Precipitation of calcium compound in solvent extraction systems

Abstract

The present invention concerns a solvent extraction process for calcium removal. In the process, an aqueous stripping solution is distributed within an organic phase having a lower density than the aqueous stripping solution. The process steps comprise distribution of droplets of the aqueous stripping solution in the upper part of a solvent extraction unit, allowing the droplets to travel through the organic phase towards the bottom part of the solvent extraction unit where the droplets are collected into a layer of aqueous phase, and removal of an aqueous slurry containing precipitated calcium compounds from the bottom portion of the layer of aqueous phase at the bottom part of the solvent extraction unit. The present invention also concerns a solvent extraction unit for carrying out the process of the invention.

Description

PRECIPITATION OF CALCIUM COMPOUND IN SOLVENT EXTRACTIONSYSTEMSFIELD

[0001] The present invention relates to a solvent extraction process and a solvent extraction unit for calcium removal.BACKGROUND

[0002] In the field of hydrometallurgy, solvent extraction is a widely employed method for the separation of metal ions. Currently such systems are designed for liquids only, whereby precipitates and solids must be avoided to all cost. The presence of solids within current solvent extraction systems will give rise to several problems, as the solids will block the flow and the equipment and may also form stable emulsions which affect the separation process negatively.

[0003] In some extraction processes, a precipitating compound may be formed upon mass transfer of ions between the two liquid phases. In order to avoid the aforementioned problem, the amount of aqueous and / or organic phase must be adjusted such that the precipitating compound is maintained in solution. This is especially the case for calcium containing compounds originating from, for example, the raw material in metal leaching processes, neutralization agents or the inlet water. Since calcium may cause problems in further steps of the separation process, removal of excess calcium is beneficial.

[0004] Calcium based neutralization chemicals are in general cheap and they are readily available. Thus, calcium based chemicals, such as lime rock (CaCCE) or lime (Ca(OH)2), could be good options for pH control in solvent extraction processes. The drawback is that calcium compounds easily may precipitate in the presence of counterions, thus blocking the apparatus. This problem becomes particularly evident in sulphate-based solutions by the formation of gypsum, which in solid form tends to stick to surfaces of the apparatus.

[0005] In the prior art, precipitation is avoided by increasing the relative volume of the aqueous phase such that the chemical equilibrium not yet is reached, thus maintaining the calcium compounds in solution. This approach may require large amounts of waterwhich in some locations, such as in Australia, Africa and some parts of Asia, may be a very limited recourse and in addition increases the amount of waste water generated in the process. Furthermore, the feedwater may also be relatively rich in calcium, especially in desert areas, thus making the calcium removal process less efficient. Current processes are generally performed at as high stage efficiency as possible, being 98% or higher. In order not to use more water than necessary, the water leaving the extraction process is almost saturated with respect to the calcium compound, e.g., calcium sulphate.

[0006] Current extraction processes also rely on strong mixing to increase the reaction surface area and the mass transfer of metals between the aqueous and the liquid phase. This generates further problems when precipitates or solid matter is present, as strong mixing tends to generate emulsions into which the solid matter is bound. Especially gypsum is prone to the formation of crud, i.e., an emulsion of organic phase, aqueous phase and solids. Newly formed gypsum is very reactive, and tends to stick to the inner surfaces of the solvent extraction unit and the pipes. When gypsum is formed under strong mixing, organic matter is typically bound to the wet precipitate and the mixing further directs the precipitate in contact with the inner surfaces of the equipment. Gypsum is accumulated in the equipment such that regular maintenance shut-downs are necessary.

[0007] There are some prior art solutions that utilize alternative flow arrangements instead of mixing. For example, EP 2614868 Bl and US 2022 / 0332751 Al both describe liquid-liquid extraction units. In EP 2614868 Bl the unit is arranged as a counterflow unit, where an aqueous phase in bubble form is admitted from an upper inlet in one sidewall and an organic phase in bubble form is admitted from a lower inlet. US 2022 / 0332751 Al concerns a multistage apparatus for liquid-liquid extraction, wherein the liquid flow is arranged from one partition to another. Neither publication do refer to removal of precipitate, although US 2022 / 0332751 Al touches the problem briefly by noting that the maintenance can be reduced as there is no piping connecting each stage.

