Method for controlling impurity quality in crystals

Abstract

To provide a crystallizer control method capable of reducing variations of impurity grade in crystals generated by the crystallization of a stock solution containing a crystallization target component.SOLUTION: In crystallization treatment, a crystallization target component is crystallized by obtaining a concentrated liquid by the pressure reduction evaporation of a stock solution containing the crystallization target component and impurities and being introduced into a crystallizer 1 under predetermined temperature and pressure conditions; concentrated slurry containing obtained crystals is drawn from a bottom, dehydrated and dried and product crystals are recovered; and a part of the concentrated liquid is drawn out from a side part. The drawn out amount of the part of the concentrated liquid required for maintaining impurity grade in the product crystals to be a predetermined value is calculated on the basis of balance calculation of the crystallization target component in a material going in and out in the system performing the crystallization treatment, impurities and the whole materials.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for controlling the quality of impurities in crystals, and more particularly to a method for controlling the quality of impurities in product crystals to a target quality when crystallization is carried out by continuously introducing a stock solution containing a component to be crystallized, such as nickel sulfate, into a crystallizer. [Background technology]

[0002] Nickel sulfate is used for various purposes, such as as a raw material for nickel plating solutions and for nickel hydroxide powder used in batteries. One known industrial method for producing nickel sulfate is the HPAL (High Pressure Acid Leaching) method, which involves treating nickel oxide ore through a series of hydroprocessing steps, including high-pressure acid leaching, to produce a nickel-cobalt mixed sulfide, which is then used as an intermediate raw material for hydroprocessing.

[0003] The hydrometallurgical process using the nickel-cobalt mixed sulfide as an intermediate raw material comprises the steps of: producing a crude nickel sulfate aqueous solution containing impurities by adding water to the nickel-cobalt mixed sulfide to prepare a slurry, and then leaching the slurry under high temperature and high pressure; removing the impurities contained in the crude nickel sulfate aqueous solution; introducing the high-purity nickel sulfate aqueous solution obtained by removing the impurities into a crystallizer for crystallization; and subjecting the slurry containing nickel sulfate crystals produced by the crystallization to post-treatments such as solid-liquid separation, drying, and sieving.

[0004] The above-mentioned impurity removal step removes iron contained mainly in the aqueous solution of crude nickel sulfate as an impurity, and therefore it was not possible to sufficiently remove thiosulfate ions that were by-produced when the reaction in the preceding leaching treatment did not proceed as expected. In this case, the purity of the nickel sulfate produced by crystallization decreases, which can cause quality problems.

[0005] As a countermeasure against this, Patent Document 1 discloses a technique of oxidatively decomposing thiosulfate ions contained in the aqueous solution of crude nickel sulfate with an oxidizing agent and then removing impurities by a solvent neutralization method or an oxidation neutralization method, which makes it possible to crystallize nickel sulfate crystals from an aqueous solution of high-purity nickel sulfate.Furthermore, in the technique of Patent Document 1, in order to suppress adverse effects on subsequent processes caused by the added oxidizing agent remaining, the ORP value of the aqueous solution of crude nickel sulfate after the oxidative decomposition treatment is measured, and the treatment conditions for removing the impurities are adjusted based on the concentration of the oxidizing agent determined from the measurement results. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2021-080123 Summary of the Invention [Problem to be solved by the invention]

[0007] It is believed that by adopting the technology of Patent Document 1, impurities such as iron and thiosulfate contained in an aqueous solution of crude nickel sulfate can be efficiently removed without adversely affecting subsequent processes. However, in reality, it is difficult to completely remove impurities contained in an aqueous solution of crude nickel sulfate, and unexpected impurities may be contained due to changes in raw material lots, etc. Therefore, even after impurity removal, the high-purity nickel sulfate solution still contains small amounts of impurities, and these small amounts of remaining impurities can cause problems during crystallization in the subsequent process.

