Patent Application
20100051548
The present application relates to a method of solvent extraction and a system for use in carrying out the method.
Solvent extraction is used inter alia for removing selected metals from aqueous solution. The process typically comprises the extraction of the metal values from aqueous solution by an organic extractant, stripping of the metal values from the metal loaded organic extractant using an aqueous stripping solution, and, optionally, a wash stage. In the extraction, stripping and wash stages, each stage consists of a mixer (or series of mixers) and a settler, where metal exchange from one phase to the other takes place. The number of extraction, strip and wash stages required to recover a desired metal value can be determined by metallurgical testing of the solutions in question. The mixer-settler of each stage consists of a mixer (or mixers) section and a settler section. In the mixer section, the aqueous solution containing the metal value is mixed with an organic extractant that selectively combines with the metal to be extracted. During the extraction stage the aqueous solution and organic are mixed to form an emulsion to provide the greatest amount of contact between the organic and aqueous phases to maximize extraction of the metal into the organic phase. In a stripping mixing stage, the metal value is re-extracted from the organic phase by the aqueous stripping solution. When an emulsion from either the extraction or strip stage enters the respective settler section, the emulsion from the mixer is introduced into the settler where the organic and aqueous phases are allowed to separate with the organic forming a layer above the aqueous phase. The two phases are in constant flow in the settler. Separate overflows for the organic and aqueous phases are provided at one end of the settler so that the organic is retrieved for subsequent treatment in the next stage of extraction or stripping. The exiting aqueous solution also reports to the respective subsequent destination in the process. Typically, both the organic and the aqueous phases flow in the same direction in the settler, i.e. there is co-current flow in the settler.
A problem with the above conventional arrangement is that stable emulsions originating and promoted by the presence of suspended solids in the aqueous solution, referred to as “crud”, form a layer between the organic and aqueous layers in the settler and a so-called “crud-run” is experienced when the crud accumulates at the organic overflow and exits with the organic, which causes problems.
U.S. Pat. No. 6,099,732, filed Apr. 19, 1999, to Dorlac discloses a solvent extraction method and apparatus in which the organic and aqueous phases flow counter-currently (in opposite directions) in the settler, rather than co-currently. The entire contents of U.S. Pat. No. 6,099,732 are incorporated herein by reference. This counter-current flow was found to result in much of the crud being maintained at the interface between the organic and aqueous phases, with a significant reduction in crud in the overflow of the organic phase. Throughput in such apparatus is dictated inter alia by phase separation and metal recovery.
Further improvements in solvent extraction are driven by industry demands for increased recovery of metal values, increased throughput and reduced capital investment as well as operating cost.
It is an object of the present invention to obviate or mitigate at least one disadvantage of previous solvent extraction methods and apparatus.
According to one aspect, there is provided a method of solvent extraction for extracting a component dissolved or dispersed in a first phase, into a second phase. The method includes mixing the first phase with the second phase, the second phase being insoluble or immiscible with the first phase and having a greater chemical affinity for the component for extracting the component from the first phase. The method also includes subjecting the mixed first and second phases to settling for separating the first and second phases, the settling including a pre-settling stage in which there is an initial separation of the first and second phases, and a subsequent main settling stage for further separating the first and second phases. The pre-settling stage includes a plurality of channels in which the first and second phases are caused to flow generally counter-currently with respect to each other.
According to another aspect, there is provided a settling system for use in solvent extraction separation of a first phase from a second phase after mixing, the second phase having a greater chemical affinity for a component dissolved or dispersed in the first phase for extracting the component from the first phase. The settling system includes a pre-settler of a plurality of channels arranged for flow of the first and second phases counter-currently with respect to each other for initial separation of the first and second phases. The settling system also includes a main settler for further separating the first and second phases.
