JP4789461B2 - Method and apparatus for fractionating mixed salt - Google Patents

Description

  The present invention relates to a method and an apparatus for separating and recovering reusable sodium chloride and potassium chloride from a mixed salt generated from a waste liquid treatment process.

  A method called fractional crystallization, in which each crystal is separated from a solution in which a plurality of salts are dissolved, is a well-known technique, and when sodium chloride and potassium chloride are mixed and dissolved in water, it is also used in handbooks. Solubility data has been published and operating methods are also described. (For example, refer nonpatent literature 1).

  According to this method, it seems that each crystal can be separated in a substantially pure form. Control for crystallization is virtually impossible. That is, even if a method of controlling the operating point by measuring the specific gravity or boiling point rise of the liquid is theoretically possible, the point to be operated is the eutectic point at which crystals of both sodium chloride and potassium chloride precipitate. This is because the eutectic point becomes a maximum value in both the specific gravity and the rise in boiling point, so that it becomes uncontrollable. Further, in crystallization operated in an unstable state of supersaturation, an error during operation is unavoidable, and an accurate operating point cannot be obtained.

  Therefore, the fractional crystallization method is not used in the concentrated crystallization separation operation in the waste liquid treatment apparatus, and all the treated products are taken out in the form of mixed salts. In order to reuse such a mixed salt, it is necessary to carry out fractional crystallization after re-dissolution, which is difficult to carry out due to the above problems.

  The operating point can be determined by analyzing the mixed salt, so that fractional crystallization is possible, but water is evaporated to dissolve the mixed salt, which consumes a lot of energy. Therefore, it is not economical even if it is carried out, and eventually such a mixed salt is usually discarded.

  As a method of producing potassium chloride without reducing the concentration of ion exchange membrane (IEM) irrigation from brine, IEM brine is concentrated in a concentration can to separate sodium chloride, Leave a certain IEM bittern at a temperature of 110-90 ° C. to separate calcium sulfate, sodium chloride, etc., cool, separate the precipitate with a slurry tank and separator, take out the bittern product, dissolve the precipitate Add water to dissolve the sodium chloride, put the slurry separated sodium chloride into a potassium chloride separator and separate into potassium chloride and mother liquor mainly containing sodium chloride, purify and dry the potassium chloride A method is described in which the product is made into a product and the mother liquor is circulated through a concentration can to further recover sodium chloride. (See Patent Document 1).

In this method, the temperature is increased to separate sodium chloride, and the temperature is decreased and water is added to separate potassium chloride. However, this method is intended for dissolved mixtures such as IEM bites that are concentrated and dissolved or precipitated and remain in the amount of sodium chloride and potassium chloride that cannot be changed from the beginning. As a result, the concentration of potassium chloride in the liquid after separation of sodium chloride at a high temperature is determined by the concentration composition of the first IEM bittern, and the concentration cannot be increased to a concentration close to the solubility. When the concentration of sodium chloride is high and not only potassium chloride precipitates when cooled, but a considerable amount of sodium chloride also precipitates, so that the amount of water for dissolving this increases, and therefore the recovery efficiency of potassium chloride is increased. Since it is not good and it is necessary to recover sodium chloride dissolved in a large amount of water, In order to return to the concentration process as a mother liquor, it is a method that has problems such as the need for an operation with a large energy consumption of concentration, etc., for separation of mixed salts where the mixing ratio of sodium chloride and potassium chloride varies greatly It is not an applicable method.
Allan S. Myerson, "Handbook of Industrial Crystallization"; Butterworth Heinemann (USA) publication (1993) (see 104. CRYSTALLIZER SELECTION AND DESIGN, especially 5.1.4. And Fig. 5.1) JP-A-55-56014 (see FIG. 1 and related explanation).

  Therefore, the present invention has an object to provide a method and an apparatus that can solve the above-mentioned problems in the prior art and that can separate a mixed salt that can greatly change the mixing ratio of sodium chloride and potassium chloride with high purity. To do.

For the present invention to solve the above problems, a first aspect of the invention, potassium chloride method of separating mixed salt, Rutotomoni has become the first temperature of the low temperature side, which is the solubility at said first temperature and A raw material mixed salt mainly composed of sodium chloride and potassium chloride is added to a low temperature side aqueous solution mainly composed of sodium chloride having a concentration close to solubility at the first temperature, and the temperature is higher than the first temperature. A heating and mixing operation to make a high temperature side mixture of the second temperature, a high temperature side separation operation to separate the sodium chloride crystals in the high temperature side mixture to make the high temperature side mixture a high temperature side aqueous solution, and Cooling crystallization operation to lower the temperature of the high-temperature side aqueous solution to the first temperature to form a low-temperature side mixture obtained by crystallizing potassium chloride, and separating the crystals of potassium chloride crystallized from the low-temperature side mixture. Te, the cold side mIXED And the low temperature side separation operation of the body to the low temperature side solution, a method of sorting mixed salt with, in the heating mixing operation, the specific gravity of the aqueous solution in the high temperature-side mixture was measured, the ratio weight is the The raw material mixed salt is added so as to have a predetermined specific gravity close to the specific gravity of the liquid at the eutectic point of the second temperature two-salt solubility curve at the second temperature.