[0008] In view of the aforesaid problems of the prior art, there is still a need for a solvent extraction process and system with more efficient use of water resources in the calcium removal process.SUMMARY OF THE INVENTION

[0009] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0010] According to a first aspect of the present invention, there is provided a solvent extraction process for calcium removal. In the process, an aqueous stripping solution is distributed within an organic phase having a lower density than the aqueous stripping solution, the process further comprising the steps of:- distribution of droplets of the aqueous stripping solution in the upper part of a solvent extraction unit,- allowing the droplets to travel through the organic phase towards the bottom part of the solvent extraction unit where the droplets are collected into a layer of aqueous phase, and- removal of an aqueous slurry containing precipitated calcium compounds from the bottom portion of the layer of aqueous phase at the bottom part of the solvent extraction unit.

[0011] The first aspect of the present invention thus concerns a process for calcium removal in solvent extraction systems. Droplets of aqueous phase are distributed within an organic phase and calcium compounds are allowed to precipitate within the aqueous droplets. The droplets travel downwards and are collected into an aqueous layer from which the precipitate is formed.

[0012] According to a second aspect of the present invention, there is provided a solvent extraction unit having, in the upper part of the unit, an inlet for aqueous stripping solution and an outlet for organic solvent. The solvent extraction unit further comprises:- a stripping solution distributor positioned at a level below the outlet for organic solvent and configured to distribute aqueous stripping solution from the inlet as aqueous droplets within the organic phase,- an inlet for organic solvent in the bottom part of the solvent extraction unit,- a concentrator section for collecting precipitate in the bottom part of the solvent extraction unit, wherein the concentrator section is located at a level below the inlet for organic solvent in the bottom part of the solvent extraction unit and comprises structures that guide precipitate towards the bottom of the solvent extraction unit, and- an outlet for slurry connected to the concentrator section.

[0013] The second aspect of the present invention thus concerns a solvent extraction unit suitable for carrying out the process of the first aspect of the invention. The solvent extraction unit is configured to produce droplets of aqueous phase within an organic phase. Furthermore, the solvent extraction system comprises means for solid matter collection and removal.

[0014] The invention is based on the discovery that the volumes of water needed for calcium removal in solvent extraction processes can be significantly reduced by utilizing an alternative reaction pattern allowing precipitate formation. Current solvent extraction systems do not tolerate precipitate, whereby calcium components have to be maintained in solution. It was discovered that problems with accumulation of precipitate within the extraction system can be avoided by limiting the calcium precipitate formation to the aqueous phase droplets, thus preventing the precipitate from being contacted with the organic phase.

[0015] Significant advantages are achieved using the invention. Problems of solvent extraction unit contamination and maintenance shutdowns due to unintentional precipitation can be avoided. Furthermore, the profitability of the process may be increased as readily available and affordable reagents may be used. Sulphuric acid is obtainable at low cost, the use of which in current systems would require large amounts of water to prevent gypsum formation in the presence of calcium ions. Hence, the water consumption in the process and system of the invention may be reduced significantly, to around 5 % of the water consumption in a conventional processes and systems. The process and system of the invention thus provides a more environmentally friendly and economical process for calcium removal in metal recovery processes.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGURE 1 illustrates a solvent extraction unit in accordance with at least some embodiments of the present invention.

[0017] FIGURE 2 illustrates a solvent extraction unit of an advantageous embodiment, wherein the unit is provided with means for recirculation of aqueous stripping solution.

[0018] In Figure 1 and 2 the aqueous solution is illustrated with diagonal lines (A) while the organic phase is illustrated as white areas (B) within the equipment. Droplets of aqueous phase are collected into a layer of aqueous phase at the bottom of the solvent extraction unit, this also being illustrated by diagonal lines. The dashed horizontal line illustrates a division between the upper portion of the aqueous layer, representing the layer directed to any recycling line, and bottom portion comprising the concentrator section from which a calcium containing slurry is withdrawn. This dashed line is only illustrative and does not represent any physical part of the equipment. Likewise, the proportion between liquid and organic phase may vary from that presented in Figure 1 and 2, whereby also the location of the inlets and outlets may vary.EMBODIMENTS

[0019] DEFINITIONS

[0020] In the present context, the term “solvent extraction” comprises all kinds of liquid-liquid extraction where two liquid phases of different properties interact, enabling mass transfer from one phase to the other.

