Technical field

This specification relates to a method and an arrangement for removing amine(s) from a thickener overflow of a mineral processing plant.

Background

Amines are a group of organic compounds widely used by the mineral processing industry. Their use especially as flotation agents is growing fast. There are, however, environmental problems related to the use of amines as flotation agents. Amines accumulate in large quantities in tailings dams wherein they present incomplete (bio)degradation. Consequently, mine tailings containing production chemicals such as amine-based collectors, pose a threat to aquatic organisms living in downstream ecosystems of the mineral processing units. Protective legislation concerning the use of amines is inevitably emerging. This will cause severe restrictions to the use of amines as flotation agents unless their removal from aqueous solutions can be improved. Consequently, there is a need to find a solution for removal of amine-based flotation chemicals from the process waters.

Summary

It is an aim of this specification is to provide a method and an arrangement for removing amine(s) from a thickener overflow of a mineral processing plant. Further aim is to provide a method and an arrangement for improving quality of the thickener overflow to such an extent that the thus formed residual process water may be led to the environment as such, or alternatively, the residual process water may be recycled back into the process for use as process water.

According to an embodiment, a method for removing amine(s)from a thickener overflow of a mineral processing plant is provided. The thickener overflow comprises process water and amine(s). The method comprises: - measuring pH of the thickener overflow and adjusting the pH to be at least 10 in order to facilitate precipitation of at least some of the amine(s),

- supplying the thickener overflow to a mixing unit and adding coagulant(s) and/or flocculant(s) and/or adsorbent(s) to the thickener overflow in the mixing unit in order to facilitate formation of floes comprising the amine(s) and in order to form a treated thickener overflow,

- supplying the treated thickener overflow to a cleaning flotation unit and subjecting the treated thickener overflow to cleaning flotation in order to separate at least some of the amine(s) as a cleaning flotation overflow and in order to form a residual process water as a cleaning flotation underflow, 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 amine(s) from a thickener overflow of a mineral processing plant is provided. The thickener overflow comprises process water and amine(s). The arrangement comprises

- a thickener arranged to dewater an overflow of a mineral flotation circuit to produce a thickener overflow and a thickener underflow,

- a sensor arranged to measure pH of the thickener overflow,

- a mixing unit arranged to provide the thickener overflow with coagulant(s) and/or flocculant(s) and/or adsorbent(s), and

- a cleaning flotation unit arranged to separate at least some of the amine(s) from the thickener overflow as a cleaning flotation overflow and to form a residual process water as a cleaning flotation underflow.

Brief description of the drawings

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

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

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

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

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

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

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

Fig. 5 illustrates, by way of an example, a schematic process flow chart according to an embodiment,

Fig. 6 illustrates, by way of an example, a schematic process flow chart according to an embodiment,

Fig. 7 illustrates, by way of an example, a schematic process flow chart according to an embodiment, and

Fig. 8 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.

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 flotation of low-grade oxidised iron ores is called reverse flotation, wherein the gangue is separated by flotation from the valuable finely grained iron ores. The valuable ore is collected from the underflow of the flotation unit. The gangue is separated from the ore with the help of collector chemicals, which typically are surface active organic reagents. Typically the beneficiation of the low-grade iron ores is performed by reverse cationic flotation. The advantages of reverse cationic flotation over anionic flotation include a higher process selectivity and rate, as well as satisfactory results when using hard water.

Typically, the gangue froth removed by the reverse flotation 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 is then 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.

In reverse flotation of Fe, typically hydrophobic amine-based flotation chemicals (collectors) are used to attach to the gangue particles and increase their hydrophobicity so that they can be removed as overflow in the reverse flotation step. Amine-based collectors are used especially in the reverse flotation of iron ore as they allow selective separation of the gangue material, for example quartz/silicate particles, from the valuable iron oxides. The separation is based on the capability of the amine-based compounds to adsorb onto particle surface, thereby forming mineralized froth that can be removed. By the use of amine-based collectors silicate content of the ore may be reduced for example from a level of higher than 2 % to a level of 0.6 % silicate in the recovered ore. For removal of silicates during reverse cationic flotation, a collector based on a mixture of a primary ether amine and a non-ionic surfactant, such as a fatty alcohol, may be used. Conventionally, the removed mineralized froth is dumped into tailings dams, as described above.

