Process for producing sodium chloride crystals

Abstract: Disclosed is a flow-through sodium chloride crystallizer comprised of a vertically-oriented crystallizer body (11) having at least two segments (12,13) of different cross-sectional area, at least two inlets for the introduction of saturated MgCl₂ brine (16,17) at a minimum of two distinct vertical locations having different cross-sectional areas along the crystallizer body, at least two inlets for the introduction of a saturated NaCl brine (18,19) at a minimum of two distinct vertical locations having different cross-sectional areas along the crystallizer body, at least one lower outlet (20) for removal of NaCl crystals and at least one upper outlet (21) for removal of spent brine. Also disclosed is an apparatus for producing hopper-shaped NaCl crystals and/or NaCl crystals of varying, segregated purity, and a process for producing the same.

Description: This invention is directed to a process which is particularly suitable for producing high purity sodium chloride. Historically, high purity sodium chloride is manufactured in heated enclosed evaporators to vaporize solvent water. Production of brine from sodium chloride requires energy, typically obtained from fossil fuels. In order to reduce fossil fuel usage in sodium chloride production, various arrangements of brine heaters and evaporators have been designed and operated. Typical arrangements to enhance efficiency employ multi-effect evaporation and vapor recompression. Sodium chloride produced in steam heated enclosed vessel evaporators is usually granular in size and cubic in crystal geometry. However, as mentioned above, sodium chloride crystal geometry is not necessarily cubic. For example, the Alberger process and the grainer process produce hopper-shaped crystals by open pan methods. Dentritic salt may be produced in enclosed vessels by introducing foreign substances (crystal habit modifiers) into the contained brine. Flat flakes may be produced by the compression of granular sodium chloride. Solar vaporization of the aqueous solvent in a brine to produce "solar salt" is an ancient, low-cost alternative to the earlier presented methods of sodium chloride production. In its simplest form, solar salt is produced by exposing aqueous sodium chloride brines to sunlight, evaporating some or all of the aqueous solvent to cause concentration and thus crystallizing dissolved constituents (such as sodium chloride). The major disadvantage of solar salt is the reduced sodium chloride purity when compared to vacuum evaporated sodium chloride. Another well-known process, sometimes, referred to as the "salting out" process, provides a relatively low energy, low cost method for the production of sodium chloride. In a salting out process, two or more aqueous solutions, each containing a single solute, or more typically, multiple solutes, are combined. When combined, the resulting aqueous mixture contains two or more solutes such that the solubility of one or more of the solutes is exceeded. For example, U.S.-A-3 832 143 discloses methods for making table-grade sodium chloride by mixing two brines having two distinct magnesium chloride concentrations, but each substantially saturated with respect to sodium chloride, to form a crystal crop of table-grade sodium chloride and a brine depleted in sodium chloride. The saturated brines may be prepared by solar evaporation of initial or starting brines such as ocean brines and Great Salt Lake brines. Specifically, Example II of U.S.-A-3 832 143 teaches mixing in a reactor crystallizer a first brine containing, among other things, 1,2 wt. percent Mg, 8,0 wt. percent Na and 14,8 wt. percent Cl and a second brine containing, among other things, 7,4 wt. percent Mg, 0,6 wt. percent Na and 20,2 wt. percent Cl. First a slurry, then substantially pure NaCl, are produced. U.S.-A-3 832 143 also discloses recycle of the depleted brine through the solar evaporation system. It should additionally be noted that U.S.-A-3 772 202 discloses use of a solar pond to concentrate a bitterns brine which contains NaCl but predominates in magnesium chloride. U.S.