Technical field

This specification relates to a method and an arrangement for removing Si from aqueous stream(s) of a minerals processing plant. Particularly, the specification relates to a method and an arrangement for removing soluble and/or colloidal Si-compounds from an aqueous stream of a minerals processing plant.

Background

During different process phases of minerals processing pH value of the slurry tends to drop due to oxidation of sulfide minerals and absence of carbonate minerals effectively buffering the pH. Said sulfur oxidation and concomitant pH decrease also continues during the tailings processing, leading to elevated concentrations of dissolved ions, such as SO ₄ , Mg, Fe, Ni, Ca, K, Na, in the liquid phase. In addition, Si and Al are released to the liquid phase due to dissolution of (alumino)silicate minerals at low pH conditions.

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 Si and metals. As the different water streams get mixed and enter the flotation process having a high pH, solubility of various chemical compounds changes. Consequently, depending on the incoming concentration of dissolved Si as well as on the pH profile of the process, the dissolved silicates may start polymerizing and forming gels and colloids either through polymerization or by reacting with different metal ions or solid surfaces.

Gels and colloids formed by the silicates 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.

Consequently, there is a need to find a solution for removal of Si-compounds from aqueous streams of a minerals processing plant. In particular, there is a need for a solution for removal of soluble and/or colloidal Si-compounds from aqueous streams of a minerals processing plant, and thus preventing the formation of gels and colloids containing silicates, as well as the problems caused by them.

Summary

Aim of this specification is to provide a method and an arrangement for removing soluble and/or colloidal Si-compounds from an aqueous stream of a minerals processing plant, thereby improving the recovery and quality of the recovered product, and consequently the overall plant performance.

According to an embodiment, a method for removing soluble and/or colloidal Si-compounds from an aqueous stream of a minerals processing plant. The method comprises adding coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to the aqueous stream in order to facilitate formation of floes comprising at least some of the Si-compounds, 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 Si-compounds as a cleaning flotation overflow, and removing the cleaning flotation overflow. 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 removing soluble and/or colloidal Si-compounds from an aqueous stream of a minerals processing plant is provided. The arrangement comprises a mixing system arranged to provide the aqueous stream with coagulant(s) and/or flocculant(s) and/or flotation chemical(s), and a cleaning flotation unit arranged to separate at least some of the Si-compounds from the aqueous stream as a cleaning flotation overflow and to form a residual process water as a cleaning flotation underflow.

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.

Amount of gangue in ore is increasing. Of gangue minerals, amount of Si- containing minerals is increasing. Majority of these Si-containing minerals can be found as Feldspar (KAISbOs - NaAISbOs - CaA^S Os) and Quartz (S1O4 / S1O ₂ ) that are the two most abundant minerals in Earth’s continental crust. The backbone of these minerals comprises mainly a continuous framework of silicon-oxygen tetrahedra with commonly Fe, Al, Mn or Mg as impurities in the structure. The otherwise very rigid structure may be vulnerable to external conditions such as pH changes because of the impurities. Consequently, the structure may split to smaller fractions. When this takes place in the process, the silicates may become solubilized or colloidal. If pH increases, the Si- containing particle may react with different compounds thus forming new structures. As the silicate rebuilds its structure, formation of for example ribbon-like structures may result. These structures may cause increase in viscosity, thus causing problems in beneficiation/concentration.

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, the process water may become at times supersaturated with respect to silicates. It has been shown that major part of the total Si is present as colloids, instead of soluble compounds.

As mentioned above, gels and colloids formed by the silicates 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). Silicates are known to form hydrophilic layers on valuable mineral surfaces, thus making them less floatable.

Further, Si in aqueous solutions is prone to formation of complexes with Fe. The complexes formed may be fine colloidal particles. The fine colloidal particles containing Si and Fe cause excess fines load, which may have a negative influence on froth flotation. Si and Fe together may form coatings/layers onto the ore surface, thus altering the flotation properties of the ore and eventually causing decrease in the ore recovery. The complexes containing Si and Fe may cause redox-properties of the ore to change, thus altering the ore’s frothing properties. For example, pentlandite (an iron-nickel sulfide) may get oxidised by the influence of complexes containing Si and Fe. The frothing properties of the oxidised pentlandite differ from those of the unoxidized one, which may have a negative effect on the ore recovery.

Fe itself may affect especially the recovery of Ni by flotation. Excess iron contained by the process water may cause formation of Fe-hydroxide based coatings onto the pentlandite ore used for Ni recovery, thus causing decrease in the Ni recovery.

The amount of soluble and/or colloidal silicon compounds in the minerals processing waters may be over 100 ppm, often even as high as 300-400 ppm. Soluble silicon compounds may also be called dissolved silicon compounds. The existence of soluble and/or colloidal silicon compounds in aqueous streams of the minerals processing plant has not generally been identified before. For water analysis, it has been a general practice to filter the water sample before performing the elementary analysis. Typical filters having a pore size of 0.45 pm cause the colloidal silicon compounds to be filtered from the sample. Further, the above discussed disadvantages of the soluble and/or colloidal silicon to the minerals processing have not been understood.