[0008] In other words, in the crystallization process, a solution containing a substance to be crystallized (also called a stock solution) introduced into a crystallizer is heated to evaporate the solvent, concentrating the substance as a solute to a supersaturated state, thereby crystallizing a portion of the substance to be crystallized and continuously withdrawing it from the bottom of the crystallizer in the form of a concentrated slurry. As the substance to be crystallized is concentrated, impurities also concentrate in the crystallizer, reducing the impurity quality of the crystals. Therefore, while withdrawing the concentrated solution from the crystallizer, an operation is performed in which an amount of stock solution corresponding to the amount withdrawn is replenished.

[0009] However, the impurity concentration in the concentrate can fluctuate due to various disturbances, and the impurity quality in the crystals can also vary greatly. That is, when the impurity concentration in the concentrate increases excessively, the impurity quality of the product crystals exceeds the quality standard, and the product crystals may need to be re-dissolved to be treated as defective. Conversely, when the impurity concentration in the concentrate decreases, the impurity quality of the product crystals may be excessively low compared to the quality standard, which means that excessive concentrate is withdrawn from the crystallizer, resulting in low productivity and high steam consumption, resulting in high-cost operation.

[0010] As described above, in a crystallization process in which a crystallizer is used to partially crystallize a component to be crystallized contained in a stock solution to produce a product, the quality of impurities in the crystals tends to vary greatly, and it has been desired to stably control this. The present invention has been made in view of the above circumstances, and its object is to provide a method for controlling a crystallizer that can suppress the variation in the quality of impurities in the crystals produced by continuously introducing a stock solution containing a component to be crystallized, such as nickel sulfate, into a crystallizer to perform crystallization. [Means for solving the problem]

[0011] The present inventors have found that in order to accurately grasp changes in the impurity concentration in the concentrated liquid in a crystallizer, a crystallizer equipped with equipment capable of measuring various operating parameters, which was not particularly required in the past, is used, and by controlling the operation of the crystallizer using these operating parameters, it is possible to efficiently suppress variations in the quality of impurities in the crystals, thereby completing the present invention.

[0012] That is, the method of controlling the quality of impurities in crystals according to the present invention involves crystallizing the component to be crystallized by evaporating a stock solution containing the component to be crystallized and impurities under reduced pressure under prescribed temperature and pressure conditions to form a concentrated solution, and then withdrawing the resulting concentrated slurry containing the crystals from the bottom, dehydrating and drying it to recover it as product crystals, while withdrawing a portion of the concentrated solution from the side. This method is characterized in that, in the crystallization process, the amount of the concentrated solution withdrawn that is necessary to maintain the quality of impurities in the product crystals at a prescribed value is determined based on a balance calculation of the component to be crystallized, impurities, and all substances that enter and leave the system in which the crystallization process is carried out. [Effects of the Invention]

[0013] According to the present invention, the variability of impurity quality in crystals produced by crystallizing a stock solution containing a component to be crystallized can be suppressed, and therefore the industrial value of the present invention is extremely great. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic flow diagram of a crystallizer to which the method for controlling the impurity grade in a crystal according to the present invention is suitably applied. [Figure 2] 1 is a graph showing the relationship between the impurity concentration of the concentrated liquid in the crystallizer and the impurity quality of the crystals crystallized in the concentrated liquid. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the method for controlling the quality of impurities in crystals according to the present invention will be described. In this method for controlling the quality of impurities according to the present invention, a stock solution containing a component to be crystallized and impurities is continuously introduced into a crystallizer, and the stock solution is evaporated under reduced pressure under predetermined temperature and pressure conditions to form a saturated concentrated solution, thereby crystallizing the component to be crystallized. The resulting concentrated slurry containing the crystals is withdrawn from the bottom, dehydrated, and dried to recover the product crystals, and a portion of the concentrated solution is withdrawn from the side to control the quality of impurities.

[0016] The solvent constituting the above-mentioned stock solution is not particularly limited, and may be water or an organic solvent. Furthermore, the component to be crystallized contained in the above-mentioned stock solution is also not particularly limited, and examples thereof include nickel sulfate, ammonium sulfate, sodium carbonate, caprolactam, and trehalose. Hereinafter, a stock solution consisting of an aqueous nickel sulfate solution containing nickel sulfate as the component to be crystallized will be described as an example.