-A-3 852 044 discloses a solar evaporation system which produces sodium crystals, potassium minerals and an aqueous solution concentrated at least near magnesium chloride saturation. As discussed in detail in Sodium Chloride: The Production and Properties of Salt and Brine, Dale W. Kaufmann, 1971, pages 15-21, sodium chloride crystals are generally in the form of a simple cube without any modifying faces. However, the presence of certain foreign substances in the solution from which salt is crystallizing may cause modifications. Skeleton-type crystals (either octahedrons or combinations of octahedron and cube) may result from rapid crystal growth without the presence of a foreign substance. Further, hopper-shaped cubes can be produced by rapid crystal growth which is parallel to octahedron and dodecahedron faces. By filling out the cube edges and corners, hopper-shaped depressions are formed at the center of each of the cube faces. Such hopper-shaped cubes are particularly preferred where rapid dissolving is desired, such as in some food and seasoning applications and certain agricultural and chemical uses. High purity is another frequently desired characteristic of sodium chloride. High purity may be desired in combination with rapid dissolution, as in the situations mentioned above, or high purity may be wanted for uses such as water softening. The present invention relates to a process for the production of high purity NaCl crystals. Additionally, a surprisingly high percentage of high purity NaCl crystals produced by the process of the current invention are hopper-shaped cubes. The process according to the current invention may be carried out in a flow-through sodium chloride crystallizer comprised of a vertically oriented crystallizer body having at least two segments of different horizontal cross-sectional area, at least two inlets for the introduction of saturated MgCl₂ brine at a minimum of two distinct vertical locations having different horizontal cross-sectional areas along the crystallizer body, at least two inlets for the introduction of a saturated NaCl brine at a minimum of two distinct vertical locations having different cross-sectional areas along the crystallizer body, at least one lower outlet for removal of NaCl crystals and at least one upper outlet for removal of spent brine. The current invention discloses a process for the preparation of NaCl crystals of varying, segregated purity comprising: (a) introducing an upper saturated MgCl₂ brine stream into a vertically oriented crystallizer body at an upper segment of said crystallizer body; (b) introducing a lower saturated MgCl₂ brine stream into said crystallizer body at a lower segment of said crystallizer body, said upper segment having a greater horizontal cross-sectional area than said lower segment; (c) introducing an upper saturated NaCl brine stream into said crystallizer body at said upper segment of said crystallizer body; (d) introducing a lower saturated NaCl brine stream into said crystallizer body at said lower segment of said crystallizer body; (e) allowing said brine streams introduced in steps (a) through (d) to mix in said crystallizer body to produce NaCl crystals and a spent brine stream; (f) removing said spent brine stream from the upper portion of said crystallizer body; and (g) removing said NaCl crystals from the lower portion of said crystallizer body. As used in this description of the invention, "saturated MgCl₂ brine" and "saturated NaCl brine" have the definitions provided below. The phrase "saturated NaCl brine" refers to a substantially saturated brine having NaCl as a principal component and may contain lesser amounts of other inorganic components including but not limited to compounds and/or ions of Mg, K, Ca, SO₄ and Br. FIG. 1 illustrates schematically a preferred embodiment of a sodium chloride crystallizer for carrying out the process of the present invention. At a minimum, the sodium chloride crystallizer is comprised of a vertically oriented crystallizer body (11) and the various inlets (16) through (19) and outlets (20) and (21) further described below. Crystallizer body (11) is comprised of upper segment (12), lower segment (13) and transition segment (14). Upper segment (12) has a greater horizontal cross-sectional area than that of lower segment (13). Although the current invention is not limited to the following specifications, it is typical for the cross-sectional area ratio of upper segment (12) to lower segment (13) to be about 2:1 to about 25:1 and preferably about 3:1 to about 6:1. Transition segment (14) is provided to supply fluid communication between upper segment (12) and lower segment (13). Optionally, crystallizer body (11) may have a second transition segment (15) below lower segment (13). Upper saturated MgCl₂ brine inlet (16) and lower