In addition to the above, the invention of claim 2 is characterized by having a water addition operation for adding water to the high temperature side aqueous solution before the cooling crystallization operation.

The invention of claim 3, mainly composed of sodium chloride that is a concentration close to the solubility in potassium chloride and the first temperature that is a solubility Rutotomoni has become the first temperature of the low temperature side, at said first temperature temperature side solution to, the addition of raw material mixed salt mainly composed of sodium chloride and potassium chloride, to enable heating mixed operation to the hot side mixture of higher temperature than the first temperature second temperature heating A high-temperature side separator that enables a high-temperature-side separation operation that separates crystals of sodium chloride in the high-temperature-side mixture to make the high-temperature-side mixture into a high-temperature-side aqueous solution; A cooling crystallization apparatus that enables a cooling crystallization operation to lower the temperature to the first temperature to form a low temperature side mixture in which potassium chloride is crystallized , and the potassium chloride crystallized from the low temperature side mixture Minute crystal To the low temperature side separation apparatus that enables low temperature side separation operation of the low-temperature-side mixture in the low temperature side water solution, in the heating mixing operation, capable of measuring the specific gravity of the aqueous solution in the high temperature side mixture A specific gravity detector and the raw material mixed salt are supplied so that the specific gravity measured by the specific gravity detector is a specific gravity close to the specific gravity of the liquid at the eutectic point of the second temperature two salt solubility curve at the second temperature. And a possible raw material supply device.

In the first aspect of the present invention, the mixed salt fractionation method includes potassium chloride (hereinafter, represented by the chemical symbol “KCl”) which is at the first temperature t ₁ on the low temperature side and has solubility at t _1. Raw material mixture containing NaCl and KCl as main components in a low temperature side aqueous solution (hereinafter referred to as “low temperature solution”) containing sodium chloride (hereinafter referred to as “NaCl” in chemical symbol) having a concentration close to solubility. A heating and mixing operation is carried out to add a salt to a high temperature side mixture of t ₂ , which is a second temperature higher than t _1. At this time, the aqueous solution in the high temperature side mixture is the next high temperature side separation The specific gravity of an aqueous solution (hereinafter, both referred to as “high temperature liquid”) that is made into a high temperature side aqueous solution by operation is measured and the specific gravity is close to the specific gravity of the liquid at the eutectic point of the second temperature two salt solubility curve at t ₂ . Add the raw material mixed salt so that In this case, NaCl has reached the solubility, and most of the added NaCl is present as crystals, and KCl is in a dissolved state at a concentration close to the solubility.

That is, in an aqueous solution containing NaCl and KCl as main components, the solubility of the eutectic point is reduced to some extent with NaCl and greatly increased with KCl when the temperature of the solution rises from t ₁ to t ₂ due to the respective dissolution characteristics. Become bigger. Therefore, the high temperature solution loses the ability to dissolve NaCl, and the ability to dissolve KCl is greatly increased. As a result, when the temperature of the cryogenic liquid is increased from t ₁ to t ₂ and the raw material mixed salt is added, NaCl immediately reaches the solubility and is added in the crystalline state, and all of the added KCl is dissolved. As the raw material mixed salt is added, the specific gravity of the high temperature liquid increases.

Then, the specific gravity of the high temperature liquid is measured, and since the raw material mixed salt is added so that the value thereof becomes a predetermined specific gravity close to the specific gravity of the liquid at the eutectic point at t ₂ , the increase in specific gravity due to the addition of KCl is limited, The concentration composition of the hot liquid is limited in the vicinity without reaching the eutectic point. As a result, the high-temperature side mixture becomes a high-temperature liquid in which the amount of NaCl crystals sufficiently close to the amount added and the total amount of KCl are dissolved at a concentration close to solubility, with the NaCl reaching solubility.

  In this case, in the heating and mixing operation as described above, the specific gravity always increases without causing the maximum value as described above, so that the value can be surely reached a value close to the specific gravity of the eutectic point. it can. When the specific gravity of the eutectic point is reached, both NaCl and KCl become soluble and precipitate immediately, so the above close value is equal to the specific gravity of the eutectic point in the range where KCl does not precipitate. The high temperature liquid is made a liquid having a concentration composition close to such a eutectic point.