[0021] The principle of function of the solvent extraction system described herein is presented in a vertical extraction system. The distributed aqueous phase is traveling downwards in the vertical system due to difference in density, i.e., by means of gravity. Expressions like “upper part” or “upper portion” is thus to be understood as the upper half of the system or the upper half of the section concerned (layer, portion etc.), when viewed from the ground level. Correspondingly, the “bottom part” or “bottom section” is to be understood as the lower half of the system or the lower half of the section concerned, when viewed from the ground level.

[0022] The size (diameter) of aqueous droplets distributed in the process of the present disclosure can be measured with light diffraction methods, such as ISO 13320:2020.

[0023] The present invention concerns a solvent extraction process for calcium removal. In the process, an aqueous stripping solution is distributed within an organic phase having a lower density than the aqueous stripping solution, the process further comprising the steps of:- distributing droplets of the aqueous stripping solution in the upper part of a solvent extraction unit,- allowing the droplets to travel through the organic phase towards the bottom part of the solvent extraction unit where the droplets are collected into a layer of aqueous phase, and- removing an aqueous slurry containing precipitated calcium compounds from the bottom portion of the layer of aqueous phase at the bottom part of the solvent extraction unit.

[0024] A preferred embodiment of the present invention concerns a solvent extraction process for calcium removal, wherein an aqueous stripping solution is distributed within an organic phase having a lower density than the aqueous stripping solution. The process comprises the steps of: distributing droplets of the aqueous stripping solution in the upper part of the solvent extraction unit, wherein the diameter of the distributed droplets is in a range of 0.1 mm to 2 mm, preferably in a range of 0.5 mm to 2 mm, allowing the droplets to travel through the organic phase towards the bottom part of the solvent extraction unit where the droplets are collected into a layer of aqueous phase, and removing an aqueous slurry containing precipitated calcium compounds from the bottom portion of the layer of aqueous phase at the bottom part of the solvent extraction unit.

[0025] In such a solvent extraction process, calcium is transferred to the aqueous stripping solution. The stripping solution contains counterions capable of forming precipitate with calcium, preferably the stripping solution is acidic, even more preferably an aqueous solution of sulphuric acid. The aqueous solution travel through the calcium containing organic phase in the form of droplets. The organic phase may contain, for example, organo-phosphorous based extracting agents, such as the commercial agents D2EHPA or Cyanex 272, or C9-C10 carboxylic acids, without being limited thereto. Since the aqueous droplets are of higher density than the surrounding organic medium, the droplets are traveling downwards by means of gravity. Calcium is transferred to theaqueous stripping solution that becomes saturated, whereafter a calcium compound is precipitated within the droplet by reaction with counterions present in the aqueous stripping solution. The aim is to achieve controlled precipitation of calcium compound within the droplets, such that the precipitate is transferred together within the droplets to a layer of aqueous phase formed upon collection of droplets in the bottom part of the solvent extraction unit. The precipitate settles on the bottom of the solvent extraction unit, in a concentrator section, wherefrom it can be removed as a slurry, i.e. a suspension containing precipitated calcium compound. Since calcium is allowed to precipitate in the reaction, there is no need to add water to maintain the concentration of calcium compound below the point of solubility equilibrium. The water consumption of the process is thus significantly reduced when compared to currently employed processes. When sulphuric acid is used in the stripping solution, it has been found that the water consumption may be as low as 3~7% of the water consumption of corresponding conventional methods.