Apart from being time-consuming, the conventional treatment method (dumping into tailings dams) has significant space requirements and is also subject to problems for example due to rain, breakage and maintenance. Changing over to other tailings methods such as thickened tailings, paste, dry stacking or hybrids of these, results in much shorter sedimentation time, of for example 3-8 h. However, when for example the hydrophobic amine-based collectors or other particles are sent to a tailings thickener, they tend to float or follow water flow more easily instead of sedimenting as desired, thereby ending up in the thickener overflow. Also, other light materials with low density such as organic material, bacteria and other microbes, colloidal and soluble material will follow the water flow to the thickener overflow.

Today, water shortage, ecological demands placed by legislation and public pressure, costs and extensive space requirements of the aforementioned conventional tailings methods for process water treatment increasingly put pressure to recirculate process waters as main processes in flotation become at least partially closed-loop systems in terms of water usage. The above described thickener overflow to be reused as a process water may comprise a significant amount of silicates which, when water is recirculated back into the flotation process, use up flotation chemicals and disrupt floating of silicates from the freshly introduced slurry infeed. Silicates may end up in the recovered Fe material in the underflow, which deteriorates both yield and quality of the Fe material. Also other residual chemicals and harmful or detrimental substances ending up in the thickener overflow, and later in the recycled process water may affect negatively the main flotation process and final product quality if not properly handled prior to recycling the process water back into the main process.

Conventional solution to control the accumulation of collector chemicals and suppress microbiological growth is to send the flotation froth to the tailings dam with a long retention time. However, as mentioned above, there are environmental problems relating to the use of in particularly amines as flotation agents. Mine tailings containing amine collectors pose a threat to aquatic organisms living in downstream ecosystems of the mineral processing units. In particularly, high levels of fatty amines cannot be directly disposed into aquatic bodies. Moreover, the harmfulness or toxicity of the amine degradation products to the environment is not complete known.

This specification aims to provide a method and an arrangement that enable removal of amine-based collectors from the thickener overflow as well as improving quality of the thickener overflow to such an extent that the thus formed residual process water may be led to the environment as such, or alternatively, the residual process water may be recycled back into the process for use as process water.

An exemplary flotation arrangement for reverse flotation of for example Fe in combination with a process water treatment arrangement for removing amine(s) is illustrated in Fig. 1a.

Within context of this specification, amine(s) refer to the amine-based flotation chemical(s) (amine-based collector(s)) that instead of sedimenting in a thickener have ended up in the thickener overflow.

A mineral flotation circuit 110 is arranged to treat ore particles suspended in a slurry 111 by flotation. The mineral flotation circuit is arranged to separate the slurry 111 into an underflow of the mineral flotation circuit 115 and an overflow of the mineral flotation circuit 112. Hence, the mineral flotation circuit is arranged to operate by a reverse flotation. The underflow of the mineral flotation circuit 115 comprises recovered, for example Fe-containing, material.

The overflow of the mineral flotation circuit 112 is led into a thickener 113. In the thickener 113, the overflow of the mineral flotation circuit 112 is dewatered to produce a thickener overflow 101 and a thickener underflow 114. The thickener underflow 114 is removed from the thickener 113. The thickener underflow 114 is typically removed from the thickener as tailings 150. The thickener 113 is configured to operate as a solid-liquid separator to separate a sediment, i.e. the thickener underflow 114, from a supernatant, i.e. the thickener overflow 101. The thickener underflow 114 comprises particles having a density higher than the one of the liquid, and thus ending up in the sediment. The solids content of the thickener underflow 114 may be at least 80 wt.%.

The thickener overflow 101 comprises process water, amine(s), and Si- containing compounds and/or particles, typically silicates. The thickener overflow may further comprise other undesired, detrimental or unrecovered material or compounds such as fine particles and larger particles comprising C, P, N, Ca, K, Mn, Mg; starch-based depressants, microbes etc., suspended and/or dissolved in process water. Concentration of the amine(s) in the thickener overflow 101 may be 1-200 mg/I, preferably 30-100 mg/I, more preferably 50-60 mg/I. The thickener overflow 101 received from the thickener 113 is in a highly unstable state, when compared for example to the state of the effluent water received from the tailings area.