  Next, the mixed salt fractionation method has a high temperature side separation operation of separating the NaCl crystals in the high temperature side mixture to make the high temperature side mixture into a high temperature liquid. Most of it can be recovered. That is, since the added KCl exists in the liquid phase, only NaCl can be taken out without being mixed in the NaCl crystals. The high-temperature liquid from which the NaCl crystals have been separated has a dissolved amount of NaCl corresponding to the solubility, a dissolved content of KCl added, and a dissolved content corresponding to the solubility of KCl present in the low-temperature liquid. It has become a liquid that holds.

Then, there are a cooling crystallization operation and a low temperature side separation operation, and by these operations, the high temperature liquid is changed to a low temperature side temperature t ₁ to form a low temperature side mixture in which KCl is crystallized, and the low temperature side mixture is crystallized. Since the separated KCl is separated into the low-temperature liquid, the KCl crystals of the added material mixed salt are separated and recovered, and the low-temperature liquid at the temperature t ₁ used in the heating and mixing operation is generated again. be able to.

  That is, KCl that is added and dissolved in the low temperature liquid does not precipitate in the high temperature side mixing operation and is not separated in the separation operation. Therefore, when the high temperature liquid is cooled and returned to the low temperature liquid, it dissolves due to a significant decrease in solubility. Thus, an amount of KCl corresponding to the amount added can be precipitated again and taken out as crystals.

  On the other hand, as the temperature decreases, the solubility of KCl decreases, so the concentration of NaCl decreases to the eutectic point and KCl reaches the solubility, so that NaCl precipitates and mixes into the KCl crystal, but the specific gravity should be adjusted. As a result, the high temperature liquid is made into a liquid having a composition close to the eutectic point, so that the amount of NaCl mixed is very small, and KCl having the required purity can be obtained.

Further, when KCl is precipitated by lowering the temperature to t ₁ , NaCl that has reached the solubility at the eutectic point is separated from the eutectic point even at a constant concentration, but the NaCl concentration at the eutectic point does not change much. Therefore, even when the temperature reaches t ₁ , NaCl has a concentration close to the eutectic point and thus close to solubility. KCl reaches the solubility as described above. Therefore, the liquid after separation of KCl crystals from the high temperature liquid by the low temperature side crystallization operation and separation operation to return the temperature to t ₁ becomes a liquid having the same concentration composition as the first low temperature liquid, and the palindromic operation In a continuous operation, it becomes a liquid that is used repeatedly.

  Conventionally, by-product salt mainly composed of NaCl generated in the evaporation and concentration crystallization process of various waste liquids such as leachate, incinerator, and human waste often contains several percent of potassium chloride. KCl has a problem that it becomes an impurity when the by-product salt is recycled as a regenerated salt because of the difficulty of the separation, but according to the mixed salt separation method of the present invention, NaCl and KCl are mixed as described above. Since separation can be performed efficiently, the use of recycled products is expanded by increasing the purity of each recycled product, and the added value is increased, which can greatly contribute to the construction of a by-product salt recycling system. For example, NaCl and KCl thus separated can be made into crystals having a quality that can be supplied to an electrolysis apparatus for producing caustic and chlorine.

  In this case, by managing the specific gravity of the high-temperature liquid, the mixed salt can be separated without using a concentration operation such as evaporation, so that the raw material mixed salt can be regenerated with less energy consumption.

  According to the invention of claim 2, in addition to the above, since there is a water addition operation for adding water to the high temperature side aqueous solution before the cooling crystallization operation, the eutectic after separation of NaCl crystals by this operation is performed. It is possible to lower the NaCl and KCl concentrations of the high-temperature solution at a position close to the point, and to pass through the eutectic point to obtain a solution having a concentration composition on the solid phase side of KCl. As a result, the purity of KCl can be further improved without generating even a few NaCl crystals until the eutectic point is reached.

  In this case, the added water becomes an extra low-temperature liquid in the fractionation system, so that NaCl and KCl contained in this liquid are discarded to discharge it, but the high-temperature liquid has a composition close to the eutectic point. Therefore, since the amount of water to be added is small, discarded NaCl and KCl are negligible and do not cause a problem.

  According to invention of Claim 3, the apparatus which makes it possible to implement the above mixed salt fractionation methods can be provided.

  1 and 2 show an example of a mixed salt separation method to which the present invention is applied, and an example of a mixed salt separation apparatus capable of performing this method. FIG. 3 to FIG. 5 are diagrams showing the separation operation state of this example in the two salt solubility curves of NaCl and KCl which are also described in the handbook. 4 is an enlarged view of the eutectic point portion of FIG. 3, and FIG. 5 is a graph showing the specific gravity of the saturated solution on the solubility curve on the vertical axis with the concentration of NaCl as the horizontal axis. Note that the specific gravity in FIG. 5 corresponds to the numerical values of the examples described later.

