CN111269107A - L-lactic acid purification and refining method - Google Patents
L-lactic acid purification and refining method Download PDFInfo
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- JVTAAEKCZFNVCJ-REOHCLBHSA-N L-lactic acid Chemical compound C[C@H](O)C(O)=O JVTAAEKCZFNVCJ-REOHCLBHSA-N 0.000 title claims abstract description 390
- 238000000034 method Methods 0.000 title claims abstract description 75
- 238000007670 refining Methods 0.000 title claims abstract description 44
- 238000000746 purification Methods 0.000 title claims abstract description 26
- 238000005342 ion exchange Methods 0.000 claims abstract description 127
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 103
- 239000007788 liquid Substances 0.000 claims abstract description 96
- JVTAAEKCZFNVCJ-UHFFFAOYSA-N lactic acid Chemical compound CC(O)C(O)=O JVTAAEKCZFNVCJ-UHFFFAOYSA-N 0.000 claims abstract description 90
- 238000000855 fermentation Methods 0.000 claims abstract description 60
- 230000004151 fermentation Effects 0.000 claims abstract description 60
- 239000004310 lactic acid Substances 0.000 claims abstract description 45
- 235000014655 lactic acid Nutrition 0.000 claims abstract description 45
- 239000012528 membrane Substances 0.000 claims abstract description 36
- 238000001728 nano-filtration Methods 0.000 claims abstract description 35
- 150000001768 cations Chemical class 0.000 claims abstract description 26
- 238000000526 short-path distillation Methods 0.000 claims abstract description 25
- 239000002994 raw material Substances 0.000 claims abstract description 20
- 238000001914 filtration Methods 0.000 claims abstract description 17
- 238000000926 separation method Methods 0.000 claims abstract description 16
- 229920000747 poly(lactic acid) Polymers 0.000 claims abstract description 14
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- 238000005341 cation exchange Methods 0.000 claims abstract description 13
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- -1 iron ions Chemical class 0.000 claims description 11
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- 229910001424 calcium ion Inorganic materials 0.000 claims description 10
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- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 9
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- 239000003957 anion exchange resin Substances 0.000 claims description 4
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- 238000000502 dialysis Methods 0.000 claims description 3
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- WQZGKKKJIJFFOK-GASJEMHNSA-N Glucose Natural products OC[C@H]1OC(O)[C@H](O)[C@@H](O)[C@@H]1O WQZGKKKJIJFFOK-GASJEMHNSA-N 0.000 description 3
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- MKJXYGKVIBWPFZ-UHFFFAOYSA-L calcium lactate Chemical compound [Ca+2].CC(O)C([O-])=O.CC(O)C([O-])=O MKJXYGKVIBWPFZ-UHFFFAOYSA-L 0.000 description 3
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- 229960002401 calcium lactate Drugs 0.000 description 3
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- KRKNYBCHXYNGOX-UHFFFAOYSA-N citric acid Chemical compound OC(=O)CC(O)(C(O)=O)CC(O)=O KRKNYBCHXYNGOX-UHFFFAOYSA-N 0.000 description 3
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- 238000005903 acid hydrolysis reaction Methods 0.000 description 2
- WQZGKKKJIJFFOK-VFUOTHLCSA-N beta-D-glucose Chemical compound OC[C@H]1O[C@@H](O)[C@H](O)[C@@H](O)[C@@H]1O WQZGKKKJIJFFOK-VFUOTHLCSA-N 0.000 description 2
- MKJXYGKVIBWPFZ-CEOVSRFSSA-L calcium;(2s)-2-hydroxypropanoate Chemical compound [Ca+2].C[C@H](O)C([O-])=O.C[C@H](O)C([O-])=O MKJXYGKVIBWPFZ-CEOVSRFSSA-L 0.000 description 2
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- 229940116871 l-lactate Drugs 0.000 description 2
- 229920002521 macromolecule Polymers 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
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- 239000012492 regenerant Substances 0.000 description 2
- 235000011121 sodium hydroxide Nutrition 0.000 description 2
- NGSFWBMYFKHRBD-DKWTVANSSA-M sodium;(2s)-2-hydroxypropanoate Chemical compound [Na+].C[C@H](O)C([O-])=O NGSFWBMYFKHRBD-DKWTVANSSA-M 0.000 description 2
- 239000002910 solid waste Substances 0.000 description 2
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- 229920002472 Starch Polymers 0.000 description 1
- CZMRCDWAGMRECN-UGDNZRGBSA-N Sucrose Chemical compound O[C@H]1[C@H](O)[C@@H](CO)O[C@@]1(CO)O[C@@H]1[C@H](O)[C@@H](O)[C@H](O)[C@@H](CO)O1 CZMRCDWAGMRECN-UGDNZRGBSA-N 0.000 description 1
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- DQYBDCGIPTYXML-UHFFFAOYSA-N ethoxyethane;hydrate Chemical compound O.CCOCC DQYBDCGIPTYXML-UHFFFAOYSA-N 0.000 description 1
- LZCLXQDLBQLTDK-BYPYZUCNSA-N ethyl (2S)-lactate Chemical compound CCOC(=O)[C@H](C)O LZCLXQDLBQLTDK-BYPYZUCNSA-N 0.000 description 1
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- LPEKGGXMPWTOCB-VKHMYHEASA-N methyl (S)-lactate Chemical compound COC(=O)[C@H](C)O LPEKGGXMPWTOCB-VKHMYHEASA-N 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
- C07C51/02—Preparation of carboxylic acids or their salts, halides or anhydrides from salts of carboxylic acids
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F11/00—Compounds of calcium, strontium, or barium
- C01F11/46—Sulfates
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
- C07C51/42—Separation; Purification; Stabilisation; Use of additives
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- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
- C07C51/42—Separation; Purification; Stabilisation; Use of additives
- C07C51/43—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation
- C07C51/44—Separation; Purification; Stabilisation; Use of additives by change of the physical state, e.g. crystallisation by distillation
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- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
- C07C51/42—Separation; Purification; Stabilisation; Use of additives
- C07C51/47—Separation; Purification; Stabilisation; Use of additives by solid-liquid treatment; by chemisorption
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- Engineering & Computer Science (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Life Sciences & Earth Sciences (AREA)
