cleaning flotation treatment the turbidity was diminished by 50%. It was shown that 100% of the Cu and Ni colloids, as well as 63% of the Fe colloids were removed by the cleaning flotation. Thus obtained treated process water was utilized for minerals processing resulting in greater Cu recovery in copper flotation (A+1.4%) and greater Ni recovery in nickel flotation (A+3%) when compared to utilizing conventional untreated process water. Cu concentrate grade was shown to be lower (A-2.2%) due to increased Mg and Fe, whereas Ni concentrate grade was shown to remain unchanged. Both Cu and Ni concentrates showed low Mg recoveries, i.e. well below a critical value set for the concentrate in question.

In another example, the aqueous stream to be treated was Cu thickener overflow water. Turbidity of the water was moderate (9 FNU). With coagulant, flocculant and collector (Aerophine) addition, easily floatable flocs were formed. Some residual turbidity could still be observable after cleaning flotation. Obtained treated process water was utilized for mineral flotation resulting in greater Cu recovery in copper flotation (A+1 %) and greater Ni recovery in nickel flotation (A+6.6%) when compared to conventional untreated process water. Cu concentrate grade was shown to be higher (A+0.3%) due to decreased amount of pyrite in the copper concentrate. Ni concentrate grade was shown to be unchanged. Again, both Cu and Ni concentrates showed low Mg recoveries. For said example it was also shown that 63% reduction in bacteria counts, 64% reduction in archaea counts and 96% reduction in fungi counts was obtained by cleaning flotation.

Claims

Claims:

1. A method for improving a mineral flotation process, wherein the method comprises

- adding coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to an aqueous stream (100, 200a, 300a, 400b) of a minerals processing plant in order to facilitate formation of flocs including colloidal particles comprising electrochemically active metal(s), and in order to form a treated aqueous stream (101 , 201 , 301 , 401 ),

- subjecting the treated aqueous stream (101 , 201 , 301 , 401 ) to cleaning flotation in order to separate at least some of the colloidal particles comprising electrochemically active metal(s) and at least some of the microbes contained by the treated aqueous stream (101 , 201 , 301 , 401 ) as a cleaning flotation overflow (103, 203, 303, 403) and in order to form a cleaning flotation underflow (104, 204, 304, 404),

- removing the cleaning flotation overflow (103, 203, 303, 403), and

- utilizing at least part of the cleaning flotation underflow (104, 204, 304, 404) as process water in minerals processing; wherein the cleaning flotation comprises gas bubbles, at least 90 % of the gas bubbles having a diameter of from 0,2 to 250 pm.

2. The method according to claim 1 , wherein the aqueous stream (100, 200a, 300a, 400b) comprises water obtained from a dewatering equipment (105).

3. The method according to claim 2, wherein the dewatering equipment (105) comprises a sedimentation device or a filter.

4. The method according to claim 3, wherein the sedimentation device is a thickener (205a, 305a, 405b).

5. The method according to any of the claims 2-4, wherein the aqueous stream (100, 200a, 300a, 400b) obtained from the dewatering equipment (105) originates from a flotation arrangement (216, 316, 416) comprising a mineral flotation circuit (211 , 311a, 311 b, 411 ) arranged to treat ore particles suspended in a slurry (212, 312, 412) by flotation for recovery of ore.

6. The method according to claim 5, wherein the flotation arrangement (216, 316, 416) is arranged to recover at least one of: Fe, Cu, Ni, Mn, Pb, Zn.

7. The method according to claim 5 or 6, wherein the flotation arrangement (216, 316, 416) comprises a first mineral flotation circuit (311 a) arranged to recover first metal and a second mineral flotation circuit (311 b) arranged to recover second metal.

8. The method according to any of the claims 5-7, wherein the method comprises recirculating at least part of the cleaning flotation underflow (104, 204, 304, 404) back into the flotation arrangement (216, 316, 416) or into a process preceding flotation.

9. The method according to any of the preceding claims, wherein the cleaning flotation is a dissolved air flotation.

10. An arrangement for improving a mineral flotation process, comprising

- a mixing system arranged to provide an aqueous stream (100, 200a, 300a, 400b) of a minerals processing plant with coagulant(s) and/or flocculant(s) and/or flotation chemical(s),

- a cleaning flotation unit (102, 202, 302, 402) arranged to separate at least some of colloidal particles comprising electrochemically active metal(s) and at least some of microbes from the aqueous stream (100, 200a, 300a, 400b) as a cleaning flotation overflow (103, 203, 303, 403) and to form a residual process water as a cleaning flotation underflow (104, 204, 304, 404), and

- means for directing at least part of the cleaning flotation underflow (104, 204, 304, 404) for use as process water in minerals processing.

11. The arrangement according to claim 10, further comprising a dewatering equipment (105) arranged upstream of the mixing system.

12. The arrangement according to claim 11 , wherein the dewatering equipment (105) comprises a sedimentation device or a filter.

13. The arrangement according to claim 12, wherein the sedimentation device is a thickener (205a, 305a, 405b). 17

14. The arrangement according to any of the claims 10-13, further comprising - a flotation arrangement (216, 316, 416) comprising a mineral flotation circuit (21 1 , 311 a, 311 b, 411 ) arranged to treat ore particles suspended in a slurry (212, 312, 412) by flotation for recovery of ore.

15. The arrangement according to claim 14, wherein the flotation arrangement (216, 316, 416) is arranged to recover at least one of: Fe, Cu, Ni, Mn, Pb, Zn.

16. The arrangement according to any of the claims 10-15, wherein the cleaning flotation unit (102, 202, 302, 402) is a dissolved air flotation unit.