saturated MgCl₂ brine inlet (17) are positioned at distinct vertical locations along crystallizer body (11), with the caveat that such locations have different horizontal cross-sectional areas. Although in FIG. 1 upper saturated MgCl₂ brine inlet (16) is located in upper segment (12) and lower saturated MgCl₂ brine inlet (17) is located in lower segment (13), it is possible, for example, to position upper saturated MgCl₂ brine inlet (16) in transition segment (14) and lower saturated MgCl₂ brine inlet (17) in optional second transition segment (15). Upper saturated MgCl₂ brine inlet (16) and lower saturated MgCl₂ brine inlet (17) are fed respectively by upper MgCl₂ brine conduit (22) and lower MgCl₂ brine conduit (23). Upper saturated NaCl brine inlet (18) and lower saturated NaCl brine inlet (19) are positioned along the crystallizer body (11), with the caveat that such locations have different cross-sectional areas. Although in FIG. 1 upper saturated NaCl brine inlet (18) is located in upper segment (12) and lower saturated NaCl brine inlet (19) is located in optional second transition segment (15), it is possible, for example to position upper saturated NaCl brine inlet (18) in transition segment (14) and lower saturated NaCl brine inlet (19) in lower segment (13). Upper saturated NaCl brine inlet (18) and lower saturated NaCl brine inlet (19) are fed respectively by upper NaCl brine conduit (25) and lower NaCl brine conduit (26). Lower outlet (20) is positioned in either the optional lower transition segment (15) or (particularly if lower transition segment (15) is not present) in lower segment (13). Lower outlet (20) is provided for the removal of a slurry of high purity (typically greater than 99,80% purity) sodium chloride. Also, a surprisingly high percentage of such sodium chloride will be in the form of hopper-shaped cubes. Conduit (28) is provided for further transport of the slurry. Typically, the sodium chloride slurry is transported to dehydration equipment (such as centrifuges) for removal of water from the slurry. Upper outlet (21) is provided for the removal of spent brine from the sodium chloride crystallizer. In addition to the components described above, the crystallizer may be comprised of additional segments of varying horizontal cross-sectional area, additional transition segments and additional inlets and outlets for the MgCl₂ brines, NaCl brines, slurries, and crystallized sodium chloride. The process of the current invention may be further described with reference to FIG. 2. The apparatus is comprised of one or more sodium chloride crystallizer body (11). Sodium crystallizer body (11) is illustrated in detail at FIG. 1 and described in detail above. Each sodium chloride crystallizer body (11) is fed by at least two saturated MgCl₂ brine streams through upper saturated MgCl₂ brine inlet (16) and lower saturated MgCl₂ brine inlet (17). Inlets (16) and (17) are supplied via upper MgCl₂ brine conduit (22) and lower MgCl₂ brine conduit (23), respectively. Conduits (22) and (23) may be fed from one saturated MgCl₂ brine conduit (24) (as shown in FIG. 2) or from separate saturated MgCl₂ brine supply sources. Each sodium chloride crystallizer body (11) is also fed by at least two saturated NaCl brine streams through upper saturated NaCl brine inlet (18) and lower saturated NaCl brine inlet (19). Inlets (18) and (19) are supplied via upper NaCl brine conduit (25) and lower NaCl brine conduit (26), respectively. Conduits (25) and (26) may be fed from one saturated NaCl brine conduit or from separate saturated NaCl brine supply sources. A slurry of sodium chloride crystals in brine is removed from crystallizer body (11) via lower outlet (20) and transported via conduit(s) (28) to a system suitable and adapted for removing brine from the sodium chloride crystals. An illustrative brine removal system shown in FIG. 2 is comprised of wash vessel (29), centrifuge (30) and dryer (31). The brine removed from wash vessel (29) and centrifuge (30) may be recycled back to crystallizer body (11) via conduits (32) and (33), and, eventually conduit (25). Spent brine is removed from crystallizer body (11) via upper outlet (21) and routed for further handling via conduits (34) and (35). The spent brine is preferably reconcentrated in MgCl₂ for recycling to crystallizer (11). Many means are available to reconcentrate such streams. Preferably, the spent brine is reconcentrated by a solar evaporation system substantially similar