- Geology (AREA)
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- Crystallography & Structural Chemistry (AREA)
- Preparation Of Compounds By Using Micro-Organisms (AREA)
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Abstract
The invention belongs to the technical field of L-lactic acid production, and particularly discloses a method for purifying and refining L-lactic acid. The method for purifying and refining the L-lactic acid provided by the invention removes the thalli in the fermentation liquor by separation to obtain the thalli residues as the feed raw materials; performing acidolysis, filtering, decolorizing the clear liquid with activated carbon, introducing into a first ion exchange column, performing cation and anion exchange to obtain a primary ion exchange solution, filtering the primary ion exchange solution with a nanofiltration membrane, performing preconcentration, and performing cation and anion exchange in sequence to obtain a secondary ion exchange solution; decolorizing the secondary ion exchange liquid with activated carbon decolorizing column, and concentrating to obtain lactic acid concentrated solution with L-lactic acid content of 80-90%; the lactic acid concentrated solution is purified by short-path distillation to obtain high-purity food grade lactic acid or produce polylactic acid. The purification and refining method of the L-lactic acid improves the quality of the finished product of the L-lactic acid, and the finally obtained L-lactic acid has good quality and high yield, and particularly meets the quality requirement of the L-lactic acid used as a PLA raw material.
Description
Technical Field
The invention relates to the technical field of L-lactic acid production, in particular to a method for purifying and refining L-lactic acid.
Background
The L-lactic acid is an organic acid obtained by fermenting glucose or sucrose as a raw material by microorganisms and then purifying and refining the fermentation liquor. The molecular formula of L-lactic acid is C3H6O3L-lactic acid is a colorless, clear viscous liquid, and the aqueous solution is acidic. L-lactic acid can be optionally mixed with water, ethanol or diethyl ether, and is insoluble in chloroform. Because of the levorotatory characteristic, the compound has good biological compatibility, can be combined with mammals, can directly participate in human metabolism, has no side effect, and is widely applied to the fields of food, medicine, cosmetics, tobacco and other industries. The L-lactic acid can also be used for producing biodegradable plastic polylactic acid and green environment-friendly solvents such as L-methyl lactate, L-ethyl lactate and the like, and has wide application.
The industrial production of the domestic L-lactic acid is started from 2003, compared with other organic acids (such as citric acid), the industrial time is shorter, the product scale is not large, the production factory with the scale of ten thousand tons is 4-5, the total domestic annual energy is about 20 ten thousand tons, the maximum production capacity is the Henan Jindan lactic acid science and technology corporation, and the annual output reaches 10 ten thousand tons (dry basis). At present, the production technology and equipment level of the domestic L-lactic acid are still to be improved and perfected, and the product quality, particularly the product qualification rate of the L-lactic acid which meets the production raw material of polylactic acid, is to be improved.
At present, two methods are mainly used for purifying and refining L-lactic acid from L-lactic acid fermentation liquor, and the process routes are respectively shown in attached figures 1 and 2. The two purification and refining methods are basically the same, firstly, the pH value of the fermentation liquor is adjusted to 10-11 by lime milk, and the fermentation liquor is heated to 80-95 ℃; then flocculating the thalli by adding a flocculating agent; pumping the flocculating solution into a plate frame for filtering to remove thalli, and obtaining calcium lactate crude product solution and wet thalli residues; then, directly or after concentration, the calcium lactate crude product solution is mixed with concentrated sulfuric acid for double decomposition reaction to obtain a mixture of calcium sulfate and lactic acid; and separating solid calcium sulfate slag from the acidolysis product by a vacuum belt filter to obtain a crude lactic acid solution. The crude lactic acid solution obtained at this time contains many impurities, such as monosaccharides, oligosaccharides, heteroacids, pigments, soluble various organic substances and salts, etc., which are brought about from the fermentation process and generated by extraction.
These two purification and refining methods differ in the purification treatment after the crude lactic acid solution. The process shown in figure 1 comprises subjecting the obtained crude L-lactic acid solution to cation exchange to remove Ca2+、Mg2+An isocationic acid; then filtering by a nanofiltration membrane to remove most macromolecular substances such as pigments, sugars, proteins and the like to obtain an L-lactic acid solution with lighter color; then carrying out carbon column decolorization, anion exchange, secondary cation exchange and secondary anion exchange to obtain a pure L-lactic acid dilute solution with the concentration of 12-15% (w/v); then the dilute L-lactic acid solution is subjected to acid concentration and short-path distillation to obtain 100% (w/w) L-lactic acid which can be directly used for producing polylactic acid, or pure water is added to dilute the L-lactic acid to 80-90% and the L-lactic acid is sold as a raw material for producing polylactic acid or a food-grade L-lactic acid product. The amount of L-lactic acid in the bottom liquid obtained by short-path distillation accounts for 15-25% of the feeding amount of the short-path distillation L-lactic acid, the bottom liquid contains relatively high impurities such as pigment, sugar and the like (dry matter accounts for 20-40%), the concentration is 60-70% (w/w), and the bottom liquid is generally sold directly as feed acid or used for producing L-sodium lactate by an extraction technology.