The method of this example includes a heating and mixing operation A, a high temperature side separation operation B, a cooling crystallization operation C, a low temperature side separation operation D, a specific gravity measurement A ₁ , a supply amount adjustment A ₂ , and the like. The apparatus of this example for carrying out this method includes a heating dissolution tank 1 as a heating dissolution apparatus, a centrifuge 2 as a high temperature side separation apparatus, a cooling crystal can 3 as a cooling crystallization apparatus, and a low temperature side separation. The apparatus includes a centrifugal separator 4 as an apparatus, a hydrometer 5 as a specific gravity detector, a raw material supply apparatus 6, and the like.

In the heating and mixing operation A, the raw material mixed salt M mainly composed of NaCl and KCl is added to the low temperature liquid W ₁ that is the low temperature side aqueous solution that is at the first temperature t ₁ on the low temperature side, and the first temperature t _1. The high temperature side mixture Bm having a higher second temperature t ₂ is used. Cold liquid W ₁ is a liquid composed mainly of the NaCl that is a concentration close to the solubility and KCl which has reached the solubility at a first temperature t _1, P ₁ position (hereinafter in FIGS. 3 to 5 It is exemplified as a liquid having a concentration composition (hereinafter referred to as “composition”). In this example, the P ₁ point is located on the KCl solubility curve Lk ₁ close to the eutectic point Q ₁ on the two-salt solubility curve of NaCl and KCl at the first temperature t ₁ = 20 ° C. Since P ₁ and Q ₁ are close, NaCl has a concentration close to solubility.

  In this example, the heating / dissolving tank 1 for performing this operation is provided so as to stir the internal liquid, a heating tube 11 provided so that steam is flown and condensed to heat the internal liquid with latent heat. A stirring blade (not shown), a tank main body 12 provided with the stirring blades and provided to store the liquid, and the like. In addition, as an apparatus for heating and melting, it is possible to adopt a device having another appropriate structure, such as an electric heating type heating unit or a separate heating unit and stirring unit.

The raw material mixed salt M is mainly composed of NaCl, which is generated from the evaporation concentration crystallization step of various waste liquids, contains 1% or more of KCl, and other small amounts of impurities. The low temperature liquid W ₁ is repeatedly used as an aqueous solution in the vicinity of the eutectic point of NaCl and KCl in the mixed salt fractionation system, and continuously flows and circulates in the system when fractionated continuously.

The supply amount of the raw material mixed salt M added to the low temperature liquid W ₁ is determined by the operation of the supply amount adjustment A ₂ based on the specific gravity measurement A ₁ . In the specific gravity measurement A _1, a specific gravity meter 5, a supernatant liquid of the high-temperature side mixture Bm heating dissolution tank 1 for measuring the specific gravity of the high temperature side solution to become hot liquid W _2.

In the supply amount adjustment A ₂ , the amount of the mixed salt added to the low temperature liquid W ₁ is adjusted so that the measured specific gravity γp becomes a set specific gravity γs that is a predetermined specific gravity. That is, when NaCl and KCl are automatically separated by continuous or batch operation, the raw material supply device 6 is provided with a hopper 61 and a rotary valve 62 as shown in FIG. It is an automatic type, and the specific gravity meter 5 not only displays the detected value of specific gravity but also outputs it externally, and controls the rotational speed of the rotary valve via a controller (not shown). When separation is performed manually, the raw material mixed salt M is put into the heating dissolution tank 1 by other appropriate means so that the specific gravity γp becomes γs while manually operating the rotary valve or watching the specific gravity meter. Supply.

The second temperature t ₂ is usually about 50 ° C. or higher and 100 ° C. or lower. That is, as shown in FIGS. 3 and 4, when the temperature is increased, the solubility of NaCl is reduced to some extent at eutectic points as indicated by Q _{1 to} Q ₃ on the eutectic point curve L indicated by a two-dot chain line. On the other hand, since the solubility of KCl is greatly increased, it is advantageous to increase t ₂ in terms of fractionation efficiency, etc. However, if the temperature exceeds 100 ° C., the separation system is set to a pressure higher than atmospheric pressure in order not to evaporate water. Since it becomes necessary to make pressure, it becomes disadvantageous in terms of apparatus cost, operability, etc., so t ₂ is set to the above temperature. Practically, it is desirable that the temperature is about 60 ° C to 90 ° C.