[0017] A suitable crystallizer to which the impurity quality control method according to an embodiment of the present invention is applied is a DTB (Draft Tube Baffle) type continuous crystallizer as shown in Fig. 1. This crystallizer 1 is provided with a draft tube 1a inside, and a stirring blade 1b provided inside the draft tube 1a circulates a slurry phase consisting of a concentrated solution of nickel sulfate aqueous solution and nickel sulfate crystals as shown by the arrows. Since evaporation of water from this circulating slurry phase occurs mainly near the liquid surface, the nickel sulfate aqueous solution near the liquid surface is particularly concentrated and becomes supersaturated.

[0018] In a supersaturated state, crystals are generated and grow, and the nickel sulfate aqueous solution moves away from the liquid surface while decreasing in concentration. Circulation within the crystallizer by the agitator blades 1b returns the solution to the vicinity of the liquid surface of the slurry phase. In this way, crystallization proceeds almost uniformly within the crystallizer 1. When the crystals circulate with the concentrated nickel sulfate aqueous solution circulating within the crystallizer 1 as described above, they leave the circulation flow and settle to the bottom of the crystallizer 1 once they have grown to a certain size, and are withdrawn in the form of concentrated slurry from the classifier leg 1c installed at the bottom. The concentrated slurry withdrawn from the bottom via the slurry pump 2 is dehydrated and dried in a liquid separation means 3, such as a solid-liquid separator or a dryer, and then, if necessary, classified by a sieving means (not shown), to produce product crystals.

[0019] The slurry phase in the crystallizer 1 loses heat due to the heat of vaporization generated when the water evaporates. To compensate for this lost heat, a portion of the slurry phase is withdrawn from the settling section 1d in the crystallizer 1 via the heated liquid withdrawal pump 4, heated in the heater 5, and then returned to the crystallizer 1. In the settling section 1d, large crystals that have grown settle out of the aqueous nickel sulfate solution, and the nickel sulfate solution containing fine crystals that were not completely separated in the settling section 1d is introduced into the heater 5. These fine crystals dissolve when heated in the heater 5, allowing crystals with a uniform particle size to be produced in the crystallizer 1.

[0020] There are no particular limitations on the type of heater 5, and a general shell-and-tube heat exchanger can be suitably used. Steam, for example, can be used as the heat medium supplied to this heater 5. By adjusting the flow rate of this heat medium based on the reading of a thermometer 10 provided in the crystallizer 1, the liquid temperature in the crystallizer 1 can be controlled to preferably about 32 to 53°C.

[0021] The inside of the crystallizer 1 is maintained at a reduced pressure of, for example, about 8 kPaA by a vacuum pump 6. This lowers the boiling point of the liquid phase, thereby efficiently concentrating the stock solution and promoting crystallization. The pressure inside the crystallizer 1 may be controlled by adjusting the opening of a control valve provided upstream of the vacuum pump 6, or by adjusting the rotation speed of the drive motor of the vacuum pump 6. However, if an abnormality occurs in the operation of the vacuum pump 6, the operator will be able to detect the occurrence of a problem due to a large fluctuation in the crystal production volume, so pressure control is not necessary.

[0022] The solvent vapor evaporated in the crystallizer 1 is discharged from the top of the crystallizer 1 together with the exhaust gas by the vacuum pump 6, and a cooler 7 for condensing the vapor is provided upstream of the vacuum pump 6. The condensate condensed by the cooler 7 is extracted downstream of the cooler 7 and discharged outside the crystallization treatment system shown by the dashed dotted line. There are no particular limitations on the type of cooler 7, and a shell-and-tube heat exchanger, for example, can be suitably used, and cooling water, for example, can be used as the refrigerant.

