A method and a device for preparing brine

Abstract: The invention concerns a method of preparing brine from sodium chloride salt containing one or more water-soluble impurities having an aqueous dissolution rate lower than that of sodium chloride, the method comprising the steps of: (a) forming a salt bed in a vessel, the vessel including a grid for supporting the salt bed; (b) wetting at least a portion of the salt bed with a flow of water; (c) forming a brine solution by at least partially dissolving the wetted portion of the salt bed in the water; (d) forming small solid particles in the wetted portion of the salt bed, said small particles containing one or more water-soluble impurities but substantially no sodium chloride; (e) passing the brine and the small solid particles containing soluble impurities through the grid and out of the salt bed; (f) separating the brine from the small solid particles. The invention also concerns a device for making brine by dissolving salt in water, which device comprises a vessel (3) containing a grid support (4), means (2, 23) for supplying solid salt (1), means (32, 42) for supplying water, means (14, 15) for the outflow of brine, and means (9) for removing undissolved salt particles (8) at the bottom of the vessel (3).

Description: The present invention relates to a method of preparing brine by dissolving salt containing calcium sulfate or other impurities. The uptake of the impurities into the brine is reduced by supplying solid salt and water to a vessel including a grid for supporting a salt bed. The water flows through at least a portion of the salt bed and dissolves the sodium chloride, while a great deal of the impurities pass through the grid and are removed from the solution. The invention also concerns a device for preparing brine by the method described. Brine, i.e. an aqueous solution of sodium chloride, is used as a raw material in several electrochemical processes, such as production of sodium chlorate. The brine is often prepared by dissolving rock salt in water. Even if rock salt mainly consists of sodium chloride, it generally contains considerable amounts of other salts, such as calcium sulfate. One kg rock salt often contains as much as 20 g calcium. Calcium sulfate, as well as some other impurities, are less soluble than sodium chloride. It is desirable to obtain brine that is as concentrated as possible in respect of sodium chloride, preferably saturated. In conventional processes for preparing brine, such as described in for example US patent 2395258, the water flows through a salt bed in a dissolving zone. Some of the calcium sulfate is dissolved and included in the brine, while some is set free and accumulates in the dissolving zone. Such processes involve two main problems. Firstly, the brine produced contains a considerable amount of calcium sulfate, generally more than 400 mg calcium per liter. Especially the calcium must be removed before the brine is used in a chlorate process, which is done by reacting with soda ash. It is also desirable that the content of sulfate is as low as possible. Secondly, the calcium sulfate as well as other impurities which is not dissolved accumulates in the dissolving zone and thus decreases the amount of sodium chloride available for dissolving. In other words, the contact surface between the sodium chloride and the water is decreasing and the contact surface between the impurities and the water is increasing during operation. After some time of operation, the brine produced will therefore be less concentrated in respect of sodium chloride but having a higher content of impurities, such as calcium sulfate. The dissolving device must then be taken out of operation and cleaned from calcium sulfate and other accumulated impurities. GB patent specification 1009736 discloses a method of producing pure solid anhydrous sodium chloride from impure solid salt by partly dissolving the salt in saturated circulating brine, the solid salt being supplied to a screen basket and the undissolved crystals falling through the screen. The patent does not deal with the production of brine or any other aqueous solutions of salts. US patent 3168379 discloses a method and an apparatus for dissolving salt in which a salt bed is supported on a perforated plate. Fresh water is supplied at only one level within the salt bed. Accordingly, prior art does not disclose any method of avoiding the above mentioned problems of dissolving rock salt containing calcium sulfate or other soluble impurities. Any suitable device to be used is not disclosed either. It is therefore an object of the invention to provide an easy method of preparing brine by dissolving salt, such as rock salt, containing calcium sulfate and/or other soluble impurities, and obtaining brine with a lower content of such impurities than prior art methods. It is another object of the invention to provide a method of