According to still another aspect, there is provided a settling system for use in separating a first phase from a second phase after mixing in a solvent extraction process, the second phase having a greater chemical affinity for a component for extracting the component from the first phase. The system includes a settling tank having a respective outlet at each end thereof for promoting counter-current flow of the first and second phases with respect to each other. The settling tank has at least one baffle arranged at an angle offset from the vertical for promoting separation of the first phase from the second phase.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific in conjunction with the accompanying figures.
Embodiments of the present application will now be described, by way of example only, with reference to the attached Figures, wherein:
It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Also, the description is not to be considered as limiting the scope of the embodiments described herein.
Reference is made to the Figures to describe a settling system for use in solvent extraction separation of a first phase from a second phase after mixing. The settling system is indicated generally by the numeral 20. The second phase has a greater chemical affinity for a component dissolved or dispersed in the first phase for extracting the component from the first phase. The settling system 20 includes a pre-settler 22 of a plurality of channels arranged for flow of the first and second phases counter-currently with respect to each other for initial separation of the first and second phases. The settling system 20 also includes a main settler 24 for further separating the first and second phases.
The settling system 20 will now be further described with specific reference to the Figures. Referring to
A launder or an inlet pipe 34 enters the first channel 26 and provides a fluid connection with the mixer for the introduction of an emulsion of the first and second phases from the mixer. According to the present embodiment, the inlet pipe 34 enters the first channel through a sidewall, proximal the center thereof. A pair of generally vertical baffles 36 extend in the flow path of the emulsion, with a respective one of the pair of baffles 36 on each side of the inlet pipe 34 for reducing turbulent flow and facilitating initial separation of the first and second phases.
Each one of the channels 26, 28, 30, 32 includes an aqueous outlet 38 and an organic outlet 40 proximal respective ends thereof (also shown in
The aqueous outlets 38 for the channels 26, 28, 30, 32 are located proximal alternating ends of the pre-settler 22 in successive channels such that the aqueous outlets 38 for the first and third channels 26, 30 are located proximal one end 46 of the pre-settler 22, while the aqueous outlets 38 for the second and fourth channels 28, 32 are located proximal the opposing end 48 of the pre-settler 22. Thus, the respective aqueous inlet and respective aqueous outlet are located near opposite ends in each of the second, third and fourth channels 28, 30, 32.
Each organic outlet 40 is a weir that provides an opening in the sidewall of the respective channel 26, 28, 30, 32 at the opposite end of the channel 26, 28, 30, 32 as the aqueous outlet 38. The organic outlet 40 for each of the first, second and third channels 26, 28, 30 is located at an upper portion of a respective sidewall for fluid flow into a respective adjacent, downstream, one of the channels 28, 30, 32. Again, the pre-settler includes four channels 26, 28, 30, 32. Thus, the organic outlet 40 for the fourth channel 32 is located in the sidewall of the fourth channel 32 for fluid flow into the adjacent main settler 24. Similarly, each organic outlet 40 thereby provides an organic inlet in the respective subsequent, downstream channel or main settler 24.
Like the aqueous outlets 38, the organic outlets 40 for the channels 26, 28, 30, 32 are located proximal alternating ends of the pre-settler 22 in successive channels such that the aqueous outlets 38 for the second and fourth channels 28, 32 are located proximal one end 46 of the pre-settler 22, while the aqueous outlets 38 for the first and third channels 26, 30 are located proximal the opposing end 48 of the pre-settler 22. Thus, the respective organic inlet 38 and respective organic outlet 40 are located near opposite ends in each of the second, third and fourth channels 28, 30, 32.
The respective aqueous outlet 38 and the respective organic outlet 40 are also located near opposite ends in each of the channels 26, 28, 30, 32, thereby promoting flow in a counter-current fashion.
Continued reference is made to
The organic baffles 52 are curved in a semi-circular cross-section on each side of each organic outlet 40 of the first, second and third channels 26, 28, 30 (on each side of the weir) and therefore extend into the respective subsequent channel 28, 30, 32 to promote laminar fluid flow through the organic outlets 40. The organic baffles 52 at the organic outlet 40 for the fourth channel 32, however, are disposed in the fourth channel 32 only, and therefore do not extend into the main settler 24.