[0026] In a preferred embodiment, aqueous stripping solution is recirculated from the layer of aqueous phase in the bottom part of the solvent extraction unit to the droplet distribution stage at the upper part of the solvent extraction unit, wherein make-up water is added to the recycled aqueous stripping solution prior to the droplet distribution. In this manner, the water consumption may be reduced even further. When acidic stripping solution is used, acid may be added to the make-up water. The volume ratio of aqueous phase to organic phase is typically maintained constant in the extraction system and may be, for example, in the range of 1 : 1-1 : 10, preferably the volume ratio is in the range of 1 :2-l : 10, 1 :2-l :5 or 2:3-1 :3. Fluctuation within a predetermined range may be allowed, such as an increase or decrease of the volume of the aqueous phase by up to 10 % or up to 20%. By maintaining the volume ratio substantially constant, also the fluid circulation becomes more reliable as mixing of the phases can be avoided. When a reagent, such as sulphuric acid, is added to the stripping solution to maintain the counter ion concentration at a certain level, also the amount of reagent added may be kept constant when the volume ratio and the composition of the two phases remains constant.

[0027] To avoid mixing of phases and unnecessary transfer of solids to the recirculation line, the circulation water outlet is optimally arranged at a level close to the interface between the organic phase and the layer of aqueous phase, but still such that the outlet for the circulation solution during operation always is fully in contact with the aqueous phase alone. This means that the distance between the stripping solution outlet and the interface between the aqueous layer and the organic phase should be great enough toallow the volume of the aqueous layer to decrease within a predetermined fluctuation range, without the stripping solution outlet being contacted with the organic phase. In this way, water is directed to the circulation from the upper portion of the layer of aqueous phase within the solvent extraction unit.

[0028] Since an acidic stripping solution is neutralized upon reaction with calcium, acid may be added to the make-up water to maintain the pH of the stripping solution, or its ion concentration, at a substantially constant level, such as, within ±15% of a predetermined value or starting value. Preferably, the volume ratio of recycled aqueous stripping solution to make-up water is 1 :1-20:1, or even more preferably 5:1-20:1. Thus, the volume of recycled aqueous solution is 1-20 times the volume of fresh water, i.e., make-up water, added to the process.

[0029] The droplets formed in the droplet distribution step should be large enough to travel efficiently through the organic phase and to function as precipitation environment for the calcium compound. Precipitation is initiated when calcium is transferred from the calcium loaded organic phase to the droplet of aqueous stripping solution containing counter ions, such as sulphate ions. When the droplets are allowed to fall through the organic phase due to the difference in density, the precipitate formed will maintain within the droplet, provided that no mixing or only moderate mixing is applied. If the droplets would be atomized into mist-like water droplets, each individual droplet would not provide a large enough volume for precipitation to take place without the precipitate itself being contacted with the surrounding organic phase. Furthermore, such small droplets would travel very slowly in the organic phase. On the other hand, too large droplets will have a negative effect on the mass transfer of calcium ions from the organic phase to the aqueous stripping solution, as the relative contact surface area between the phases decreases. To achieve the desired effect, a suitable diameter of the droplets distributed in the upper part of the solvent extraction unit is in a range of 0.1 mm to 2 mm, preferably in a range of 0.5 mm to 2 mm. Such droplets may be formed, for example, by a nozzle arrangement, preferably comprising a plurality of nozzles, or a distribution system comprising a plurality of openings allowing formation of droplets of desired size within the organic solvent used. The droplets generated are thus relatively small but still larger than droplets formed by a turbine.

[0030] The size of the droplets formed in the process can be determined by laser diffraction methods. In particular, the diameter of the distributed droplets can be determined according to ISO 1330:2020.

[0031] It is preferred that the majority of the droplets distributed in the process have a diameter in said range. Thus, preferably at least 50 % of the number of droplets have a diameter in the range of 0.1 mm to 2 mm, or the preferred range of 0.5 mm to 2 mm. Even more preferably at least 75 %, at least 85 %, or at least 95 % of the droplets have a diameter in the range of 0.1 mm to 2 mm, or the preferred range of 0.5 mm to 2 mm.

[0032] In an optimal system, the droplets of aqueous stripping solution are essentially maintaining their initial size when traveling through the organic phase towards the layer of aqueous phase in the bottom part of the solvent extraction unit. This may be achieved in the absence of mixing, or when applying only moderate mixing corresponding to a mixing tip speed of up to 1 m / s, preferably up to 0.5 m / s. In still a further preferred embodiment, the diameter of the droplets when reaching the layer of aqueous phase in the bottom part of the solvent extraction unit is in a range of 0.1 mm to 2 mm, preferably in a range from 0.5 mm to 2 mm.