avoiding accumulation of impurities in the dissolving device by continuously removing undissolved matter, such as calcium sulfate, from the dissolving zone. It is still another object to provide a simple device to be used in a process according to the invention. The objects of the invention can be achieved with a method and a device as claimed. The invention is based on the fact that the dissolution rate of calcium sulfate is lower than the dissolution rate of sodium chloride. The dissolution of calcium sulfate can therefore be minimized by minimizing the contact time and the contact surface between the calcium sulfate and the water, but still keeping the contact time and the contact surface between the sodium chloride and the water large enough for obtaining brine of desired concentration in respect of sodium chloride. Thus, the claimed invention concerns a method of preparing brine from sodium chloride salt containing one or more water-soluble impurities having an aqueous dissolution rate lower than that of sodium chloride, the method comprising the steps of: (a) forming a salt bed in a vessel, the vessel including a grid for supporting the salt bed; (b) wetting at least a portion of the salt bed with a flow of water supplied through a plurality of spray nozzles distributed throughout the wetted portion of the salt bed; (c) forming a brine solution by at least partially dissolving the wetted portion of the salt bed in the water; (d) forming small solid particles in the wetted portion of the salt bed, said small particles containing one or more water-soluble impurities but substantially no sodium chloride; (e) passing the brine and the small solid particles containing soluble impurities through the grid and out of the salt bed; (f) separating the brine from-the small solid particles. Generally, the small solid particles passing through the grid mainly consist of calcium sulfate and other impurities, and contain substantially no sodium chloride. Accordingly, by the method of the invention the calcium sulfate and the other impurities are continuously removed from the wetted portion of the salt bed, i.e. the dissolving zone. It is preferred that the grid is arranged substantially horizontally in the vessel and that the water is supplied above the grid, preferably through spray nozzles. If the water is supplied within the salt bed it will then flow through and wet the lower portion of the salt bed. It is often desirable to obtain brine that is substantially saturated in respect of sodium chloride. In such cases the dissolution of impurities is reduced by keeping the contact time and the contact surface between the salt and the water just sufficient for obtaining a substantially saturated solution of sodium chloride. Suitably a steady state of brine in the salt bed is maintained through control of the water flow rate to the salt bed. Thereby the wetted portion of the salt bed, i.e. the zone between the upper level of brine in the salt bed and the grid, defines the dissolving zone, and the free volume below the grid is filled with brine. Suitable flow rate of the water depends on the desired concentration of the brine, the salt used, the temperature and pH of the water, as well as on the size and design of the dissolving device. In most cases it is preferred that the average residence time of the water in the dissolving zone is from about 1.5 to about 5 minutes, preferably from about 1.8 to about 3 minutes, most preferably from about 2 to about 2.5 minutes. If the residence time is too low the brine will be less concentrated, and if it is too high the brine will include too much calcium sulfate or other impurities. In many cases it is preferred that the average residence time is just sufficient for obtaining a substantially saturated solution of sodium chloride. The water supplied should suitably be distributed as uniformly as possible throughout the dissolving zone, which helps keeping a big contact surface between the sodium chloride and the water. Preferably it is supplied through a plurality of spray nozzles throughout the wetted portion of the salt bed, i.e. the dissolving zone. The upper nozzles are then preferably situated just above the brine level, i.e. just above the dissolving zone, while the lower nozzles preferably are situated just above the grid. It has been found suitable to use from 1 to 10 spray nozzles per 100 litre volume of the dissolving zone. The highest capacity is obtained if the openings in the grid supporting the salt bed constitutes as big share of the total area as possible, i.e. the free area is as big as possible. However obvious to any person of ordinary skill in the art that any design of the grid permits production of brine according to the invention, it is preferred that the grid has a free area of at least 20%, most preferably at least 50%. The free area of the grid, in addition of facilitating the outflow of brine, also