to the one illustrated at FIG. 2 and described below; however, the spent brine may be first removed to holding tank (36) prior to reconcentration. A solar evaporation system requires at least one solar pond. Most preferably, the solar evaporation system of the current application is comprised of first solar evaporation pond (37), second solar evaporation pond (38) and third solar evaporation pond (39). As described in detail in Example 1 below, a three stage system to reconcentrate MgCl₂ provides three batches of NaCl segregated by purity. In a preferred embodiment, the NaCl from the first solar pond has an approximately 99,0 to 99,7% purity, the NaCl from the second solar pond has an approximately 98,5% to 99,0% purity and the NaCl from the third solar pond has a purity of less than about 98,5%. In an even more preferred embodiment, the MgCl₂ brine stream produced by the first solar pond comprises approximately 5,0 to 6,0 weight % MgCl₂, the MgCl2 stream of the second pond approximately 6,0 to 7,0 weight % and the stream of the third pond 7,0 to 8,0 weight % MgCl₂. Optionally, the solar evaporation system may contain a spent brine holding tank (36) and a reconstituted storage tank (40). Alternatively, it may also be desirable to provide filter (41) and saturator (42) for solids removal and storage tank (43) for holding the reconstituted, filtered saturated MgCl₂ brine prior to supply to crystallizer body (11) via conduits (24), (22) and (23). One method of supplying NaCl saturated brine for supply to crystallizer body (11) is illustrated at FIG. 2. The method involves preparing "pickle brine" (brine which has undergone fractional crystallization in a so-called "lime pond" to remove a preponderance of the contaminant calcium sulfate) then storing it at the the proper sodium concentration in NaCl brine tank (44). Additionally, it may be desirable to provide filter (45) and saturator (46) for solids removal and storage tank (47) to hold the filtered sodium saturated brine prior to supply to crystallizer (11) via conduits (27), (26) and (25). As demonstrated by the examples and data which follow, the process of the current disclosure produces high purity NaCl crystals. Additionally, a surprisingly high percentage of high purity NaCl crystals produced by the process of the current invention are hopper-shaped cubes. Example 1: Process Material Balance The process of the current invention is demonstrated by the material balance described in this Example 1. It is understood that this material balance is only an exemplary process within the scope of the current invention. It is intended only as one working example of the current invention. Neither this Example 1 nor the following examples are intended to limit the scope of this invention. This Example 1 is discussed with reference to FIGS. 1 and 2 and Table 1. This material balance was based on a daily production of 500 tons hopper-shaped NaCl as output (through one or more conduit(s) (28) from one or more crystallizer body (11). The composition of the NaCl product stream J (in slurry form) is detailed in Table 1. Approximately 50 to 80% of the NaCl crystals are hopper-shaped cubes. The hopper-shaped NaCl product may be further treated in wash vessel (29), centrifuge (30) and dryer (31) to produce dried, purified hopper-shaped salt granules. In operation a substantially saturated NaCl brine (1020 gpm) of composition I is introduced into one or more crystallizer body (11) via conduits (25) and (26) to mix with a substantially saturated MgCl₂ brine (870 gpm) of composition H which is introduced into crystallizer body (11) via conduits (22) and (23). The spent brine of composition A is removed via conduit (34) and transported via conduit (35). Most desirably the spent brine is reconstituted and recycled to the one or more crystallizer body (11). Any reconstitution means which will provide an appropriate saturated MgCl₂ brine for the crystallizer body (11) is acceptable in the current process. However, a solar evaporation system is employed for this Example 1. In operation of the solar evaporation system, spent brine may be transported directly to first solar pond (37). Alternatively, the spent brine may be sent to spent brine holding tank (36) prior to introduction into first solar pond (37). First solar pond (37) is of sufficient size to allow concentration of the brine to composition C. First solar pond (37) produces approximately 146.842 tons per year of about 99,6% purity cubic NaCl