[0023] Nickel sulfate crystals, which are formed by boiling of the solvent constituting the slurry phase, may adhere in the form of scale to the wall surface of the gas phase within the crystallizer 1. To wash this off, a pipe for introducing a washing liquid is connected to the gas phase side of the crystallizer 1. As mentioned above, if the impurity concentration of the concentrated solution increases, the impurity quality in the crystals also increases accordingly. Therefore, to prevent the impurity concentration of the concentrated solution from becoming too high, an extraction pipe equipped with a concentrated solution extraction pump 8 for extracting a portion of the concentrated solution is connected to the top of the settling section 1d of the crystallizer 1. In an embodiment of the present invention, the amount of the concentrated solution extracted from this extraction pipe is calculated using the material balance calculation shown below.

[0024] That is, the impurity concentration C in the concentrated solution of the nickel sulfate aqueous solution saturated in the crystallizer 1 L (unit: mg / L) and the impurity grade C of the nickel sulfate crystals crystallized from this saturated nickel sulfate aqueous solution. S(unit: mass ppm) is proportional to the partition ratio K (i.e., C S / C L ) has a fixed value for each impurity element. For example, if the impurity is magnesium (Mg), the distribution ratio K obtained by dividing the Mg content in the nickel sulfate crystals by the Mg concentration in the concentrated nickel sulfate aqueous solution is 0.4, as shown in the graph in Figure 2.

[0025] Therefore, by analyzing the concentration of a specific impurity in the concentrated solution of nickel sulfate aqueous solution, the quality of the specific impurity in the nickel sulfate crystals can be predicted in advance. Conversely, in order to adjust the quality of a specific impurity in the nickel sulfate crystals to a desired value, it is possible to determine to what extent the concentration of the specific impurity in the concentrated solution of nickel sulfate aqueous solution in the crystallizer 1 should be controlled. For example, it can be seen from the graph of Figure 2 that in order to adjust the Mg quality in the nickel sulfate crystals to 8 ppm, the Mg concentration in the concentrated solution should be set to 20 mg / L.

[0026] The concentration of specific impurities in the concentrated solution of the nickel sulfate aqueous solution can be adjusted by the amount of concentrated solution withdrawn from the crystallizer 1. The amount of concentrated solution withdrawn can be determined based on balance calculations of the components to be crystallized, the specific impurities, and the total amount of material that enter and leave the crystallization treatment system where crystallization treatment is performed, as shown by the dashed-dotted line in Figure 1. Specifically, the materials that enter and leave the crystallization treatment system include the raw solution introduced into the crystallizer 1, the washing liquid used to wash the inside of the crystallizer 1, the crystallized product, the concentrated solution withdrawn from the side of the crystallizer 1, the condensed steam withdrawn from the top of the crystallizer 1, and the separated liquid separated by the liquid separation means 3. Therefore, it is possible to establish the balance equation for nickel (Equation 1), the balance equation for specific impurities (Equation 2), and the balance equation for the total amount of material (Equation 3).

[0027] [Formula 1] X.N X =Y·N Y +10000 Z N Z [Formula 2] X C X =Y·C L+1000 Z C S [Formula 3] X ρ X +1000·A=Y·ρ Y +1000·Z+1000·(B1+B2)

[0028] Here, as shown in Table 1 below, X, Y, A, B1, and B2 are the flow rates measured by the flow meters 11 to 15, respectively, Z is the mass measured by the weighing device 9 such as a load cell, and N X , N Y , N Z is the nickel concentration or nickel grade obtained by analysis, and C X is the concentration of a specific impurity obtained by analysis, and ρ X and ρ Y is the specific gravity obtained by analysis.

[0029] [Table 1]

[0030] As mentioned above, the impurity concentration in the concentrate, C L is the impurity grade C in the nickel sulfate crystals mentioned above. S By setting the target value of the above, it can be calculated from the distribution ratio K. Therefore, the above various parameters A, B1, B2, N X , N Y , N Z , C X , ρ X , ρ Y , C S and C L Substituting these equations into Equations 1 to 3 results in simultaneous equations with three variables, X, Y, and Z, and therefore, once the feedstock introduction amount X and the product crystal production amount Z are determined, it becomes possible to derive the concentrated solution withdrawal amount Y. Nickel sulfate crystals having a target impurity quality can be refined by periodically or continuously withdrawing the nickel sulfate concentrated solution from the crystallizer 1 at the obtained concentrated solution withdrawal amount Y.