As shown in FIGS. 3 and 4, when t ₂ is 80 ° C., for example, the eutectic point Q ₂ is in the position shown in the figure, and the solubility of NaCl is 28 than the position of the eutectic point Q _{1 at} 20 ° C. From about 0.7% to about 27.8%, while the KCl solubility increases by about 15% from about 15% to 30%. At 80 ° C., the solubility curves Ln ₂ of NaCl and KCl and Lk ₂ is as shown in the figure. Due to the solubility characteristics of the two salts, when the low temperature liquid W ₁ is heated to circulate to 80 ° C. and the raw material mixed salt M is added thereto, the high temperature side mixture Bm becomes the NaCl solubility of the two salt solubility curve at 80 ° C. Most of the NaCl crystals added to the high-temperature liquid W ₂ , which is the liquid having the composition at the P ₂ position on the curve L n ₂ , remain. The position of point P ₂ is determined by the position of point P ₁ and the amount of added KCl.

In other words, even if the normal raw material has a large amount of NaCl, even if NaCl and KCl in the raw material mixed salt to be added have the same ratio, if the mixed salt is gradually added after the temperature of the low temperature liquid W ₁ is raised, the liquid While the concentration position is below the solubility curves Ln ₂ and Lk ₂ , the solution passes through the virtual dissolution line L ₁ while dissolving NaCl and KCl, as shown by the thin dashed line in FIG. When the NaCl solubility curve Ln ₂ at the temperature t ₂ is reached, the P ₂ point is obtained via the crystallization line L ₂ indicated by a two-dot chain line on Ln ₂ while precipitating NaCl once while dissolving KCl. To reach. In practice, since the heating and the supply of the mixed salt proceed in parallel while stirring the high temperature side mixture Bm, the dissolution from the NaCl and the crystallization are not distinguished from the P ₁ point to the P ₂ point. It moves on the curve swollen to the concentration increasing side.

According to such a change in state, NaCl always remains in the form of crystals and the specific gravity tends to always increase by the amount of dissolved KCl, and the dissolved concentration of the mixed salt in the state before reaching the eutectic point Q ₂ is increased. Since it is unsaturated and stable, the supply amount of the mixed salt can be reliably controlled. At this time, it stirs until the crystal | crystallization which remains among the added raw material mixed salt becomes only NaCl.

The set specific gravity γs in the supply amount adjustment A ₂ that determines the specific gravity γp and determines the amount of the mixed salt added to the low temperature liquid W ₁ is the eutectic point Q ₂ of the second temperature two salt solubility curve at the second temperature t ₂ . The specific gravity of the liquid is close to γq. In this case, since the specific gravity of the liquid becomes a value corresponding to the density of the liquid, if the P ₂ point in a position close to the eutectic point Q _2, to the γs specific gravity of the liquid having a composition of P ₂ points Can do. Therefore, by setting γs to a predetermined specific gravity close to the specific gravity of the liquid at the Q ₂ point, the P ₂ point can be set to a predetermined position close to the Q ₂ point.

As shown in FIG. 5, γs also exists as γs ′ having the same specific gravity value at the R point on the solubility curve Lk ₂ of KCl. However, as described above, in order to shift from the P ₁ point to the P ₂ point, if a certain amount of NaCl is present, it passes through a path on the side where the NaCl concentration becomes higher, such as the dissolution line L ₁ and the crystallization line L ₂ . If the point P ₁ is in a range close to the eutectic point Q ₁ to some extent, the feed amount of raw material salt is adjusted so as to reach the point P _{2 as} described above by making γs the reference value for adjustment. can do. And the R point is not reached beyond the eutectic point Q ₂ . Therefore, no matter how much the ratio of NaCl to KCl in the raw material mixed salt varies, the supply amount can be determined reliably.

In the high temperature side separation operation B, NaCl crystals in the high temperature side mixture Bm are separated to convert Bm into a high temperature liquid W ₂ that is a high temperature side aqueous solution. That is, as shown in FIG. 4, by subtracting the amount corresponding to the NaCl concentration difference between the P ₂ point and the P ₁ point from the added raw material mixed salt M, most of the remaining NaCl is separated and recovered. and, a liquid portion after separation can be a high temperature fluid W _2.

This high temperature side separation operation B is performed by the centrifuge 2 in this example. The centrifuge 2 rotates the high temperature side mixture Bm to generate a centrifugal force, passes the high temperature liquid W ₂ , which is a liquid component, through the screen 21, and removes the crystal of NaCl, which is a solid component. Is a device that separates and collects on the screen 21. The high temperature liquid W ₂ is a liquid having a concentration close to the solubility in which the crystal content is separated and NaCl becomes soluble as described above, and KCl becomes sufficiently high on the high temperature side. As the high temperature side separation device, a stationary filtration type device or the like may be employed in addition to or instead of the centrifuge 2.