having composition B, a brine of composition C and about 587 million litres (155 million gallons) per year water of evaporation. Brine of composition C is further concentrated, which is accomplished in this example by use of secondary solar pond (38) and tertiary solar pond (39). The brine of composition C is transferred to secondary solar pond (38) having dimensions to produce a brine of composition E. Secondary solar pond (38) produces 108.216 tons per year of about 99,5% purity cubic NaCl having composition D, a brine of composition E and about 382 million litres (101 million gallons) per year water of evaporation. The brine of composition E is transferred to tertiary solar pond (39) having dimensions to produce a brine composition G. Tertiary solar pond (39) produces 45.360 tons per year of mixed salts having composition F, a brine of composition G and about 95 million litres (25 million gallons) per year water of evaporation. The mixed salts are predominantly chloride and sulfate salts of sodium, potassium and magnesium. They may be discarded or treated for further recovery of the salts. The brine of composition G is typically sent to reconstituted storage tank (40) prior to treatment in filter (41) and saturator (42), then holding in storage tank (43) prior to use in crystallizer body (11). It should be noted that in addition to producing a saturated MgCl₂ brine stream, the tertiary solar evaporation system of this Example produces NaCl of size and shape equivalent to that produced by conventional solar evaporation but segregated into three distinct purities. In its preferred embodiment, the saturated NaCl brine stream I is prepared by producing a so-called "pickle" brine in a "lime pond" (not shown) to remove the impurity calcium sulfate from the brine by fractional crystallization. The pickle brine of composition P may be stored in NaCl brine tank (44), treated in filter (45) and saturator (46), then stored in storage tank (47). The brine transferred from saturator (46) to storage tank (47) may have the composition I or it may have a composition so that when mixed with composition K in storage tank (47) the output of storage tank (47) will have the composition I. In order to produce "make-up" saturated MgCl₂ brine of concentrations to replenish the recycled saturated MgCl₂ brine for feed to crystallizer body (11), a portion of the "pickle brine" of composition P may be treated by solar evaporation processes similar to the evaporation process of solar ponds (37), (38) and (39). The portion of "pickle brine" is treated seriatum in solar ponds (50), (51) and (52) to produce brines of composition Q, R and S. Additionally, approximately 210.600 tons per year of "conventional" solar salt is produced from solar pond (50). A portion of the "conventional" solar salt crop (approximately 31.200 tons per year) may be dissolved in tank (48) to produce a substantially saturated NaCl solution of Composition N. Brine filter (49) may be used to remove suspended particulate contaminants. Brine of composition N may be mixed with brine of composition M removed from centrifuge (30) to provide co-mingled brine of composition L. This co-mingled brine may be used to wash the salt produced in crystallizer body (11) free of adhering mother liquor to produce a washed, purified NaCl of purity greater than 99,8%. Description of flowstreams of the material balance of Example 1 Flow stream Composition (% by weight) Flowrate (tons/yr) Mg⁺² Na⁺ K⁺ Ca⁺² Cl⁻¹ SO₄⁻² H₂O A 4,56 3,18 0,69 16,74 3,03 71,76 3.821.810 B ,06 39,14 ,05 0,06 60,53 0,16 155.880 C 5,76 1,93 1,57 18,10 3,66 68,98 2.888.496 D 0,11 39,09 0,08 0,08 60,38 0,26 112.392 E 6,94 1,08 1,48 19,60 4,35 66,55 2.247.192 F 0,08 39,03 0,06 0,10 60,43 0,30 99.648 G 7,50 0,64 0,87 21,00 3,72 66,27 1.995.840 H 7,50 0,68 0,77 21,04 3,84 66,17 287,136* I 1,02 8,75 0,56 15,56 1,90 72,21 306,662* J 1,05 8,73 0,49 15,70 1,71 72,31 42,155* K 0,01 9,20 0,36 15,85 1,24 72,58 57,138* L NOT DETERMINED M NOT DETERMINED N 0,01 10,51 0,01 0,01 16,23 0,01 73,22 15.43 * O 0,02 0,01 0,03 99,94 11.102* P 1,08 8,65 0,60 15,50 2,05 72,12 1.796.400 Q 1,50 7,40 0,80 15,10 3,00 72,20 R 3,50 5,50 2,20 14,20 6,70 67,66 S 7,50 0,50 0,80 20,50 3,80 66,90 71.539 Indicates "tons/h" rather than "tons/yr". Example 2: Crystallizer of the current invention A crystallizer similar to that illustrated in FIG. 1 was employed. With reference to FIG. 1, the crystallizer body had the following dimensions. Upper Segment (12) 69 cm (27") long x 5,1 cm (2") I.D. Lower Segment (13) 112 cm (44") long x 2,5 cm (1") I.D. Transition Segment (14) 7 cm (2,75") long x 5,1 cm (2") I.D. x 2,5 cm (1") I.D. Second Transition Segment (15) 4,4 cm (1,75") long x 2,5 cm (1") I.D. x 0,95 cm (0,375") I.D. The saturated MgCl₂ brine streams and the saturated NaCl brine streams were introduced at locations as illustrated in FIG. 1. The operating data are provided at Table 2 for these separate runs through the above-described crystallizer. Runs A-C were conducted at ambient temperature and pressure. The NaCl product from Runs A-C, as determined by microscopic evaluation was approximately 75 wt.