In a method according to an embodiment, pH of the thickener overflow 101 is measured and the pH is adjusted to be at least 10. Thus, the pH is adjusted to be 10 or higher. The pH is measured by a sensor 190. The sensor 190 may be located for example between the thickener 113 and a mixing unit 105, as illustrated in Figs. 1-8. Alternatively or additionally, the sensor may be placed in any convenient location, e.g. in the mixing unit 105. The pH may be adjusted for example in a thickener overflow tank. Adjusting the pH to be at least 10 facilitates precipitation of at least some of the amine(s). The pH may be adjusted by a pH adjusting agent that is also arranged to act as a precipitating agent. One example of this kind of pH adjusting agent is lime, i.e. calcium- containing inorganic mineral. The pH of the thickener overflow 101 may be adjusted by adding lime and/or aluminate. Aluminate refers to a compound containing an oxyanion of aluminium, sodium aluminate being an example. Ettringite (Ca6Al ₂ (S0 ₄ )3(0H)i ₂ -26H ₂ 0) may be formed in a case both lime and aluminate are utilized for adjusting the pH of the thickener overflow to be at least 12,5. The amine(s) may be precipitated or attached to the formed ettringite floes. In a case solely aluminate is used for the pH adjustment, pH of at least 10 is enough for aluminates to be available in anionic form, i.e. as aluminium hydroxide(s), and thus for facilitating precipitation of the amine(s).

The thickener overflow 101 is supplied to a mixing unit 105. In the mixing unit

105 coagulant(s) and/or flocculant(s) and/or absorbent(s) are added to the thickener overflow 101. Thus, formation of floes comprising amine(s) is facilitated. The pH of the thickener overflow 101 may also be adjusted in the mixing unit 105. In the mixing unit 105 a treated thickener overflow 106 is formed.

The coagulant may be at least one of the following: an inorganic collector, an aluminium salt, an iron salt and an organic coagulant. The flocculant may be a natural polymer and/or a synthetic flocculant. As the adsorbent, colloidal silica may be used.

The treated thickener overflow 106 is supplied to a cleaning flotation unit 102. The treated thickener overflow 106 is subjected to cleaning flotation in the 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 amine(s) 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 amine(s) may be removed as tailings 150.

As illustrated in Fig. 1b, prior to subjecting the thickener overflow 101 to the mixing unit 105, the thickener overflow may be led to a thickener overflow tank 107 in order to stabilize the thickener overflow 101 . As mentioned above, the pH of the thickener overflow 101 may be adjusted in the thickener overflow tank 107.

According to an embodiment, prior to subjecting the treated thickener overflow

106 to cleaning flotation, temperature of the treated thickener overflow is from 0 to 50 °C. Preferably, the temperature of the treated thickener overflow is from 0 to 35 °C. Lower temperature of the treated thickener overflow decreases the solubility of the amine(s), and thus enables their separation by the cleaning flotation. According to an embodiment, the cleaning flotation is a 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 are 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. Chemicals may sometimes be needed to aid flocculation and increase solids removal efficiency. Typically, colloids removal is possible with efficient coagulation.

The cleaning flotation underflow 104 may be supplied to further cleaning unit(s) in order to further separate at least some of the amine(s). As illustrated by Figs. 2a-b, 3a-b and 4a-b, the further cleaning unit may be a sand filter unit (330, 430) and/or an electrocoagulation unit (220, 420).

According to an embodiment and as illustrated in Fig. 2a, the cleaning flotation underflow 204 collected from the cleaning flotation unit 202 is supplied to an electrocoagulation unit 220. In the electrocoagulation unit 220 the cleaning flotation underflow 204 is subjected to electrocoagulation in order to further separate at least some of the amine(s) as an electrocoagulation overflow 222. An electrocoagulation underflow 221 thus formed comprises a secondary residual process water. By electrocoagulation, at least some of the amines contained by the cleaning flotation underflow 204 may get removed as the electrocoagulation overflow 222. Additionally or alternatively, chemistry of at least some of the amines of the cleaning flotation underflow 204 may get changed. The electrocoagulation overflow 222 may be removed as tailings 250, as illustrated in Fig. 2a. Fig. 2b illustrates an embodiment, wherein prior to supplying to the mixing unit 205, the thickener overflow 201 is led to a thickener overflow tank 207 in order to stabilize the thickener overflow 201 .