Referring now to
Each inclined baffle 54 includes an impervious, or solid, central portion 56, a pervious upper portion 58 and a pervious lower portion 60, as best shown in
Referring again to
Inclined baffles 66 also extend across the entire width of the main settler 24, in the flow path of the partially separated phases. Each inclined baffle 66 in the main settler 24 is located proximal a respective end 46 of the main settler 24. Thus, an inclined baffle 66 is located proximal the aqueous outlet 62 of the main settler 24 and another inclined baffle 66 is located proximal the organic outlet 64 of the main settler 24. Each inclined baffle 66 in the main settler 24 is arranged at an angle offset from the vertical for promoting separation of the first phase from the second phase. The angle can be any suitable angle such as, for example, up to 45 degrees offset from the vertical. Although the angle can be any suitable angle, in the present embodiment, each inclined baffle 66 in the main settler 24 is arranged at an angle of about 20 degrees from the vertical, with a top edge of the inclined baffle 66 displaced longitudinally from the bottom edge of the inclined baffle 66 such that the top edge is spaced a greater longitudinal distance from the organic outlet 64 than the bottom edge.
As with the inclined baffles 54 in the channels, each inclined baffle 66 in the main settler 24 includes an impervious, or solid, central portion and pervious upper and lower portions. The inclined baffles 66 are spaced from the bottom of the main settler 24 to permit free flow of fluid under the inclined baffle 54. Similarly, each inclined baffle 66 is spaced from the top of the main settler 24 and from the top of the fluid level to permit free flow of fluid above the inclined baffle 66.
In use, the emulsion of first and second phases from the mixer is centrally introduced to the first channel 26 via the inlet pipe 34. During flow, separation of the first and second phases is initiated in the first channel 26 as the organic and aqueous phases begin forming layers and the initially separated phases flow in opposite directions in the first channel 26 as the top, primarily organic phase flows toward the organic outlet 40 and the bottom, primarily aqueous phase flows toward the aqueous outlet 38. The direction of flow of the organic phase switches in each subsequent channel, thereby alternating flow directions in each subsequent channel. Similarly, the direction of flow of the aqueous phase switches in each subsequent channel, thereby alternating flow directions in each subsequent channel. Thus, the phases flow in counter-current fashion in each of the channels 26, 28, 30, 32.
Phase separation is incrementally improved in each subsequent channel 28, 30, 32 as the phases further separate during flow through each channel. The aqueous and organic baffles 50, 52 aid in promoting laminar flow, reducing turbulence to maintain separation of the phases as the bottom, primarily aqueous phase flows through each aqueous outlet 38 and the top, primarily organic phase flows through each organic outlet 40.
The inclined baffles 54 in the channels 26, 28, 30, 32 further aid in promoting separation of the phases as the phases in the pre-settler 22 flow up against the inclined baffles 54 urging droplets of the organic phase in the aqueous flow upwardly and droplets of the aqueous phase in the organic flow downwardly.
The aqueous phase with some remaining emulsion exits from the aqueous outlet 38 in the fourth channel 26 and into the main settler 24. Similarly, the organic phase with some remaining emulsion exits from the organic outlet 40 in the fourth channel 26 and into the main settler 24. Again, the organic phase 68 and the aqueous phase 70 are layered with the emulsion 72 at the organic/aqueous interface. The organic and aqueous phases 68, 70, respectively, flow in counter-current fashion toward the respective one of the aqueous outlet 62 and organic outlet 64 at opposing ends of the main settler 24, as best shown in
In operation of the above-described settling system, the crud-like material tends toward the interface between the organic and aqueous phases during counter-current flow of the phases through the channels of the pre-settler 22 and the main settler 24. Control of the interfacial crud layer can be effected in any suitable manner, such as by suction of some of the material using a suction pump and pipe (not shown) at the interfacial level in a semi-continuous or continuous operation. The crud can then be separated from any organic and aqueous phases suctioned off and the organic and aqueous phases returned to the pre-settler 22. The crud-like material is inhibited from rising and floating over the weir (organic outlet 64) in the main settler 24 and is inhibited from passing to the aqueous phase when the interfacial crud layer is controlled to an acceptable level. When controlled, the crud layer acts as a filter between the organic and aqueous phases.