allows passage of undissolved particles of calcium sulfate and other impurities from the dissolving zone. Suitable size of the openings depends on the size distribution of the salt particles to be dissolved. If the salt particles supplied substantially ranges from about 1 to about 10 mm a preferred grid has openings with an effective size from about 1 to about 10 mm, most preferably from about 3 to about 8 mm. The shape of the openings is not critical. It is preferred that the small particles passing through the grid are collected at the bottom of the vessel, wherefrom they are removed, continuously or discontinuously, e.g. with a scraper or by flushing water. Preferably they are removed discontinuously by stopping the supply of water to the dissolving zone, flushing water from one end at the bottom and pumping out the slurry from the other end at the bottom. It is preferred to separate the particles from the slurry and reuse the water for flushing. Preferably the brine is passed out from the vessel through a pipe with an opening situated below the grid but preferably above the bottom of the vessel, most preferably above the top level of the possible bed of undissolved solid particles settled at the bottom of the vessel, and optionally through a mesh filter for stopping solid particles. It is also preferred to pass the brine through a sedimentation tank to ensure that no solid particles are carried on to the process in which the brine is used, such as a process of preparing sodium chlorate. It has been found that the pick up of calcium sulfate into the brine can be further minimized if the water supplied has a pH value above about 7. It is generally not economic to keep the pH value above about 12. A preferred range is from about 9 up to about 11. Suitable pH value can be obtained by adding alkali metal hydroxide, such as sodium hydroxide, to the fresh water. The pick up of calcium sulfate also decreases with increasing temperature, but above about 40°C the improvement is small. Even if any temperature of the water supplied between 0 and 100°C is possible the preferred temperature range is from about 20 to about 40°C, most preferably from about 30 to about 35°C. The pick up of calcium sulfate may also be decreased if the water supplied contains sodium sulfate through common ion effect. It is possible to recirculate a portion of the brine for obtaining water containing sodium sulfate. The invention also concerns a device for preparing brine by dissolving salt, such as rock salt, in water. The device comprises a vessel containing a preferably substantially horizontally arranged grid support, preferably with a free area of at least 20%, most preferably at least 50%, preferably above the grid situated means for supplying solid salt and means for supplying water, including a plurality of spray nozzles distributed throughout the wetted portion of the salt bed. Further, it comprises means for the outflow of brine, preferably including a pipe with an opening situated below the grid, a mesh filter, and a sedimentation tank. Most preferably the opening of the pipe for the outflow of brine is situated above the bottom of the vessel. The device also comprises means for removing undissolved particles from the bottom of the vessel, preferably including at least one spray nozzle for flushing water and means for pumping out the slurry from the vessel. By performing the present invention the brine produced contains only about 10 to 30 percent by weight of the calcium from the salt used as a raw material. The lower content of calcium will reduce the amount of soda ash consumed for purifying the brine prior to use. Some other impurities, such as barium and strontium will also be excluded to a high extent from the brine produced. A device according to the invention has a very high capacity and does not have to be taken out of operation for cleaning very often and the cleaning goes very quick. A preferred embodiment of the invention will now be described in detail with reference to the drawings. However, it is obvious to any person with ordinary skill in the art that many other embodiments are possible within the scope of the invention defined by the claims. Figure 1 schematically shows a sectional side view of a plant for preparing brine by dissolving rock salt in water, figure 2 shows a top section of the dissolving device and figure 3 shows a side section of a portion of the dissolving device. Referring to fig 1, a silo 2 for storing ground rock salt is situated above a cylinder shaped vessel 3 containing a horizontal grid 4 covering the entire cross section. The silo includes a knife gate 23 for supplying the salt to the vessel. A preferred grid 4 comprises a standard steel grating covered with a wire mesh determining the effective size of the openings. However, any construction of a grid with suitable free area and size of the openings can be used. Each one