Within context of this specification, electrocoagulation (EC) refers to a process, wherein a liquid, typically water, is treated electrochemically, namely by passing by electrically charged electrodes, in order to remove impurities. Multiple reactions may take place simultaneously as water to be treated is arranged to pass through an electrocoagulation cell. In its simplest form, an EC reactor may be made up of an electrolytic cell comprising one anode and one cathode. Oxidation of ions or neutral molecules takes place at the anode. Reduction of the ions or neutral molecules takes place at the cathode. Loss of electrons is called oxidation, while electron gain is called reduction. The electrolytic cell is an electrochemical cell that drives a redox reaction through the application of electrical energy. First, a metal ion may be driven into the water. On the surface of the cathode, water may be hydrolysed into hydrogen gas and hydroxide ions (OH ^' ). Meanwhile, electrons flow freely to destabilize surface charges on suspended solids. As the reaction continues, floes may be formed that entrain suspended solids and other contaminants. Finally, the floes may be removed.

In electrocoagulation, a coagulant may be generated in situ by electrolytic oxidation of an appropriate anode material. Consumable metal plates, such as iron or aluminium, may be used as sacrificial electrodes to produce ions in the water to treated. The released ions may remove undesirable contaminants either by chemical reaction and/or precipitation, or by causing colloidal materials to coalesce. Additionally or alternatively, ionization, electrolysis, hydrolysis and/or free-radical formation may take place, thus altering the physical and chemical properties of the treated matter.

EC systems are typically constructed of at least one set of electrodes (usually in form of metal plates), through which water to be treated flows between the spaces of the electrodes. Typically, direct current is used in EC systems.

According to an embodiment, the electrocoagulation unit 220 comprises consumable metal plates, such as iron (Fe) or aluminium (Al) as the electrodes. The electrodes are arranged to produce ions into the cleaning flotation underflow. The produced ions may enable precipitation of amine(s) of the cleaning flotation underflow. The hydroxide ions produced at the cathode may raise the pH of the treated matter, i.e. the cleaning flotation underflow, and thus may further enable precipitation of the amine(s).

According to another embodiment, the electrocoagulation unit 220 comprises non-consumable metal plates, such as titanium (Ti) or stainless steel as the electrodes. Some of the amine(s) may get oxidized at the anode, while some may get reduced at the cathode. These reactions may have the effect of decreasing the solubility of the compounds, thus enabling their precipitation. The reactions may have the effect of altering chemistry of the amines. The hydroxide ions produced at the cathode may raise the pH of the treated matter, i.e. the cleaning flotation underflow, and thus may further enable precipitation of the amine(s).

According to an embodiment and as illustrated in Fig. 3a, the cleaning flotation underflow 304 collected from the cleaning flotation unit 302 is supplied to a sand filter unit 330. The cleaning flotation underflow 304 is filtered by allowing it to flow through a sand filter in the sand filter unit 330 in order to produce a filtered cleaning flotation underflow 331. The sand filter may be a sand bed. The sand filter comprises sand, i.e. large silica (S1O2) particles. The large silica particles are capable of collecting at least some of the amine(s) of the cleaning flotation underflow 304. Thus, a filtered cleaning flotation underflow 331 is produced.

The filtered cleaning flotation underflow 331 preferably has smaller content of amine(s) when compared to the cleaning flotation underflow 304. The sand filter collects at least some of the amine(s) of the cleaning flotation underflow 304. Eventually this will lead to the sand filter being saturated by the amine(s). The sand filter may be cleaned, i.e. the collected amine(s) may be removed by subjecting the filter to a washing liquid. As illustrated by the dashed arrow in Fig. 3a, the washing liquid 341 comprising the removed amine(s) may be led into the cleaning flotation unit 302 for cleaning flotation. The thickener underflow 314 and the cleaning flotation overflow 303 may be removed as tailings 350. Fig. 3b illustrates the embodiment, wherein prior to supplying to the mixing unit 305, the thickener overflow 301 is led to a thickener overflow tank 307 in order to stabilize the thickener overflow 301 .

According to an embodiment and as illustrated by Fig. 4a, the filtered cleaning flotation underflow 431 is supplied into an electrocoagulation unit 420 as described above. The filtered cleaning flotation underflow 431 is subjected to electrocoagulation in the electrocoagulation unit 420 in order to further separate at least some of the amine(s) as an electrocoagulation overflow 422 and to form a secondary residual process water as an electrocoagulation underflow 421. The electrocoagulation overflow 422 may be removed as tailings 450. Fig. 4b illustrates the embodiment, wherein prior to supplying to the mixing unit 405, the thickener overflow 401 is led to a thickener overflow tank 407 in order to stabilize the thickener overflow 401 .