In the cooling crystallization operation C, the high temperature liquid W _{2 is set} to the low temperature side temperature t ₁ to obtain a low temperature side mixture Cm in which KCl is crystallized. This operation is performed by the cooling crystal can 3. In this example, the cooling crystal can 3 includes a cooling pipe 31 provided to cool the internal liquid by flowing cooling water, a stirring blade (not shown) provided to stir the internal liquid, and these are arranged. It is comprised by the can body 32 etc. which were installed and were provided so that a liquid might be stored. As the cooling water, seawater, river water, industrial water, cooling tower (not shown), cooling water cooled by a refrigeration apparatus, or the like is used. In addition, as a cooling crystallizer, the thing of other suitable structures, such as making a cooling part and a stirring part into a different body, is employable. Although the temperature t ₁ varies depending on the cooling medium, it is desirable that the temperature t ₁ is set to 40 ° C. or lower and 30 ° C. or lower from the balance of the separation efficiency and the apparatus cost.

According to this cooling crystallization operation C, KCl can be separated based on the di-salt solubility curve at high and low temperatures t ₂ and t ₁ . That is, if the high temperature liquid W ₂ is at the P ₂ point on the NaCl solubility curve, KCl does not reach the solubility at first, so it does not precipitate even if the temperature is lowered, while its concentration does not change. Is on the solubility curve L n ₂ , but the temperature transitions to a eutectic point curve L while generating a small amount of NaCl crystals on the solubility curve at a temperature that has been lowered successively with a constant KCl concentration by lowering the temperature. When reaching the P ₄ position, NaCl is now below the solubility due to the characteristics of the eutectic point curve L, and away from the eutectic point curve L at a constant concentration, while KCl generates crystals and lowers the concentration. The temperature gradually changes on the solubility curve and reaches the point P ₅ on the KCl solubility curve at the temperature t ₁ .

Although this P ₅ point is far from the eutectic point Q _{1 at the} temperature t ₁ , it is at the same NaCl concentration position as the P ₄ point sufficiently close to the eutectic point Q ₂ and the eutectic point curve L is NaCl. Since the NaCl concentration difference between the eutectic points Q ₁ and Q ₂ is 1% or less, the P ₅ point becomes the eutectic simply by lowering the temperature from t ₂ to t _1. It becomes in a position close to the point Q _1.

The NaCl and KCl precipitated in the cooling crystallization operation C are in amounts corresponding to the concentration differences between P ₂ -P ₄ and P ₄ -P ₅ . In the example of the figure, they are about 0.3% and 14%, respectively. However, in order to make it easy to understand in the figure, the point P ₂ is shown slightly separated from the point Q _2, so the amount of NaCl mixed into KCl is reduced to 0 by actually reducing the amount of NaCl precipitated. It is possible to make it about 1%. On the other hand, when γs is brought close to γq ₂ and the P ₂ point is brought close to the Q ₂ point, the amount of NaCl mixed in KCl can be reduced, but it becomes difficult to determine the amount by adjustment and control, The KCl in the mixed salt M is not completely dissolved in the high temperature side mixture, and the amount of KCl crystals mixed into the NaCl crystals when the high temperature side separation operation is performed increases.

NaCl and KCl separated by this method can be used in various applications, and there are acceptable limits of the amount of each salt mixed, that is, the purity of the salt required by the application. For example, there are applications where the amount of NaCl mixed into KCl is more severely limited than the amount of KCl mixed into NaCl, and vice versa. Therefore, the set specific gravity γs, which is a predetermined specific gravity, is usually set to a position where the purity of both P ₂ is approximately the same, and is the specific gravity of the liquid at that position. The value is varied to some extent according to the required purity. According to the experiments by the inventors, it is desirable that γs is a value lower by about 0.001 to 0.004 than the specific gravity γq ₂ of the liquid at the eutectic point Q ₂ , particularly about 0.0015 to 0.003.

In the low temperature side separation operation D, the KCl crystals crystallized as described above from the low temperature side mixture Cm are separated into the low temperature liquid W ₁ which is a low temperature side aqueous solution. This low temperature liquid W ₁ is a liquid having a composition of point P ₅ as described above. P ₅ points, setting the concentration P ₂ points determined by γs as above, thus determined by the P ₄ points, NaCl close to the eutectic point Q ₁ is turned concentration close to the solubility be on the solubility of KCl at a temperature t ₁ Liquid. Then, for the next heating and mixing operation A, the low temperature liquid W _{1 at} point P ₅ is supplied. In order to make the explanation easier to understand, the point P ₅ is different from the point P _1. However, if the operation with γs kept constant is continued, the point P ₅ coincides with the point P _1. .