The process shown in the attached figure 2 is to decolorize the obtained crude L-lactic acid solution, and since the pigment content of the crude L-lactic acid solution is very high (about 3000. sup. & 6000. sup. Hazen), powdered activated carbon is added for decolorization; the decolored solution is sequentially exchanged by cations and anions to remove anions and cations; then further decolorizing by a carbon column; the decolored solution is concentrated to 80-90% by acid and can be directly used as food grade lactic acid for sale, or short-path distillation is carried out to obtain 100% (w/w) L-lactic acid for producing PLA, or pure water is added to dilute to 80-90% for sale. Wherein the treatment of the short-path distillation still bottom liquid is the same as the process shown in figure 1, and the short-path distillation still bottom liquid is generally sold directly as feed acid or used for producing L-sodium lactate by extraction technology.
The two purification methods described above have some disadvantages as follows.
Firstly, the fermentation liquor is made of lime milk, namely Ca (OH)2Adjusting the pH of the solutionWhen the value is 10-11, the fermentation liquor is heated to 80-95 ℃, residual sugar, protein and the like in the fermentation liquor are easy to generate Maillard reaction under the alkaline and high-temperature conditions, so that the pigment of the fermentation liquor is further deepened, the burden is brought to subsequent refining and decoloration, and the production cost is increased; the pH value of the fermentation liquor is adjusted to 10-11, so that the bacterial slag is alkaline, the bacterial slag obtained by the treatment process can not be used as feed and can only be used as organic matters of compound fertilizer, and the additional value of the bacterial slag is reduced.
Secondly, the acidolysis solution contains saturated CaSO according to the process shown in figure 14General Ca2+The content is 700-1400mg/L, SO4 2-The content is 1500-; after the acidolysis solution is subjected to cation exchange, the sulfuric acid content in the acidolysis solution reaches 1.5-2.2% (w/v), the sulfuric acid concentration is very high, great influence is brought to the selection of the nanofiltration membrane material and the service life of the nanofiltration membrane in the subsequent process, and the selection difficulty, the investment and the operation cost of the membrane are increased. The pigment of the crude L-lactic acid filtered by the nanofiltration membrane is still higher (average chroma is 600-1200Hazen), the load of carbon column decolorization is very large, the consumption of water and liquid alkali is high, the generation amount of alkaline wastewater is very large, and the subsequent treatment pressure is relatively large.
Thirdly, the process shown in figure 2, in which powdered activated carbon is directly used for decolorization, has large consumption of activated carbon (60-80 kg per ton of finished lactic acid), poor production environment and high treatment cost of waste carbon according to solid waste. The decolorizing of the powdered carbon and the primary ion exchange cannot achieve the ideal separation effect on the purification of pigment, residual sugar and various ions, and the separation effect on residual oligosaccharide and glucose in the fermentation liquor is very limited. The pigment in the acid concentration process can be further deepened due to the residue of impurities, so that only low-end food-grade L-lactic acid can be produced, and the quality and the yield of the high-quality L-lactic acid obtained after short-path distillation are greatly influenced, even the quality standard of the heat-resistant L-lactic acid cannot be reached.
Disclosure of Invention
The invention mainly solves the technical problem of providing a method for purifying and refining L-lactic acid so as to improve the quality of L-lactic acid products.
In order to solve the technical problems, the invention adopts a technical scheme that: a method for purifying and refining L-lactic acid comprises the steps of heating fermentation liquor containing L-lactic acid to 60-70 ℃, carrying out solid-liquid separation, and collecting fermentation clear liquid;
carrying out double decomposition reaction on the collected fermentation clear liquid in the environment with the pH value of 1.5-2.0, carrying out solid-liquid separation, and collecting the liquid phase to obtain acidolysis filter clear liquid;
carrying out primary decolorization on the collected acidolysis filtrate clear liquid, and then carrying out primary ion exchange treatment; filtering the obtained ion exchange solution by a filter membrane, concentrating, performing secondary ion exchange treatment, and performing secondary decolorization;
the ion exchange column adopted in the first and/or second ion exchange treatment is formed by connecting a cation column and an anion column in series.
Further, the invention provides a method for purifying and refining L-lactic acid, which comprises the following steps:
(1) heating fermentation liquor containing L-lactic acid obtained by fermentation to 60-70 ℃, then carrying out centrifugal separation to obtain fermentation clear liquid, controlling the OD value of the fermentation clear liquid to be less than or equal to 2, and drying solid phase obtained by centrifugal separation to obtain a feed raw material;
(2) concentrating the fermentation clear liquid to obtain a fermentation clear liquid concentrated solution, adding concentrated sulfuric acid into the obtained fermentation clear liquid concentrated solution to perform double decomposition reaction to obtain an acidolysis solution, wherein the pH value of the acidolysis solution is controlled to be 1.5-2.0, and the acidolysis solution contains L-lactic acid and calcium sulfate; then carrying out solid-liquid separation on the acidolysis solution to obtain calcium sulfate solid and acidolysis filter clear solution, and drying the calcium sulfate solid at high temperature to obtain gypsum for buildings;
(3) performing primary decolorization on the acidolysis filtered clear liquid by using an activated carbon decolorizing column to obtain primary decolorized liquid, allowing the primary decolorized liquid to enter a first ion exchange column in a continuous ion exchange system for primary ion exchange, wherein the first ion exchange column is formed by connecting a cation column and an anion column in series, and the primary decolorized liquid is subjected to cation exchange and anion exchange in sequence to obtain primary ion exchange liquid; filtering the primary ion exchange liquid by a nanofiltration membrane and then pre-concentrating to obtain an L-lactic acid pre-concentrated solution; the L-lactic acid preconcentrated solution enters a second ion exchange column in a continuous ion exchange system for secondary ion exchange, the second ion exchange column is formed by connecting a cation column and an anion column in series, and the L-lactic acid preconcentrated solution is subjected to cation exchange and anion exchange in sequence to obtain secondary ion exchange solution; and (3) carrying out secondary decolorization on the secondary ion exchange liquid by using an activated carbon decolorization column, and then carrying out concentration treatment to obtain a lactic acid concentrated solution with the L-lactic acid content of 80-90%, wherein the lactic acid concentrated solution is food-grade L-lactic acid.