In a case further purification of the electrocoagulation underflow is preferred, the electrocoagulation underflow may be supplied to a secondary cleaning flotation unit 560, 760 or to a (secondary) sand filter unit 670, 870 for further purification, as illustrated in Figs. 5-8.

According to an embodiment and as illustrated by Fig. 5, the electrocoagulation underflow 521 is supplied to a secondary cleaning flotation unit 560 and subjected to a cleaning flotation in order to separate a secondary cleaning flotation overflow 562 and in order to form a secondary cleaning flotation underflow 561. Again, the cleaning flotation comprises gas bubbles, at least 90 % of the gas bubbles having a diameter of from 0,2 to 250 pm. The secondary cleaning flotation overflow 562 may comprise at least some amine(s) and/or other undesired substances, and may be removed as tailings 550. Fig. 7 illustrates an embodiment, wherein a sand filter unit 730 is arranged between a cleaning flotation unit 702 and an electrocoagulation unit 720. The thickener underflow 714, the cleaning flotation overflow 703, the electrocoagulation overflow 722 and the secondary cleaning flotation overflow 762 may be removed as tailings. As illustrated by dashed lines in Figs. 5 and 7, the thickener overflow tank 707 is optional.

According to another embodiment and as illustrated by Fig. 6, the electrocoagulation underflow 621 is supplied to a (secondary) sand filter unit 670 and filtered by allowing it to flow through a sand filter in the (secondary) sand filter unit 670 in order to produce a filtered electrocoagulation underflow 671. The definition “secondary” herein refers to a situation wherein prior to electrocoagulation the thickener overflow has already been subjected to a pre treatment in a sand filter unit 430, as illustrated by Figs. 4a and 4b. The thickener underflow 614, the cleaning flotation overflow 603 and the electrocoagulation overflow 622 may be removed as tailings 650. Fig. 8 illustrates an embodiment, wherein a sand filter unit 830 is arranged between a cleaning flotation unit 802 and an electrocoagulation unit 820. The thickener underflow 814, the cleaning flotation overflow 803 and the electrocoagulation overflow 822 may be removed as tailings 850. As illustrated by dashed lines in Figs. 6 and 8, the thickener overflow tank 807 is optional.

According to an embodiment, the cleaning flotation underflow 104, 204, 304, 404, 504, 604, 704, 804 and/or the filtered cleaning flotation underflow 331 , 431 , 731 , 831 and/or the electrocoagulation underflow 221 , 421 , 521 , 621 , 721 , 821 and/or the secondary cleaning flotation underflow 561 , 761 and/or the filtered electrocoagulation underflow 671 , 871 are led via a tailings area into the environment. The tailings area may comprise for example an intermediate tank for stabilizing the pH of the underflow in question. Once the pH has stabilized it is possible to release the contents of the intermediate tank into the environment.

Alternatively, the cleaning flotation underflow 104, 204, 304, 404, 504, 604, 704, 804 and/or the filtered cleaning flotation underflow 331 , 431 , 731 , 831 and/or the electrocoagulation underflow 221 , 421 , 521 , 621 , 721 , 821 and/or the secondary cleaning flotation underflow 561 , 761 and/or the filtered electrocoagulation underflow 671 , 871 may be recirculated back into the process for use as process water.

The thickener overflow 101 , 201 , 301 , 401 , 501 , 601 , 701 , 801 may comprise amine(s) adsorbed onto silicate(s). Thus, by the method described herein, it is possible to remove at least some of the silicate(s) comprised by the thickener overflow 101 , 201 , 301 , 401 , 501 , 601 , 701 , 801 .

As mentioned above, in closed-loop systems, wherein water is to be recirculated back into the process, silicates may use up flotation chemicals and disrupt floating of silicates from the freshly introduced slurry infeed. Silicates may end up in the recovered ore, thus deteriorating both yield and quality of the recovered ore. Thus, the method described herein has the effect that the treated process water, i.e. the liquid obtained from the cleaning flotation unit and/or the sand filter unit and/or the electrocoagulation unit, is insofar pure in terms of amine and/or silicate content that it is possible to reuse the treated process water without negatively influencing the outcome of the process. Further, it is possible to lead the treated process water into the environment without causing harm to the environment.