In alternative embodiments of the present invention, the pre-settler may include other numbers of channels, rather than four channels as in the above-described embodiment. Further, the pre-settler and the main settler may include any number of inclined baffles or, alternatively, may be without inclined baffles. In a particular alternative embodiment, the baffles are vertical rather than inclined. One such alternative baffle arrangement is shown in the side view of
In other embodiments, the baffles can be any other suitable angle offset from the vertical. The baffles can extend through the entire channel such that the bottom portion of the baffles extends to the bottom of the respective settler. With the baffles spaced from the bottom of the settler and from the top of the organic phase, the baffles can also be impervious. In other alternative embodiments, the pre-settler may be without curved baffles at the aqueous outlets or at the organic outlets. In still other embodiments, the mixer may be in direct connection with the first channel of the pre-settler, for example, so that the mixed phases enter the pre-settler directly, rather than entering through the pipe as described. In yet further embodiments, the main settler can be arranged for co-current flow, rather than counter-current flow of phases.
The present invention provides advantages over prior art co-current settlers in which the crud tends to float to the organic surface and overflow with the organic to the next stage. In such prior art systems, the crud is thereby beaten to a finer material causing a tighter emulsion that does not permit separation in the time during flow through a pre-settler and main settler which results in poor separation and can cause complications such as flooding in which the aqueous phase is carried over the organic outlet weir in large volumes and the organic phase is carried out the aqueous outlet in large volumes.
It will be appreciated that emulsions with high organic to aqueous ratio rise to the top while emulsions with low organic ratio descend in the settler. Emulsions with organic to aqueous ratio nearing 1:1, in which coalescence is minor or negligible, stay in the central zone, between the top and the bottom. In the embodiments described herein, emulsions with high organic to aqueous ratio and low organic to aqueous ratio are transferred from one channel into the next (or into the main settler) in preference to emulsions that have an organic to aqueous ratio nearing 1:1. Thus, emulsions that have an organic to aqueous ratio nearing 1:1 generally stay longer in the channels until coalescence advances. Therefore, stable emulsions have an extended retention time in each channel and in the main settler, aiding in reducing entrainments in phases leaving the settler. Further, control of the interfacial crud layer can be effected in any suitable manner, such as by suction of some of the material using a suction pump and pipe at the interfacial level in a semi-continuous or continuous operation, as described above.
The use of channels in a pre-settler in which the phases flow in a counter-current fashion facilitates the separation of phases and permits increased flow rates, thus increasing throughput of the settling system by comparison to a system employing a pre-settler absent such channels. Thus, increased throughput can be realized in a settling system having an equivalent footprint (or surface area). Alternatively, a similar throughput can be realized in a settling system having a smaller footprint. The addition of inclined baffles further promotes separation of phases permitting a further increase in throughput. It will be appreciated that the capital investment for such a settling system can be reduced with a reduced footprint as the smaller settler system is less costly to manufacture. Using the baffles as described, a much cleaner aqueous phase exits the system.
While the embodiments described herein are directed to particular implementations of the invention, it will be understood that modifications and variations to these embodiments are within the scope and sphere of the present application. For example, the size and shape of many of the features may vary while still performing the same function. Further, the particular arrangement of certain features can vary. For example, the depth of the channels may differ from the depth of the main settler. Many other alterations, modifications and variations can be effected. Those of skill in the art can effect such alterations, modifications and variations to the particular embodiments without departing from the scope of the present application, which is defined by the claims appended hereto.