[0031] The value of the withdrawal amount Y of the nickel sulfate concentrate from the crystallizer 1 obtained by the above balance calculation is the withdrawal amount per day. This amount may be withdrawn from the crystallizer 1 only once per day (24 hours). However, if the withdrawal frequency is extremely low, impurities in the nickel sulfate concentrate in the crystallizer will accumulate, and the impurity quality in the nickel sulfate crystals may exceed the target value. Therefore, taking into account the typical factory work schedule of "8 hours per shift," it is preferable to withdraw the nickel sulfate concentrate at least once per 8 hours or multiple times at equal intervals (e.g., once per 4 hours, once per 2 hours, etc.). This ensures that the work content of each shift is the same, further reducing the variation in the impurity quality of the product crystals due to differences in work performed by each shift. Alternatively, as shown in Figure 1, the concentrated solution may be continuously withdrawn by providing a flow control valve on the discharge side of the concentrated solution withdrawal pump 8 in the concentrated solution withdrawal pipe and controlling its opening using a control means such as a DCS.

[0032] Conventionally, the impurity quality of the product crystals was adjusted while monitoring the operating conditions of the crystallizer, for example, by increasing the withdrawal amount by 1 to 5 times if the Mg concentration in the nickel sulfate concentrate was greater than 20 mg / L, and decreasing the withdrawal amount by 1 to 1 / 5 times if it was less than 20 mg / L. As a result, the amount of concentrated solution withdrawn from the crystallizer was either excessive or insufficient, resulting in large fluctuations in the impurity quality of the product crystals. However, as described above, the control method of an embodiment of the present invention determines the amount of concentrated solution withdrawn from the crystallizer based on a balance calculation of the components to be crystallized, specific impurities, and the entire substance that enters and leaves the crystallization treatment system, making it possible to suppress variations in the impurity quality in the product crystals. [Example]

[0033] (Example) A stock solution of nickel sulfate containing nickel sulfate as the component to be crystallized and magnesium (Mg) as an impurity was introduced into a crystallizer 1 as shown in Figure 1, and evaporated under reduced pressure at a liquid temperature of 45 to 46°C to form a concentrated solution, thereby crystallizing the component to be crystallized. The concentrated slurry containing the obtained crystals was extracted from the bottom of the crystallizer 1, dehydrated, and dried, thereby producing product crystals at a production rate (Z) of 14 to 56 tons per day. The target Mg content (C) in the product crystals was 100%. S ) was set to 8 mass ppm, and the amount of concentrated liquid (Y) required to be extracted in this case was derived from the balance calculations of the above equations 1 to 3.

[0034] The values ​​of each parameter used in this balance calculation were obtained as follows: The Ni concentration (N X ) 135-144g / L, and Mg concentration (C X ) 4 to 8 mg / L, respectively, and the Ni concentration (N Y ) 180g / L was obtained. The Ni content (N) in the nickel sulfate product crystals was calculated from the latest value and the average value for the last 4 hours. Z ) 22.3 mass%. Furthermore, the specific gravity (ρ X ) and the specific gravity of the concentrated liquid (ρ Y ) was calculated from the following relational expression 4. [Formula 4] Specific gravity of nickel sulfate solution (g / cm 3 )=1+Ni concentration (g / L)÷40

[0035] Mg concentration of concentrated nickel sulfate solution (C L ) is the distribution ratio K=C S / C L In the proportional relationship, the distribution ratio K for magnesium (Mg) is set to 0.4, and the target value of the Mg content in the product crystal (C S) was substituted to obtain 20 mg / L. Furthermore, the flow rate (X) of the raw solution introduced into the crystallizer 1, the flow rate (A) of the washing water used to wash the inside of the crystallizer 1, the flow rate (B1) of the steam condensate extracted from the top of the crystallizer 1, and the flow rate (B2) of the separated liquid separated by the liquid separation means 3 were determined by readings from flowmeters 11, 13-15 installed in the pipes through which these fluids flow, and the production amount of the crystal product (Z) was determined by the value weighed by the weigher 9.