of two pipes 30, 40 is connected to a separate set of spray nozzles for fresh water. The nozzles are distributed substantially uniformly substantially within a zone above the grid 4, i.e. the dissolving zone. The bottom of the vessel 3 includes spray nozzles 9 for flushing water and at least one outlet 10 at the opposite side connected to a pump 11. Downstream the pump 11, means 12 are provided for separation of solid particles from water, preferably including a sedimentation tank and a filter press. The vessel 3 further includes a funnel shaped brine outlet 14 with the opening faced downwards and situated below the grid 4. Preferably a mesh filter covers the opening of the funnel shaped outlet 14, which is connected to a pipe 15 ending in a sedimentation tank 16. A pipe 19 connects the bottom of the tank 16 to the pump 11. Another pipe 20 connects the tank 16 to a brine storage tank not shown. Fig 2 shows a top view of the upper of the two sets of spray nozzles in the vessel 3. The water supply pipe 30 leads to a connection 31 in the center of the vessel 3. The connection forms the center of a star consisting of six pipes 32, each with a plurality of holes 33 functioning as spray nozzles, extending against the mantle of the vessel 3. The lower set of spray nozzles has a similar star-like shape from above except that there are eight pipes 42 extending from the connection 41 in the center of the vessel 3. Fig 3 shows a side section of a portion of the vessel 3 from the center to the mantle. Only one of six upper pipes 32 and one of eight lower pipes 42 are shown. The salt particles in the bed are also omitted. It appears that each one of the upper pipes 32 has a horizontal portion 35 and a vertical portion 36. The holes 33 are uniformly distributed along the pipes 32 in both the sections 35, 36. The vertical portion 36 ends with a conical spray nozzle 37. The lower set of spray nozzles consists of a water supply pipe 40 leading too a connection 41 in the center of the vessel 3 and pipes 42, each with a plurality of uniformly distributed holes 43 functioning as spray nozzles, extending from the connection 41 against the mantle of the vessel 3. The pipes 42 have no vertical portions and no spray nozzles at their ends. Preferred size of the holes 33, 43 is from about 1 to about 6 mm. However, it is obvious to any person with ordinary skill in the art that any construction of the water supply means that gives an essentially uniform distribution of water in the lower portion of the salt bed, i.e. the dissolving zone, may be used in the present invention. For instance, the number of pipes 32, 42 extending from the connections 31, 41 could be anything from 3 to 25 in each set of spray nozzles depending on the size of the vessel 3. A preferred method of preparing brine according to the invention will now be described with reference to the figures 1, 2 and 3. Ground rock salt stored in the silo 2 is supplied to the vessel 3 and forms a salt bed 1 filling the entire volume between the grid 4 and the silo 2. Water is distributed uniformly throughout the dissolving zone by spraying through the holes or nozzles 33, 37, 42 in the pipes 32, 41. The water flows through and wets the lower portion of the salt bed 1, i.e. the dissolving zone, and dissolves at least the sodium chloride. The brine thus obtained fills the volume below the grid 4, i.e. the settling zone, as well as the free volume of the dissolving zone within the salt bed 1. Its level 7 is just below the horizontal portion 35 of the pipes 32 in the upper set of spray nozzles. The water flow is just sufficient to obtain a substantially saturated sodium chloride solution, whereby small particles mainly consisting of calcium sulfate form. These particles 8, as well as small particles of insoluble material, such as sand and small stones etc, continuously fall through the grid 4 and settle at the bottom. Big insoluble material, such as wood chips, wires, stones etc, are collected on the grid. Brine flows through the outlet 14 to the sedimentation tank 16, where it is separated from particles carried on, and finally flows through the pipe 20 to a storage tank. At steady state the brine in the vessel 3 and in the sedimentation tank 16 will automatically be at the same level 7, 17. Before the top level of the particles 8 collected at the bottom of the vessel 3 comes too close to the brine outlet 14, generally about every 5 to 20 hours, they have to be removed. At this point, the inlet water to the dissolving zone of the vessel 3 is stopped and flush water is sprayed through the nozzles 9 and the slurry is pumped out through the outlet 10 to the device 12 where the solid particles are separated from the water which is recirculated. The brine produced flows through the outlet 14 and the pipe 15 to the sedimentation tank 16, where solid particles carried on fall to the bottom. These particles 18 are removed in a