% hopper-shaped cubes and 25 wt.% simple cubes. Example 3: Comparative Example of the Salting Out Process This Example 3 demonstrates a "salting out" process generally of the type described in U.S.-A-3 832 143. A funnel-shaped vertical reactor was employed. From top to bottom, the reactor comprised a 15,2 cm (6,0") long x 45,7 cm (18,0") I.D. top section, a 30,5 cm (12") long x 45,7 cm (18,0") I.D. x 8,26 cm (3,25") I.D. transition section and a 61 cm (24") long x 3,25 elutrating leg. Substantially saturated MgCl₂ brine and substantially saturated NaCl were simultaneously fed through separate inputs into the base of the vertical reactor. The mixed brine was then circulated from the top of the reactor into the bottom of the elutrating leg. The time of the runs varied between 4 to 12 hours. The saturated MgCl₂ brine input and the saturated NaCl input were each fed to the reactor at a rate of 0,250 ℓpm (0,066 gpm). The total flowrate to the reactor varied between 8,06 ℓpm (2,13 gpm) and 11,85 ℓpm (3,13 gpm). The circulation flowrate to the elutrating leg varied between 8,10 ℓpm (2,14 gpm) and 11,36 ℓpm (3,0 gpm). The NaCl product produced this salting out process as determined by microscopic evaluation was approximately 40 wt.% simple cubes, 60 wt.% agglomerated cubes and essentially no hopper-shaped cubes.

Claims: A process for the preparation of NaCl crystals of varying, segregated purity comprising: (a) introducing an upper saturated MgCl₂ brine stream into a vertically-oriented crystallizer body at an upper segment of said crystallizer body; (b) introducing a lower saturated MgCl₂ brine stream into said crystallizer at a lower segment of said crystallizer, said upper segment having a greater horizontal cross-sectional area than said lower segment; (c) introducing an upper saturated NaCl brine stream into said crystallizer at said upper segment of said crystallizer; (d) introducing a lower saturated NaCl brine stream into said crystallizer at said lower segment area of said crystallizer; (e) allowing said brine streams introduced in steps (a) through (d) to mix in said crystallizer to produce NaCl crystals and a spent brine stream; (f) removing said spent brine stream from the upper portion of said crystallizer; and (g) removing said NaCl crystals from the lower portion of said crystallizer. The process of claim 1 further comprising: (h) introducing said spent brine stream into a first solar pond to produce NaCl crystals of approximately 99,0% to 99,7% purity and a first increased MgCl₂ brine stream of approximately 5,0 to 6,0 weight % MgCl₂; (i) introducing said first increased MgCl₂ brine stream into a second solar pond to produce NaCl crystals of approximately 98,5% to 99,0% purity and a second increased MgCl₂ brine stream of approximately 6,0 to 7,0 weight % MgCl₂; and (j) introducing said second increased MgCl₂ brine stream into a third solar pond to produce NaCl crystals of less than about 98,5% purity and a third increased MgCl₂ brine stream of approximately 7,0 to 8,0 weight % MgCl₂. The process of claim 2 further comprising dividing said third increased MgCl₂ brine stream into a first and a second portion, introducing said first portion into said crystallizer as said upper saturated MgCl₂ brine stream and introducing said second portion into said crystallizer as said lower saturated MgCl₂ brine stream. The process of claims 2 wherein said third increased MgCl₂ brine stream is introduced into said crystallizer as said upper saturated MgCl₂ brine stream. The process of claim 2 wherein said third increased MgCl₂ brine stream is introduced into said crystallizer as said lower saturated MgCl₂ brine stream.

Inventors: ASCHAUER MARTIN NICHOLAS, ASCHAUER, MARTIN NICHOLAS
Applicants: AKZO NOBEL NV, AKZO NOBEL N.V.
Application year: 1991
Main classification: B01D9
IPC codes: B01D9, C01D3