Further, the obtained food-grade L-lactic acid can be refined by short-range distillation to obtain high-purity lactic acid with the L-lactic acid content of 98-100%, and the high-purity lactic acid can be used for producing polylactic acid or used as high-purity food-grade lactic acid. The obtained lactic acid with the L-lactic acid content of 98-100 percent can be diluted by pure water to obtain food-grade L-lactic acid for sale.
Preferably, the temperature of the fermentation clear liquid concentrated solution in the step (2) is 75-80 ℃, wherein the mass volume ratio concentration of the L-lactic acid is 20-25%.
Preferably, the acidolysis filtered clear solution in the step (3) is firstly decolorized by an activated carbon decolorizing column filled with granular activated carbon, the activated carbon decolorizing column is connected in parallel, that is, the acidolysis filtered clear solution is uniformly divided into several equal parts, for example, 5 to 6 equal parts, after passing through the main pipeline, and then simultaneously and respectively enters different activated carbon decolorizing columns arranged in parallel, and flows through the activated carbon columns for decolorization in a laminar flow manner by using a liquid distributor from bottom to top; the feeding mode is low-in and high-out, namely the acid hydrolysis filtering clear liquid enters from the bottom of the decolorizing column and flows out from the top, the retention time of the acid hydrolysis filtering clear liquid in the activated carbon decolorizing column is 6-10 hours, and the chroma of the obtained primary decolorizing liquid is less than 30 hazen; wherein, the filled granular activated carbon can be regenerated, and the regeneration mode adopts thermal regeneration.
As a preferred embodiment, the continuous ion exchange system comprises a first ion exchange column and a second ion exchange column, the first ion exchange column and the second ion exchange column are independently arranged, each of the first ion exchange column and the second ion exchange column is composed of a cation column and an anion column in series, the cation column adopts strong acid cation exchange resin as a filler, and the anion column adopts weak base anion exchange resin as a filler. The first ion exchange column and the second ion exchange column are arranged in a continuous ion exchange system, and two times of ion exchange treatment can be completed through the continuous ion exchange system, so that the number of equipment can be reduced, the equipment investment can be reduced, the occupied area can be saved, the consumption of regenerant hydrochloric acid, liquid caustic soda and pure water can be reduced, and the operation cost can be reduced.
Preferably, the concentration of calcium ions in the primary ion exchange liquid is less than or equal to 2ppm, the concentration of iron ions is less than or equal to 1ppm, the concentration of sulfate ions is less than or equal to 2ppm, and the concentration of chloride ions is less than or equal to 2 ppm.
Preferably, the concentration of calcium ions in the secondary ion exchange liquid is less than or equal to 2ppm, the concentration of iron ions is less than or equal to 1ppm, the concentration of sulfate ions is less than or equal to 2ppm, and the concentration of chloride ions is less than or equal to 2 ppm.
In a preferred embodiment, the nanofiltration membrane is used for filtering, the concentration multiple of the nanofiltration membrane is 10-15 times, the addition amount of the dialysis water is 15% -25%, and the content of the L-lactic acid in the nanofiltration membrane filtrate is 12-16%.
Preferably, the content of the L-lactic acid in the pre-concentrated L-lactic acid solution obtained by pre-concentration is 35-45%.
As a preferred embodiment, the activated carbon decolorizing column adopted in the secondary ion exchange liquid in the secondary decolorizing is filled with granular activated carbon, the activated carbon decolorizing column is also in a parallel connection mode, the feeding mode is a low-inlet and high-outlet mode, the retention time of the secondary ion exchange liquid in the activated carbon decolorizing column is 6-10 hours, and the chromaticity of the obtained decolorizing liquid is less than 30 hazen.
According to the method for purifying and refining the L-lactic acid, on the first hand, when the thalli are separated from the L-lactic acid fermentation liquor, a flocculating agent is not required to be added, the pH value is not regulated by lime milk, but the fermentation liquor is heated to 60-70 ℃ by heating, preferably 65-70 ℃ so that the thalli are separated from the fermentation liquor, so that equipment investment such as flocculating agent allocation and the like and consumption of auxiliary materials and steam are saved, the deepening of chroma caused by pH adjustment (for example, pH value is adjusted to 10-11) and high-temperature heating can be avoided, and the burden of subsequent dehydration is reduced. Meanwhile, the thalli separated by the method can be dried to be used as protein feed, the additional value of byproducts is improved, and the production cost of the L-lactic acid is further offset.
In the second aspect, the acidolysis filtrate is firstly decolorized by an activated carbon decolorizing column and then is subjected to ion exchange, and the decolorization adopts a granular activated carbon + thermal regeneration process, for example, a multi-stage regeneration furnace is adopted to regenerate the activated carbon. Thus, the high cost of the existing powdered carbon decolorization can be effectively changed, and the production environment is improved; the existing single granular activated carbon and alkali regeneration process is changed, the consumption of a large amount of liquid alkali and pure water is overcome, the generation of waste water is reduced, and the method has a great significance particularly for areas lacking water resources.