[0033] In some embodiments, the stripping solution is distributed within the organic phase at a level below the level of an outlet for organic solvent.

[0034] In a further embodiment of the present disclosure, the droplets are distributed by means of a stripping solution distributor arranged in horizontal direction of the solvent extraction unit at a level below the level of an outlet for organic solvent. In the aforementioned embodiment, the distributing device directs droplets downwards within the organic phase over an area that is at least 70% of the total area of the horizontal cross section of the solvent extraction unit 1, preferably over at least 85% and even more preferably over at least 95% of the total area of the horizontal cross section of the solvent extraction unit 1. This ensures even distribution of droplets within the organic phase.

[0035] In an especially preferred embodiment of the present disclosure, the acidic aqueous phase contains sulfuric acid (H2SO4). Sulphate ions form gypsum upon reaction with calcium, whereby the precipitated calcium compound in the aqueous slurry is gypsum (CaSO4 • 2H2O). Sulphuric acid is the cheapest mineral acid available, why being able to use this in a solvent extraction process is highly favorable. In traditional systems where precipitation is not tolerated, this may not be possible or it might be necessary to add great volumes of water to maintain the calcium sulphate formed in solution. Simply, by being able to utilize sulphuric acid in the process of the present disclosure, the profitability of the process is increased.

[0036] The calcium precipitate formed in the aqueous droplets is transferred to the bottom part of the solvent extraction unit, which preferably functions as a concentratordirecting the solid matter to the bottom of the solvent extraction unit. Aqueous slurry containing precipitated calcium compound is removed through a slurry outlet in the bottom part of the unit, preferably from the lowermost section of the concentrator. The solid content may be, for example, from 10, 15 or 20% up to 25, 30 or 40%. According to one embodiment of the present disclosure, the aqueous slurry containing precipitated calcium compound removed from the solvent extraction unit has a solids content of 10-60%, such as 10-40% or 10-20%. An aqueous slurry of gypsum can relatively easily be removed even at higher solids content.

[0037] In the step of collection of precipitate and slurry removal, the problem with the calcium precipitate sticking to the inner surfaces of the equipment may become a problem, especially in the case of gypsum. Thus, it is preferable to have at least the bottom part of the solvent extraction unit, which is in contact with the layer of collected aqueous phase and the calcium precipitate slurry, coated with an agent that decreases the friction of the surface, such as polymer coatings, for example, fluoropolymer coatings, such as polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PF A) or polytetrafluoroethylene (PTFE) coating. For the same reason, it is beneficial to have the solvent extraction unit designed such that the shape of the bottom segment of the solvent extraction units directs the flow towards the slurry outlet. In order to protect the pump line from blockage and wear, it is beneficial to arrange a guide plate in connection with the slurry outlet line, directing solid matter away from any pump in connection with the concentrator section.

[0038] As noted above, the droplets of aqueous solution should optimally remain intact as they are transferred towards the bottom part of the solvent extraction unit. In order to achieve this, it is preferable to carry out the process in the absence of mixing.Alternatively, a moderate mixing may be applied, such that the mixing does not break the droplets formed in the upper part into smaller ones. Naturally any such gentle mixing or stirring may in the process of the present disclosure be carried out by any mixing means known in the art, such as by using one or more rotating mixing blades at low speed or by direction of liquid flows. In a preferred embodiment, any mechanical mixing is performed using a tip-speed of up to 1 m / s, preferably up to 0.5 m / s. This is a very low mixing rate compared to mixing rates of, for example 5 m / s, used in traditional solvent extraction units aiming for as high stage efficiency as possible, e.g, >98%. In traditional units, a mixing rate of around 5 m / s may be applied to increase the contact surface between the two phases and thus make the mass transfer more efficient, while a mixing rate in the similar range in the system of the present disclosure would break down the droplets of aqueous strippingsolution into smaller ones, whereby the risk of the precipitate being contacted with the organic phase increases. Thus, also the precipitate would more easily become contacted with the organic phase. The problems of calcium precipitate being deposited on the inner surfaces of the unit would arise when such regular mixing is used. Likewise, the amount of organic phase lost in the stripping stage is minimized when the aqueous droplets remain intact.