[0036] The nickel sulfate concentrate solution was withdrawn from the side of the crystallizer 1 every four hours so that the withdrawal amount (Y) obtained from the balance calculation of equations 1 to 3 above was withdrawn per 24 hours. As a result, the magnesium content in the nickel sulfate crystals deviated from the target value of 8 ppm by mass on only one day during the 30-day operation, but never reached the upper control limit of 10 ppm by mass. The average magnesium content during this period was 8.0 ppm by mass, with a standard deviation of 0.2 ppm by mass, and the process capability index Cpk relative to the upper control limit was 3.65, demonstrating sufficient process capability.

[0037] (Comparative Example) Ni concentration of nickel sulfate stock solution (N X ) is 138-147g / L, and the Mg concentration of the nickel sulfate stock solution (C X Crystallization was carried out in the same manner as in the above Examples, except that the Mg content in the product crystals was 5 to 10 mg / L, the daily production amount of the product crystals (Z) was 30 to 60 tons, and the amount of concentrated solution withdrawn was calculated as follows without using the balance calculations of Equations 1 to 3 above.

[0038] That is, in this comparative example, the withdrawal amount of the nickel sulfate concentrate was calculated according to the analytical value of the Mg concentration in the nickel sulfate concentrate. Specifically, since the Mg concentration in the nickel sulfate concentrate that results in a Mg grade of 8 mass ppm in the product crystal is 20 mg / L as described above, when the Mg concentration in the nickel sulfate concentrate is higher than 20 mg / L, the withdrawal amount was increased by 1 to 5 times, and when it is lower than 20 mg / L, the withdrawal amount was reduced by 1 to 1 / 5 times. The Ni and Mg concentrations of the nickel sulfate stock solution and the Mg concentration of the nickel sulfate concentrate were analyzed every 8 hours, and the operation of withdrawing the nickel sulfate concentrate from the crystallizer was carried out every 4 hours.

[0039] As a result, the magnesium content in the nickel sulfate crystals deviated from the target value of 8 ppm by mass on 15 days out of 30 days of operation, and reached the upper limit of the control value of 10 ppm by mass on two days. The average magnesium content during this period was 8.2 ppm by mass, with a standard deviation of 0.8 ppm by mass, and the process capability index Cpk for the upper limit of the control value was 0.73, indicating that the process capability was insufficient and that improvement was required. [Explanation of symbols]

[0040] 1 crystallizer 1a Draft tube 1b Mixing blade 1c classification leg 1d Set ring part 2. Slurry pump 3 Liquid separation means 4. Heated liquid extraction pump 5 Heater 6. Vacuum pump 7 Cooler 8 Concentrate extraction pump 9 Weighing scale 10 Thermometer 11~15 Flowmeter

Claims

1. A crystallization process in which a stock solution containing a component to be crystallized and impurities introduced into a crystallizer is evaporated under reduced pressure under predetermined temperature and pressure conditions to form a concentrated solution, thereby crystallizing the component to be crystallized, and the resulting concentrated slurry containing the crystals is withdrawn from the bottom, dehydrated, and dried to recover product crystals, while a portion of the concentrated solution is withdrawn from the side, A method for controlling the quality of impurities in crystals, characterized in that the amount of the portion of the concentrated solution to be withdrawn necessary to maintain the quality of impurities in the product crystals at a predetermined value is determined based on a balance calculation of the components to be crystallized, impurities, and the entire substance that enters and leaves the system in which the crystallization treatment is carried out.

2. 2. The method for controlling impurity quality according to claim 1, wherein the substances flowing in and out of the system where the crystallization treatment is carried out are the raw solution, a washing liquid for washing the inside of the crystallization vessel, the product crystals, the concentrated liquid withdrawn from the side, a condensate obtained by condensing steam withdrawn from the top of the crystallization vessel, and a separated liquid separated by the dehydration and drying.

3. 3. The method for controlling impurity quality according to claim 1, wherein a proportional relationship between the concentration of the impurity in the concentrated solution and the quality of the impurity in the crystals crystallized from the concentrated solution is reflected in the balance calculation.

Images