In the third aspect, the acidolysis filtered clear solution is firstly decolorized for the first time by using an activated carbon decolorizing column and then is subjected to ion exchange for the second time, and the primary decolorized solution is subjected to cation exchange and anion exchange sequentially to obtain a primary ion exchange solution in the primary ion exchange, namely, the cation and anion ion exchange process is adopted, so that the condition that the L-lactic acid solution contains 2-4% of sulfuric acid due to the existing cation exchange decalcification process is avoided, the corrosion to equipment in the nanofiltration membrane process is avoided, the requirement on the nanofiltration membrane is also reduced, the nanofiltration membrane can be selected to be relatively universal, the cost is reduced, and the nanofiltration membrane is low in operation cost.
And in the fourth aspect, the ion exchange liquid after the primary ion exchange is filtered by a nanofiltration membrane, and macromolecular substances which are brought in fermentation and cannot be completely removed in the decoloring and ion exchange processes, such as pigments, proteins, degradation products thereof, sugar and the like and have larger molecular weight than that of the L-lactic acid are further separated, so that the impurities are prevented from being brought into short-range distillation, and the quality of the finished product L-lactic acid and the yield of the finished product L-lactic acid are not influenced. Filtering by a nanofiltration membrane, then pre-concentrating, concentrating the dilute solution of the L-lactic acid from 12-16% to 35-45%, enriching anions, cations and pigments which cannot be completely removed by primary ion exchange and primary decolorization, and then performing secondary ion exchange and secondary decolorization purification for removing impurities.
In the fifth aspect, the load of the secondary ion exchange is very light, the second ion exchange column and the first ion exchange column of the primary ion exchange are arranged in a continuous ion exchange system, and the first ion exchange column and the second ion exchange column are independently arranged and are provided with two inlets and two outlets.
In the sixth aspect, the load of the secondary decolorization is very light, the secondary decolorization and the primary decolorization can share one thermal regeneration furnace for activated carbon regeneration, and the thermal alkali regeneration can also be independently adopted.
The invention is a purification and refining process of L-lactic acid, which is obtained by improving the characteristics of the components of L-lactic acid fermentation liquor on the basis of summarizing and analyzing the prior domestic L-lactic acid process. In the purification and refining process, firstly, carrying out primary decolorization on acidolysis solution, then removing most impurities through primary cation and anion ion exchange, then carrying out nanofiltration and preconcentration to enrich the impurities, then removing the impurities again through secondary cation and anion ion exchange, then carrying out secondary decolorization to obtain an L-lactic acid product with high purity, and then concentrating to obtain a lactic acid concentrated solution with the L-lactic acid content of 80-90%, namely food-grade L-lactic acid, which can be directly sold; or food-grade L-lactic acid is distilled in a short distance to obtain high-purity lactic acid with the L-lactic acid content of 98-100 percent, and the high-purity lactic acid is used for producing polylactic acid. The purification and refining method is suitable for industrial production, can reduce the production cost of the existing L-lactic acid, improves the product quality of the L-lactic acid, not only greatly improves the quality index of the food-grade L-lactic acid, but also greatly improves the yield of the L-lactic acid product according with the polylactic acid raw material, meets the national call of plastic restriction and plastic prohibition, and provides raw material guarantee for large-scale industrialization of degradable Plastics (PLA).
In the purification and refining process of the L-lactic acid, wherein the residue of the short-path distillation step is also called still bottom liquid and black lactic acid, the amount of the L-lactic acid in the black lactic acid is related to the quality of the L-lactic acid subjected to the short-path distillation, and the amount of the L-lactic acid in the black lactic acid generally accounts for 15-30% of the feeding amount of the short-path distillation L-lactic acid due to the influence of the quality of the L-lactic acid subjected to the short-path distillation at present. The experiment shows that the quality of the L-lactic acid subjected to short-path distillation obtained by adopting the L-lactic acid purification and refining method provided by the invention is improved, the impurity content is low, and the amount of the L-lactic acid in the black lactic acid can be reduced to be less than 10% of the feeding amount of the short-path distillation L-lactic acid. The short-path distillation of the black lactic acid is used as feed acid or used for producing food-grade L-lactic acid or L-lactate by extraction. The L-lactic acid purification and refining method provided by the invention improves the quality of the finished product of the L-lactic acid, and the finally obtained L-lactic acid has good quality and high yield, and is particularly suitable for the current industry development trend, namely meets the quality requirement of the L-lactic acid used as a PLA raw material.
Drawings
FIG. 1 is a process scheme of a conventional method for purifying and refining L-lactic acid from an L-lactic acid fermentation broth;
FIG. 2 is a process scheme of another conventional method for purifying and refining L-lactic acid from an L-lactic acid fermentation broth;
FIG. 3 is a process scheme diagram of a method for purifying and refining L-lactic acid from L-lactic acid fermentation broth according to the present invention.
Detailed Description
The technical solution of the present invention will be explained in detail below.
The invention provides a method for purifying and refining L-lactic acid from L-lactic acid fermentation liquor, which refers to a process route shown in figure 3 and comprises the following steps:
s1: pretreatment of L-lactic acid fermentation broth
(1) The pH value of the L-lactic acid fermentation liquor obtained by fermentation is 6.5-6.8, and after the fermentation is finished, the fermentation liquor is heated to 60-70 ℃.
(2) And (3) conveying the heated fermentation liquor into a disc centrifuge for centrifugal separation treatment, wherein the obtained centrifugal clear liquid is fermentation clear liquid, the OD value of the fermentation clear liquid is controlled to be less than or equal to 2, and the solid phase obtained by centrifugal separation is subjected to the following treatment.