similar way. Water is flushed through the nozzle 24 and the particles 18 are pumped out through the pipe 19 and the pump 11. The brine from the sedimentation tank 16 is passed through the pipe 20 to a brine storage tank. After dissolving about 300 to 1000 tons of rock salt the grid 4 has to be cleaned. At this point, the supply of rock salt is stopped by closing the knife gate 23 and the water supply is continued to dissolve the remaining salt above the grid. When there is no soluble salt left, the grid is cleaned manually and the unit restarted. EXAMPLE : A full scale dissolver according to the drawings was used for dissolving ground rock salt containing 1.0 to 1.2 percent CaSO₄ by weight on an average. Less than 10 % of the particles were bigger than 10 mm and less than 20 % were smaller than 1 mm. The volume of the dissolving zone was 970 liters, the volume of the settling zone was 2200 liters and the cross section area was 1.8 m². The grid was made from standard steel gratings of 25 x 25 x 102 mm size covered with wire mesh reducing the effective opening to 6 mm. The height of the salt bed was 1.2 m. Water was supplied with a flow varying from 341 to 416 liters/minute through the upper set of spray nozzles and from 76 to 151 liters/minute through the lower set. The total flow was varying from 420 to 541 liters/minute. Thus the average residence time of water in the dissolving zone was varying from 2.0 to 2.6 minutes. The concentration of NaCl in the brine produced varied from 290 to 300 g/litre. When the pH of the brine varied from 6.1 to 6.7 the concentration of calcium in the brine varied from 195 to 308 mg/litre. When the pH varied from 9.1 to 11.1 the calcium content varied from 74.4 to 141 mg/litre.

Claims: A method of preparing brine from sodium chloride salt containing one or more water-soluble impurities having an aqueous dissolution rate lower than that of sodium chloride, comprising the steps of: (a) forming a salt bed in a vessel, the vessel including a grid for supporting the salt bed; (b) wetting at least a portion of the salt bed with a flow of water; (c) forming a brine solution by at least partially dissolving the wetted portion of the salt bed in the water; (d) forming small solid particles in the wetted portion of the salt bed, said small particles containing one or more water-soluble impurities but substantially no sodium chloride; (e) passing the brine and the small solid particles containing soluble impurities through the grid and out of the salt bed; (f) separating the brine from the small solid particles; characterised in that the water in step (b) is supplied through a plurality of spray nozzles distributed throughout the wetted portion of the salt bed. A method as claimed in claim 1, wherein the grid is arranged substantially horizontally and the water is supplied above the grid. A method as claimed in any of the claims 1 or 2, wherein a steady state of brine in the salt bed is maintained through control of the water flow rate to the salt bed, wherein the wetted portion of the salt bed defines the dissolving zone and the free volume below the grid is filled with brine. A method as claimed in any of the claims 1-3, wherein the average residence time of water in the wetted portion of the salt bed is just sufficient for obtaining a substantially saturated solution of sodium chloride. A method as claimed in any of the claims 1-4, wherein the small solid particles passing through the grid are collected at the bottom of the vessel, wherefrom they are removed. A method as claimed in any of the claims 1-5, wherein the brine is passed out from the vessel through a pipe with an opening situated below the grid but above the bottom of the vessel. A device for making brine by dissolving salt in water the comprising a vessel (3) containing a grid support (4), means (2, 23) for supplying solid salt (1), means (32, 42) for supplying water, means (14, 15) for the outflow of brine, and means (9) for removing undissolved salt particles (8) at the bottom of the vessel (3), characterised in that the means (32, 42) for supplying water comprise a plurality of spray nozzles arranged throughout the wetted portion of the salt bed. A device as claimed in claim 7, wherein the grid (4) is arranged substantially horizontally and the means (2, 23, 32, 42) for supplying salt and water are situated above the grid (4). A device as claimed in any of the claims 7-8, wherein the means for supplying water includes a plurality of spray nozzles (33, 37, 43) distributed essentially uniformly in a zone above the grid (4). A device as claimed in any of the claims 7-9, wherein the means for the outflow of brine includes a pipe (15) with an opening (14) situated below the grid (4) but above the bottom of the vessel (3).

Inventors: TEWARI MOHAN PRASAD, TEWARI, MOHAN PRASAD
Applicants: EKA NOBEL AB
Application year: 1991
Main classification: C01D3
IPC codes: C01D3