[0039] When no mixing or moderate mixing is applied, the stage efficiency decreases. It was however found that, in contrary to extraction systems intended for metal recovery, i.e., production extraction, a relatively low stage efficiency in the range of 10-50% and even 10-20% is sufficient in a system of the present disclosure, where the primary aim is to remove calcium that may cause problems later in the process. There is no need to remove all of the calcium in one process stage, as long as the calcium contained in the organic phase is not allowed to accumulate. Lower stage efficiency can also be compensated with the aqueous phase internal circulation.

[0040] The present invention also concerns a solvent extraction unit 1 having, in the upper part of the unit 1, an inlet 3 for aqueous stripping solution and an outlet 7 for organic solvent. In addition, the solvent extraction unit 1 comprises a stripping solution distributor 2 positioned at a level below the outlet 7 for organic solvent and configured to distribute aqueous stripping solution from the inlet 3 as aqueous droplets 4 within the organic phase, an inlet 6 for organic solvent in the bottom part of the solvent extraction unit 1 , a concentrator section 8 for collecting precipitate in the bottom part of the solvent extraction unit 1, wherein the concentrator section 8 is located at a level below the inlet 6 for organic solvent in the bottom part of the solvent extraction unit 1 and comprises structures that guide precipitate towards the bottom of the solvent extraction unit 1, and an outlet 5 for slurry connected to the concentrator section 8.

[0041] Such a solvent extraction unit is illustrated in Figure 1 and is suitable for carrying out the method of the invention. In contrast to prior at solutions, the solvent extraction unit of the present invention is configured to tolerate solid matter, typically in the form of precipitates. When precipitate is allowed to form during extraction or stripping processes, the water feed may be reduced as there is no need to adjust the amount of solvent to maintain the calcium compound in solution.

[0042] The solid matter formed in the process is collected in a concentrator that is located in the bottom part of the solvent extraction unit and from which a slurry, i.e., suspension of solid matter and aqueous solution, is removed through the slurry outlet 5 in the bottom part of the solvent extraction unit 1. The concentrator 8 guides solid matter towards the bottom of the reactor. This is preferably achieved by providing the concentrator with a structure that tapers towards the bottom, such as a rounded or conical shape. The slurry outlet 5 may be connected to the lowermost part of such a concentrator section. The concentrator may comprise further structures, such as guide plates, baffles, or shields directing the solid matter in a desired direction, typically towards the bottom of the solvent extraction unit, and / or protecting any pumps or additional outlets located in the concentrator section from precipitate. Since the solid calcium compound may be of a character that easily sticks to the walls of the equipment, such as gypsum, it is preferred to coat any surfaces with coatings that decreases the friction of the surfaces. Especially steel surfaces are prone to contamination, whereby gypsum tends to accumulate on the surfaces and block flow connections. By providing the inner surfaces of the solvent extraction unit 1, in particular the inner surfaces of the concentrator section 8, with such low-friction coatings, the need for system maintenance is decreased and the flow of the slurry is improved. Different kinds of low-friction coatings are known in the art. Many polymer coatings, such as polytetrafluoroethylene (PTFE), paints, and liners are suitable for the purpose.

[0043] In an especially preferred embodiment of the present disclosure, the solvent extraction unit 1 further comprises a stripping solution outlet 9 positioned in the bottom part of the solvent extraction unit at a level below the inlet 6 for organic solvent and above the outlet 5 for slurry, wherein the stripping solution outlet 9 is in fluid connection with a supply line 10 for make-up water and the stripping solution inlet 3. The stripping solution outlet 9 in fluid connection with the stripping solution inlet 3 provides for the possibility to circulate aqueous stripping solution, thus further reducing the water consumption of the system. Because at least a part of the aqueous solution introduced via the inlet 3 is removed through the slurry outlet 5, there is a need to introduce fresh make-up water compensating for the aqueous solution removed together with the slurry, thus keeping the system in balance. The make-up water may be introduced via supply line 10, i.e., make-up water inlet, which also is in fluid connection with the stripping solution inlet 3. Acid or another aqueous solution containing reactive counterions may be added to the make-up water to maintain the precipitation reaction at a desired level.