(3) And (3) feeding the separated solid phase into a double-phase horizontal spiral separator, wherein the solid content of the obtained mushroom dregs is more than or equal to 50% (w/w), then concentrating and spray-drying the mushroom dregs to obtain commercial mushroom dregs, and drying and packaging the commercial mushroom dregs to be sold as feed raw materials.
S2: l-lactic acid extraction
(1) Concentrating the fermented clear liquid containing L-calcium lactate (the L-calcium lactate comes from the fermentation process), controlling the temperature of the concentrated discharging liquid at 75-80 ℃, and concentrating to obtain the concentrated liquid of the fermented clear liquid, wherein the concentration of the L-lactic acid is controlled at 20-25% (w/v).
(2) Concentrated fermentation liquor is added with concentrated sulfuric acid to carry out double decomposition reaction, the pH value of acidolysis solution is controlled to be 1.5-2.0, the temperature of an acidolysis pot is controlled to be 75-90 ℃, and acidolysis solution obtained through double decomposition reaction contains L-lactic acid and calcium sulfate.
(3) The acidolysis solution is subjected to solid-liquid separation treatment by a belt filter (or a plate frame filter, a vertical belt filter and the like), the obtained calcium sulfate solid can be sold as gypsum for construction through high-temperature drying, and the obtained acidolysis filtration clear solution enters the next treatment step.
S3: l-lactic acid purification
(1) Removing pigment from the acidolysis filtered clear liquid through granular activated carbon, filling the granular activated carbon in an activated carbon decoloring column, adopting a parallel connection mode for the decoloring column, adopting a low-in-high-out feeding mode, controlling the chroma of the primary decoloring liquid after decoloring to be less than 30hazen, and controlling the retention time of the acidolysis filtered clear liquid in the activated carbon decoloring column to be 6-10 h. The regeneration mode of the granular activated carbon adopts a multi-section regeneration furnace for thermal regeneration, acid and alkali consumption is not generated, and the environmental pollution is reduced.
(2) The primary decolorized solution enters a first ion exchange column in a continuous ion exchange system for primary ion exchange, the first ion exchange column is formed by connecting a cation column and an anion column in series, wherein the cation column and the anion column can be arranged in parallel, the cation column adopts strong acid cation exchange resin as a filler, the anion column adopts weak base anion exchange resin as a filler, the primary decolorized solution sequentially undergoes cation exchange and anion exchange to obtain the primary ion exchange solution, and the ion exchange aims at removing anions and cations in the primary decolorized solution and a small amount of pigments, so that the product purity is improved, the pollution to the nanofiltration membrane of the later process can be reduced, the service cycle of the nanofiltration membrane is prolonged, and the production cost is reduced. The calcium ion, the iron ion, the sulfate ion and the chloride ion in the feed liquid after the primary ion exchange are less than or equal to 2ppm, less than or equal to 1ppm, less than or equal to 2ppm and less than or equal to 2ppm respectively.
(3) And filtering the primary ion exchange liquid by using a nanofiltration membrane, wherein the concentration multiple of the primary ion exchange liquid is 10-15 times that of the nanofiltration membrane, the addition amount of dialysis water is 15-25%, the content of L-lactic acid in the filtrate of the nanofiltration membrane is 12-16% (w/w), and the filtering purpose of the nanofiltration membrane is to remove residual sugar, macromolecular protein, inorganic salt, pigment and the like in the filtrate, so that the purity of the L-lactic acid is further improved, and the quality of the final L-lactic acid product is guaranteed.
(4) Pre-concentrating the nanofiltration membrane filtrate filtered by the nanofiltration membrane to obtain L-lactic acid pre-concentrated solution; the pre-concentration is to evaporate the nanofiltration membrane filtrate by an evaporator to increase the content of L-lactic acid in an L-lactic acid pre-concentrated solution to 35-45% (w/w), and the pre-concentration aims to enrich a small amount of ions, pigments and impurities remained in the previous process, so as to be convenient for further removal.
(5) And (2) allowing the L-lactic acid preconcentrated solution to enter a second ion exchange column in the continuous ion exchange system for secondary ion exchange, wherein the second ion exchange column is formed by connecting a cation column and an anion column in series, a plurality of the cation columns and a plurality of the anion columns can be arranged in parallel, the cation column adopts a strong acid cation exchange resin as a filler, the anion column adopts a weak base anion exchange resin as a filler, the L-lactic acid preconcentrated solution is subjected to cation exchange and anion exchange in sequence to obtain a secondary ion exchange solution, the secondary ion exchange can remove a small amount of enriched ions and impurities, further purify the L-lactic acid, and the calcium ions, the iron ions, the sulfate ions and the chloride ions in the feed solution after the secondary ion exchange are less than or equal to 2ppm, the iron ions are less than or equal to 1 ppm. The first ion exchange column and the second ion exchange column are arranged in a continuous ion exchange system, and two times of ion exchange treatment can be completed through the continuous ion exchange system, so that the consumption of regenerant hydrochloric acid, liquid caustic soda and pure water can be greatly reduced, the operation flow is simplified, and the cost is reduced.
(6) The secondary ion exchange liquid is decolorized for the second time by an active carbon decolorizing column, the secondary ion exchange liquid is decolorized by the active carbon decolorizing column, concentrated and enriched pigments are removed, the product quality is improved, the chroma of the secondary decolorized liquid is less than 30hazen, and the retention time of the secondary ion exchange liquid in the active carbon decolorizing column is controlled to be 6-10 h; because the concentration of L-lactic acid in the secondary ion exchange liquid is high and the pigment amount is small, the treatment period of the activated carbon decoloring column used for secondary decoloring is long, and the activated carbon regeneration treatment is carried out only at longer intervals, so that the regeneration mode of the activated carbon decoloring column used for secondary decoloring can adopt chemical regeneration.