This specification aims to provide a method and an arrangement for removal of soluble and/or colloidal Si-compounds from aqueous streams of a minerals processing plant, and thus preventing the formation of gels and colloids containing Si-compounds, as well as the problems caused by them. Further, as the aqueous streams typically comprise Fe complexed with Si, the method also provides a route for removing at least some of the iron comprised by the aqueous streams of the minerals processing plant.

Within context of this specification, Si-compounds refers to compounds containing silicon and appearing in the substances related to the minerals processing. Si-compounds include silicates. Term silicate may refer to any member of a family of anions consisting of silicon and oxygen, any salt of such anions, or any ester of such anions. Term silicate also includes those anions that comprise a continuous framework of S1O ₄ tetrahedra having metals, typically Fe, Al, Mn or Mg, as impurities in the structure. Term silicate also includes silicate minerals as well as rock types comprising predominantly such minerals. Quartz (S1O ₂ ) may also be included in the silicates. Silicates also include minerals wherein aluminium or other tetravalent atom(s) replace some of the silicon atoms, such as aluminosilicates.

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. Thus, herein term colloidal Si-compounds refers to silicon containing compounds that are suspended in water forming a mixture where insoluble or soluble particles do not settle or would take a very long time to settle.

Within context of this specification, “removal” or “removing” of soluble and/or colloidal Si-compounds may refer to a process of entirely eliminating said compounds or to a process wherein the amount of said compounds is reduced, i.e. the amount of Si-compounds in the aqueous stream to be treated is higher than the amount of said compounds in the stream obtained after performing the method disclosed herein.

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 are 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 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 soluble and/or colloidal Si-compounds 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 soluble and/or colloidal Si-compounds may be removed as tailings. The cleaning flotation underflow 104 may be recirculated back into the process for use as process water. The aqueous stream wherefrom the soluble and/or colloidal Si-compounds 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 Ni and/or Cu. As illustrated in Fig. 3, the flotation arrangement 316 may comprise a first mineral flotation circuit 311a arranged to recover Cu and a second mineral flotation circuit 311 b arranged to recover 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 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 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 soluble and/or colloidal Si-compounds 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 and soluble and/or colloidal Si- compounds. The thickener overflow may further comprise Si-containing particles as well as other undesired, detrimental or unrecovered material or compounds such as fine particles and larger particles, starch-based depressants, microbes 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) 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) to the aqueous stream 100, 200a, 300a, 400b is ensured. For example, the coagulant(s) and/or flocculant(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.

As a result of the method described, turbidity of the aqueous stream may be diminished 50-99 %. 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.

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. Figure 3 illustrates an arrangement wherein the cleaning flotation underflow 104, 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. In a case the water for flotation or for the process preceding flotation, such as grinding, 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 precipitate. The precipitates, for one, cause problems for the flotation process. The metals and other impurities contained by the water used may also cause lowering of the quality of the mineral surface already in grinding. Thus, the above described problems may be avoided by recirculating the cleaning flotation underflow back into the flotation or even into a process preceding flotation, such as grinding. 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 floe. Flocculation refers to a process wherein destabilized particles are actually combined into even larger aggregates known as floes 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 floe. 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 floes comprising at least some of the Si-compounds, 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.

According to an embodiment, pH of the aqueous stream 100, 200a, 300a, 400b is adjusted to be in a range of 4.5 - 10 prior to subjecting the treated aqueous stream 101 , 201 , 301 , 401 to the cleaning flotation. The pH of the aqueous stream may be adjusted prior to adding coagulant(s) and/or flocculant(s) and/or flotation chemical(s) to the aqueous stream. Thus, pH of treated aqueous stream 101 , 201 , 301 , 401 is in a range of 4.5 - 10 prior to subjecting it to cleaning flotation.

According to an embodiment an iron salt is used as a coagulant. In that case, pH of the aqueous stream 100, 200a, 300a, 400b may be adjusted to be in a range of 5-8 prior to subjecting the treated aqueous stream 101 , 201 , 301 , 401 to the cleaning flotation in order to obtain efficient coagulation. The pH range of 5-8 may be preferred, as iron hydroxide is known to precipitate in said pH range. The pH of the aqueous stream may be adjusted in any suitable manner. The pH may be adjusted for example in a mixing unit. Using iron salt as the coagulant is beneficial as the aqueous stream already contains Fe, and thus smaller amount of coagulant may be needed in order to obtain the desired coagulation. Further, floes formed by the use of iron salt as coagulant may be more durable under cleaning flotation conditions, thus improving the efficiency of cleaning flotation.

The method described herein has the effect that the treated process water, i.e. the liquid obtained from the cleaning flotation unit is insofar pure in terms of soluble and/or colloidal Si-compound content that it is possible to reuse the treated process water without negatively influencing the outcome of the process. Si-removal rates of between 55-90 % may be obtained depending on the composition of the aqueous stream in question.

Further, as the aqueous streams typically comprise Fe complexed with Si, the method also provides a route for removing at least some of the iron comprised by the aqueous streams of the minerals processing plant. Thus the problems caused by the Si and Fe containing complexes as well as the excess iron itself may be avoided. Depending on the coagulant dosage and composition of the aqueous stream about 60-90 % of the iron contained by aqueous streams may be removed by the method disclosed herein.