[0044] In some embodiments, the stripping solution distributor 2 is configured to distribute aqueous stripping solution as aqueous droplets 4 having a diameter in a range of 0.1 mm to 2 mm, preferably in a range of 0.5 mm to 2 mm.

[0045] In a further embodiment of the present disclosure, the stripping solution distributor 2 comprises a plurality of fluid distribution openings arranged at a distance from each other, the ratio of the distance between two adjacent opening and the diameter of such an opening being in the range of 2 : 1 -20 : 1 , preferably 3:1-10:1 or 5:1-10:1. The distance between openings is thus sufficient for the formation of individual droplets, i.e, the droplets are not so close to each other that the would combine, but still in a range where a sufficient efficiency can be achieved. For the formation of droplets in a desired size range of 0.1-2 mm, the openings may be within a similar range of 0.05-2 mm, or 0.1-2 mm. An arrangement with evenly distributed droplet openings minimizes the degree of interaction between droplets and may maximize the droplet distribution within the volume of organic phase as the droplets are spread and generated in a controlled manner.

[0046] In a further preferred embodiment, the stripping solution distributor 2 extends over at least 70% of the total area of the horizontal cross section of the solvent extraction unit, preferably over at least 85% and even more preferably over at least 95% of the total area of the horizontal cross section of the solvent extraction unit. By arranging the distributor such that an area as wide as possible is accessed, the efficiency of the system is improved. Further, by having the distributor arranged such that the droplets are allowed to fall straight in vertical direction, the interaction between surrounding droplets is minimized. When compared to point distribution, for example using a single nozzle system arranged in the area of the vertical center axis of the system, the droplets formed in point distribution are spread also in horizontal or partially horizontal direction, making the droplet flow less predictable. The distributor system may be arranged such that organic phase is allowed to pass through the distributor system arrangement, for example by arrangement of a plurality of distribution systems or a network of distributor openings.

[0047] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0048] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0049] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0050] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0051] The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention.EXAMPLESExample 1 - Calcium removalA 35 m3 / h of aqueous feed is fed to a solvent extraction process. Calcium is extracted to the organic phase in the extraction units. Calcium containing organic phase is directed to liquid-liquid stripping step(s). Aqueous stripping solution is dropwise distributed within the organic solvent in a volume ratio of aqueous solution to organic solvent 1 :10. Gypsum precipitate slurry is collected from the bottom of the solvent extraction unit in a flow of 1 m3 / h. The gypsum slurry has a solid content of 10%. Aqueous solution is circulated fromthe bottom part of the solvent extraction unit to the inlet and with addition of make-up water in an amount of 1 m3 / h, corresponding to the volume of removed slurry.In a conventional system not allowing precipitation, the stripping solution flow must be equal to the aqueous feed flow, i.e., 35 m3 / h.INDUSTRIAL APPLICABILITY

[0052] The present process and system can be used to replace conventional alternatives for solvent extraction in calcium removal.

[0053] In particular, the process provides a resource efficient procedure for calcium removal in metal separation processes. The amount of water needed is only a fraction of that of conventional processes and further the process makes it possible to utilize affordable reagents.Reference Signs List1 Solvent extraction unit2 Stripping solution distributor3 Stripping solution inlet4 Aqueous droplets (formed in the stripping solution distributor)5 Slurry outlet6 Organic solvent inlet7 Organic solvent outlet8 Concentrator section9 Stripping solution outlet10 Supply line for make-up water11 Stripping solution circulation pumpCitation ListPatent Literature: EP 2614868 BlUS 2022 / 0332751 Al

Claims

Claims1. A solvent extraction process for calcium removal, wherein an aqueous stripping solution is distributed within an organic phase having a lower density than the aqueous stripping solution, characterized in that the process comprises: distributing droplets of the aqueous stripping solution in the upper part of a solvent extraction unit, wherein the diameter of the distributed droplets is in a range of 0.1 mm to 2 mm, preferably in a range of 0.5 mm to 2 mm, allowing the droplets to travel through the organic phase towards the bottom part of the solvent extraction unit where the droplets are collected into a layer of aqueous phase, and removing an aqueous slurry containing precipitated calcium compounds from the bottom portion of the layer of aqueous phase at the bottom part of the solvent extraction unit.