(7) And concentrating the secondary decolorized solution to obtain a lactic acid concentrated solution with the L-lactic acid content of 80-90%, wherein the lactic acid concentrated solution is food-grade L-lactic acid, one part of the lactic acid concentrated solution can be directly sold as the food-grade L-lactic acid, and the other part of the lactic acid concentrated solution can be subjected to the following steps.
(8) And (3) short-path distillation, namely continuously concentrating the food-grade L-lactic acid obtained by concentration by adopting scraper evaporation until the content of the L-lactic acid is 98-100%, and refining by molecular distillation to obtain an L-lactic acid product which meets the production raw material of polylactic acid. The L-lactic acid product can also be diluted by pure water to obtain food-grade L-lactic acid for sale.
In the process, the yield of the finished product of the L-lactic acid which conforms to the one-step polylactic acid process after short-path distillation is more than or equal to 75 percent (wherein the finished product requires that the chromaticity rise value is not more than 30(hazen) under the conditions that the concentration of the L-lactic acid is 88 percent, the heating condition is 200 ℃ and the heating time is 2 hours). The yield of the finished product of the L-lactic acid which is in line with the one-step polylactic acid process is less than or equal to 64 percent by adopting the prior process method shown in the attached figure 1 at present; the yield of the finished product of the L-lactic acid which is in line with the one-step polylactic acid process is less than 50 percent by adopting the prior process method shown in the attached figure 2.
Example 1
Taking annual production of 1 ten thousand tons of L-lactic acid as an example, the purification and refining process provided by the invention is adopted, and the purification and refining method provided by the embodiment specifically comprises the following steps:
(1) providing 8.669m of L-lactic acid fermentation liquor3H, the L-lactic acid content in the fermentation liquor is 16 percent (mass volume ratio m/v), and the temperature of the fermentation liquor is heated to 65 ℃ from 50 ℃ by utilizing steam;
(2) pumping the heated fermentation liquor obtained in the step (1) into a disc centrifuge, wherein the addition of washing water accounts for 17% of the feeding amount, the volume content of the suspension in the light phase is 0.18% v/v, and the OD is less than or equal to 2. Continuously treating the heavy phase by a horizontal screw centrifuge to obtain solid residues, wherein the solid content of the solid residues is 51.25%, and returning the obtained clear liquid into a feeding tank of the disc centrifuge;
(3) evaporating and concentrating the light phase of the disc centrifuge obtained in the step (2), controlling the temperature of the concentrated discharging material at 75-80 ℃, and concentrating to obtain a concentrated solution of a fermented clear liquid, wherein the concentration of calcium lactate in the concentrated feed liquid is 24.44% (w/w), and the concentration of folded L-lactic acid is 20.18% (w/w);
(4) sending the concentrated solution of the fermentation clear liquid obtained in the step (3) into an acidolysis pot, and adding concentrated sulfuric acid into the acidolysis pot, wherein the concentrated sulfuric acid amount is 0.788t/h, the concentrated sulfuric acid addition amount is 2% in excess, and the temperature of the acidolysis pot is controlled at 75-80 ℃;
(5) enabling the acidolysis solution obtained in the step (4) to pass through a vacuum belt type suction filter, wherein the addition of washing water accounts for 25% of the feeding amount, and obtaining calcium sulfate solid waste residue and belt filter clear liquid, wherein the surface water content of the calcium sulfate solid is 40%, and the L-lactic acid content in the belt filter clear liquid is 18.73%;
(6) the acidolysis filtrate obtained in the step (5) is cooled to 45 ℃ by using circulating water at 70 ℃, and then enters an activated carbon decolorizing column (filled with granular activated carbon) for primary decolorizing treatment, wherein the feeding chromaticity is 5500hazen, the discharging chromaticity is 20hazen, the regenerated activated carbon is thermally regenerated by a regenerating furnace, and the consumption of natural gas is reduced by L-lactic acid unit consumption to 2 (w/w);
(7) pumping the primary decolorized solution obtained in the step (6) into a first ion exchange column in a continuous ion exchange system to carry out primary ion exchange (the first ion exchange column comprises an anode column → an anion column) to remove anions and cations and a small amount of pigment, wherein calcium ions in the feed liquid after the primary ion exchange are less than or equal to 2ppm, iron ions are less than or equal to 1ppm, sulfate ions are less than or equal to 2ppm, and chloride ions are less than or equal to 2 ppm;
(8) pumping the feed liquid obtained after the primary ion exchange in the step (7) into a nanofiltration membrane for filtration, wherein the addition of the nanofiltration membrane dialysis water is 20%, the concentration multiple is 12 times, the operation temperature of the nanofiltration membrane is controlled within 50 ℃, the yield of the nanofiltration membrane reaches 99.50%, and the content of L-lactic acid in the filtrate of the nanofiltration membrane is 14.27%;
(9) pre-concentrating the nanofiltration membrane filtrate obtained in the step (8) by evaporation, increasing the content of L-lactic acid to 40%, enriching impurities, and consuming steam at 0.152 t/h;
(10) carrying out secondary ion exchange on the pre-concentrated solution obtained in the step (9) through a second ion exchange column in the continuous ion exchange system (the second ion exchange column comprises a positive column → a negative column) to remove positive and negative ions, impurities and pigments, treating the temperature of the material before the material enters the second ion exchange column, ensuring that the temperature of the material entering the second ion exchange column is less than or equal to 50 ℃, and ensuring that calcium ions, iron ions, sulfate ions and chloride ions in the material liquid are less than or equal to 2ppm, 1ppm and 2ppm respectively after the secondary ion exchange;
(11) performing secondary decolorization on the feed liquid treated in the step (10) through an activated carbon decolorizing column, wherein the feeding chromaticity is 520hazen, the discharging chromaticity is 27hazen, and the regenerated activated carbon is subjected to regeneration treatment by adopting a chemical method;
(12) evaporating and concentrating the decolored L-lactic acid purified liquid obtained in the step (11), wherein the concentration of the L-lactic acid before concentration is 27.78%, the L-lactic acid is evaporated and concentrated to 88% and the steam consumption is 0.082t/h, so that a food-grade L-lactic acid product is obtained;
the food-grade L-lactic acid product is refined by short-path distillation to obtain high-purity L-lactic acid, and the short-path distillation comprises scraper evaporation and molecular distillation operations, and specifically comprises the following steps:
(13) the L-lactic acid concentrated solution obtained in the step (12) is further concentrated to 100% by evaporation with a scraper, the amount of the generated scraper concentrated solution is 1.25t/h, and the steam consumption is 0.239 t/h;
(14) and (4) further purifying the L-lactic acid by using the scraper concentrated solution obtained in the step (13) through a molecular distillation system, so that the high-quality raw material L-lactic acid which meets the heat-resistant grade can be produced.