Similarly to the high temperature side separation operation B, this low temperature side separation operation D is also performed in the centrifuge 4 in this example. The centrifuge 4 rotates the low temperature side mixture Cm to generate centrifugal force, and the screen 41 passes the low temperature liquid W ₁ , which is a liquid component, to take out the crystal. A device for separating and collecting on the screen 41. In addition to or in place of the centrifugal separator 4, a stationary filtration type apparatus or the like may be employed as the low temperature side separation apparatus.

FIGS. 6 to 8 show other examples of the mixed salt separation method and apparatus to which the present invention is applied, and correspond to FIGS. 1, 2, and 4, respectively.
In addition to the method of FIG. 1, the method of this example includes a water addition operation C ₁ for adding water to the high temperature liquid W ₂ by the water supply system 3a before the cooling crystallization operation C. According to this operation, as shown in FIG. 8, the NaCl crystal is separated, and the hot liquid W ₂ at the point P ₂ in which NaCl is a solution and has a concentration of solubility is slightly diluted with water. From the solution to the KCl solid phase side, the solution of the P ₆ point exceeding the eutectic point curve L is cooled, and then cooled to the solution of the P ₇ point, thereby precipitating KCl without precipitating NaCl and increasing the purity of KCl. It can be made even better. Also, well separated by a distance necessary and P ₂ point, Q ₂ points without reducing the purity of KCl to be separated, or to facilitate adjustment and control of the supply amount of the raw material mixed salts, the purity of NaCl to separate can do. The amount of water supplied may be an amount that moves the P ₆ point to the solid phase side of KCl, but is about 2 to 20% by weight with respect to the crystal separation amount of NaCl and KCl.

When water is added to the high-temperature liquid W ₂ in this way, the total amount of water in the separation system increases, so that excess water is discharged from the water discharge system 4a by the water discharge operation D ₁ . According to this operation, NaCl having a concentration close to solubility and KCl having solubility are discharged out of the system together with water, so these salts are lost by separation, but the amount is small. It doesn't matter. On the other hand, since the raw material mixed salt often contains trace amounts of soluble salts that do not form solid solutions with NaCl and KCl, such as sodium sulfate and sodium nitrate, as impurities, these are discharged at a certain concentration by this water discharge operation D ₁ . Maintaining the following, it is possible to prevent influence on the purity of NaCl and KCl to be collected separately.

In the water supply operation C ₁ , as shown by a two-dot chain line in FIG. 6, instead of the water supply system 3 a, the cleaning water supply systems 2 a and 4 b are provided so that the cleaning water is sent to the centrifuges 2 and 4. Also good. Further, the water discharge system 4a may be provided in the centrifuge 4 as a blow system 4c. In this way, it is necessary to supply water with pressure, and the amount of water increases and the amount of salt discharged increases, but water is supplied to the screens 21 and 41 of the centrifuges 2 and 4. By washing the NaCl and KCl crystals, cleaner crystals can be obtained.

  Note that the method of the present invention as described above can be carried out by any of batch, continuous, automatic, and manual methods. However, if the system is continuous, the control becomes more stable and the equipment can be downsized.

The mixed salt fractionation method of the present invention was carried out batchwise. A neutral salt solution containing 27.5 g of NaCl and 16.0 g of KCl per 100 g of water was prepared as the low temperature liquid W ₁ . The specific gravity of this solution was 1.231, and the measurement temperature was 24 ° C. While maintaining 1000 parts of this solution heated at 75 ° C. and mixing with a stirrer, KCl = 14.3%, NaCl = 80.8%, the high temperature side mixture Bm consisting of the remaining water was 1.239 at a measurement temperature of 70 ° C. Gradually added until. The amount of the mixed salt added at this time was 460 parts. After confirming that the specific gravity did not change, the residual crystals were dehydrated and separated using a centrifuge.

After adding 20 parts of water to the high-temperature liquid W ₂ after separation, cooling and crystallization was gradually performed to 20 ° C. with stirring. When the temperature reached 55 ° C., crystal precipitation was confirmed, and the crystal increased thereafter. When the temperature was stabilized, the precipitated crystals were dehydrated and separated with a centrifuge.

The separated amount and composition (dry basis) is
First stage high temperature separation: 380 parts NaCl = 99.4% KCl = 0.55%
Second stage cold separation: 61 parts NaCl = 0.61% KCl = 99.4%
Met. According to this result, it was judged that any salt had a purity sufficiently lower than the target 1% of impurities, and was a reusable crystal.

  Salt-containing wastewater from general factory wastewater, leachate from a final disposal site, scrubber wastewater from a garbage incinerator, and wastewater temporary treatment wastewater from a combined septic tank are conventionally discharged if the discharge standard is satisfied by water treatment. I came. In recent years, however, there has been a problem of salt damage to crops, etc., as the salt concentration has increased, and it has become necessary to desalinate these salt wastewaters for release or recycling, and many desalination plants have been installed and operated. Has been. By-product salt is generated from these desalination plants, and some of them are reused in special fields, but most of them do not have effective reuse as impurities-containing salts. The application development for is waiting. The present invention is used as a technique in the field of refining such a by-product salt as a regenerated salt as an industrial raw material.