2. Process according to claim 1, characterized in that aqueous stripping solution is recirculated from the layer of aqueous phase in the bottom part of the solvent extraction unit to the droplet distribution stage at the upper part of the solvent extraction unit, wherein make-up water is added to the recycled aqueous stripping solution prior to the droplet distribution.

3. Process according to claim 2, characterized in that the volume ratio of recycled aqueous stripping solution to make-up water is 1 : 1-20: 1.

4. Process according to any one of the preceding claims, characterized in that the diameter of the droplets when reaching the layer of aqueous phase in the bottom part of the solvent extraction unit is in a range of 0.1 mm to 2 mm, preferably in a range of 0.5 mm to 2 mm.

5. Process according to any of the preceding claims, characterized in that the droplets are distributed by means of a stripping solution distributor arranged in horizontal direction of the solvent extraction unit at a level below the level of an outlet for organic solvent, wherein the distributing device directs droplets downwards within the organic phase over an area that is at least 70% of the total area of the horizontal cross section of the solventextraction unit, preferably over at least 85% of the total area of the horizontal cross section of the solvent extraction unit.

6. Process according to any of the preceding claims, characterized in that the aqueous phase contains sulfuric acid (H2SO4), whereby the precipitated calcium compound in the aqueous slurry is gypsum (CaSO4 • 2H2O).

7. Process according to any of the preceding claims, characterized in that the aqueous slurry containing precipitated calcium compound removed from the solvent extraction unit has a solids content of 10-60%, such as 10-40% or 10-20%.

8. Process according to any of the preceding claims, characterized in that the process is carried out in the absence of mixing.

9. Process according to any of claims 1-7, characterized in that any mechanical mixing is performed using a tip-speed of up to 1 m / s, preferably up to 0.5 m / s.

10. Solvent extraction unit (1) having, in the upper part of the unit (1), an inlet (3) for aqueous stripping solution and an outlet (7) for organic solvent, characterized in that the solvent extraction unit (1) further comprises:- a stripping solution distributor (2) positioned at a level below the outlet (7) for organic solvent and configured to distribute aqueous stripping solution from the inlet (3) as aqueous droplets (4) within the organic phase,- an inlet (6) for organic solvent in the bottom part of the solvent extraction unit (1),- a concentrator section (8) for collecting precipitate in the bottom part of the solvent extraction unit (1), wherein the concentrator section (8) is located at a level below the inlet (6) for organic solvent in the bottom part of the solvent extraction unit (1) and comprises structures that guide precipitate towards the bottom of the solvent extraction unit (1), and- an outlet (5) for slurry connected to the concentrator section (8).

11. Solvent extraction unit (1) according to claim 10, characterized in that the solvent extraction unit (1) further comprises a stripping solution outlet (9) positioned in the bottom part of the solvent extraction unit on a level below the inlet (6) for organic solvent andabove the outlet (5) for slurry, wherein the stripping solution outlet (9) is in fluid connection with a supply line (10) for make-up water and the stripping solution inlet (3).

12. Solvent extraction unit (1) according to claim 10 or 11, characterized in that the stripping solution distributor (2) comprises a plurality of fluid distribution openings arranged at a distance from each other, the ratio of the distance between two adjacent opening and the diameter of such an opening being in the range of 2: 1-20: 1 , preferably 3:1-10:1.

13. Solvent extraction unit (1) according to any one of claims 10-12, characterized in that the stripping solution distributor (2) extends over at least 70% of the total area of the horizontal cross section of the solvent extraction unit, preferably over at least 85% of the total area of the horizontal cross section of the solvent extraction unit.

14. Solvent extraction unit (1) according to any one of claims 10-13, characterized in that the concentrator section (8) comprises guide plates arranged to shield any pumps in connection with the concentrator section (8) from precipitate.

15. Solvent extraction unit (1) according to any one of claims 10-14, characterized in that inner surfaces of the solvent extraction unit (1), in particular the surfaces of the concentrator section (8), are coated with polymer coating preventing precipitate from sticking to the inner surfaces of the unit (1), preferably the surfaces are coated with fluoropolymer coatings, such as polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PF A) or polytetrafluoroethylene (PTFE) coating.

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