It is explanatory drawing which shows an example of schematic structure of the mixed salt fractionation method to which this invention is applied. It is an example of the apparatus which can implement the said method, and is a flowchart which shows typically the processing flow in the separation apparatus for fractionating a mixed salt into a sodium chloride crystal and a potassium chloride crystal. It is the graph which illustrated the operating point of this invention in the mutual solubility curve of NaCl-KCl-H2O ternary system. It is the graph which expanded the eutectic point part of FIG. FIG. 5 is a diagram showing operating points of Example 1 on a graph showing the relationship between NaCl concentration and solution specific gravity for the ternary solution. It is explanatory drawing which shows the other example of schematic structure of the mixed salt fractionation method to which this invention is applied. It is a flowchart which shows an example of the apparatus which can implement the said method. It is a graph which shows an example of the operating point which implemented the said method, and respond | corresponds to FIG.

Explanation of symbols

1 Heating dissolution tank (heating dissolution equipment)
2 Centrifuge (High temperature side separator)
3 Cooling crystal can (cooling crystal equipment)
4 Centrifuge (low temperature side separator)
5 Specific gravity meter (specific gravity detector)
6 Raw material supply device A Heating and mixing operation A ₁ Specific gravity measurement A ₂ Supply amount adjustment B High temperature side separation operation Bm High temperature side mixture C Cooling crystallization operation Cm Low temperature side mixture C ₁ Water supply operation D Low temperature side separation operation M Raw material mixing Salt Q ₂ eutectic point t _{1 First} temperature on the low temperature side W ₁ Low temperature liquid (low temperature side aqueous solution)
W ₂ high temperature liquid (high temperature side aqueous solution)
Specific gravity of γp aqueous solution
γq Specific gravity of liquid at eutectic point γs Specific gravity (predetermined specific gravity)

Claims (3)

Rutotomoni has become the first temperature of the low temperature side, the sodium chloride that is a concentration close to the solubility in potassium chloride and the first temperature that is a solubility first temperature to a low temperature side water solution mainly, It added material mixed salt mainly composed of sodium chloride and potassium chloride, and heating mixed operation to the hot side mixture of higher the temperature than the first temperature second temperature,
Separating the sodium chloride crystals in the high temperature side mixture to make the high temperature side mixture a high temperature side aqueous solution;
Cooling crystallization operation to lower the temperature of the aqueous solution on the high temperature side to the first temperature to form a low temperature side mixture crystallized with potassium chloride;
Separating the crystal of the potassium chloride crystallized from the low temperature side mixture, and making the low temperature side mixture into a low temperature side aqueous solution, and separating the mixed salt,
In the heating and mixing operation, the specific gravity of the aqueous solution in the high temperature side mixture is measured, and the specific gravity is close to the specific gravity of the liquid at the eutectic point of the second temperature two-salt solubility curve at the second temperature. The mixed salt fractionation method is characterized in that the raw material mixed salt is added. The mixed salt fractionation method according to claim 1 , further comprising a water addition operation of adding water to the high temperature side aqueous solution before the cooling crystallization operation. Rutotomoni has become the first temperature of the low temperature side, the sodium chloride that is a concentration close to the solubility in potassium chloride and the first temperature that is a solubility first temperature to a low temperature side water solution mainly, It added material mixed salt mainly composed of sodium chloride and potassium chloride, a heating dissolution apparatus for enabling heating mixed operation to the hot side mixture of higher temperature than the first temperature second temperature,
A high-temperature-side separation device that separates sodium chloride crystals in the high-temperature-side mixture and makes the high-temperature-side mixture into a high-temperature-side aqueous solution and enables a high-temperature-side separation operation;
A cooling and crystallization apparatus that enables a cooling and crystallization operation to lower the temperature of the high-temperature side aqueous solution to the first temperature to form a low-temperature side mixture in which potassium chloride is crystallized;
Separating a crystal of the potassium chloride crystallized from the low temperature side mixture, and allowing the low temperature side separation operation to make the low temperature side mixture the low temperature side aqueous solution;
In the heating and mixing operation, a specific gravity detector capable of measuring the specific gravity of the aqueous solution in the high temperature side mixture,
A raw material supply apparatus capable of supplying the raw material mixed salt so that the specific gravity measured by the specific gravity detector has a predetermined specific gravity close to the specific gravity of the liquid at the eutectic point of the second temperature dual salt solubility curve at the second temperature. And a mixed salt fractionating device.

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