The obtained high-quality L-lactic acid which is the raw material of the lactic acid and meets the heat-resistant grade can also be added with pure water and then diluted to 80-90 percent to produce the high-quality food grade lactic acid.
The total yield of the L-lactic acid in the embodiment reaches 90.12 percent, wherein the raw material yield of the lactic acid meeting the heat-resistant grade reaches 76.61 percent. The definition of heat-resistant grade lactic acid is: the color of the L-lactic acid does not rise more than 30hazen after the L-lactic acid with the content of 88% (w/w) is heated to 200 ℃ and maintained for 2 hours.
At present, the total yield of the L-lactic acid is 88-89% by adopting the purification and refining process shown in figure 1, wherein the yield of the raw material which meets the requirement of heat-resistant grade lactic acid is less than or equal to 64%.
At present, the total yield of the L-lactic acid is 85-86% by adopting the purification and refining process shown in figure 2, wherein the yield of the raw material which meets the requirement of heat-resistant grade lactic acid is less than or equal to 50%.
The comparison shows that the total yield of the L-lactic acid product obtained by the purification and refining method is higher than the current same industry level, and the raw material yield of the heat-resistant lactic acid is obviously higher than the current same industry level. The process has the characteristics of stable quality, high yield, high raw material yield of lactic acid conforming to heat-resistant grade, high product quality and low consumption of raw and auxiliary materials.
The residues of molecular distillation are also called still bottom solution and black lactic acid. The amount of the L-lactic acid is determined by the mass of the L-lactic acid subjected to short-path distillation, the mass of the L-lactic acid subjected to short-path distillation is poor, the amount of the black lactic acid is large, otherwise, the amount of the black lactic acid is small, and the amount of the L-lactic acid in the black lactic acid accounts for 15-30% of the feeding amount of the short-path distillation L-lactic acid. The process flow of the application not only reduces the amount of the part to be less than 10 percent, but also greatly improves the quality of the finished product of the L-lactic acid. The black lactic acid is used as feed acid, or the extraction process is used for producing food-grade L-lactic acid or L-lactate.
The amount of L-lactic acid produced in the bottom liquid of this example was 1000 tons/year, wherein the amount of L-lactic acid was 9.1% (w/w) of the amount of L-lactic acid entering the short path distillation.
Similarly, taking 1 ten thousand tons of L-lactic acid produced annually as an example, the production amount of L-lactic acid in the bottom liquid of the kettle is 1500 tons/year by adopting the purification and refining process shown in the attached figure 1. By adopting the purification and refining process shown in the attached figure 2, the production amount of the L-lactic acid in the bottom liquid of the kettle is 2500 tons/year.
The index comparison table of the food-grade L-lactic acid product obtained by the purification and refining process of the invention and the food-grade L-lactic acid product obtained by the existing process is shown below. The food grade L-lactic acid product herein is an L-lactic acid product before short path distillation.
TABLE 1
The process for purifying and refining the L-lactic acid has better effect on the purification of the fermentation liquor which takes the self-made liquid glucose of the starch as the fermentation raw material.
The above description is only an example of the present invention, and is not intended to limit the scope of the present invention. Although the invention has been described in detail hereinabove with respect to a general description and specific embodiments thereof, it will be apparent to those skilled in the art that many modifications and improvements can be made based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (10)
1. A method for purifying and refining L-lactic acid is characterized in that fermentation liquor containing L-lactic acid is heated to 60-70 ℃, solid-liquid separation is carried out, and fermentation clear liquid is collected;
carrying out double decomposition reaction on the collected fermentation clear liquid in the environment with the pH value of 1.5-2.0, carrying out solid-liquid separation, and collecting the liquid phase to obtain acidolysis filter clear liquid;
carrying out primary decolorization on the collected acidolysis filtrate clear liquid, and then carrying out primary ion exchange treatment; filtering the obtained ion exchange solution by a filter membrane, concentrating, performing secondary ion exchange treatment, and performing secondary decolorization;
the ion exchange column adopted in the first and/or second ion exchange treatment is formed by connecting a cation column and an anion column in series.
2. The method for purifying and refining L-lactic acid according to claim 1, comprising the steps of:
(1) heating fermentation liquor containing L-lactic acid obtained by fermentation to 60-70 ℃, then carrying out centrifugal separation to obtain fermentation clear liquid, controlling the OD value of the fermentation clear liquid to be less than or equal to 2, and drying solid phase obtained by centrifugal separation to obtain a feed raw material;
(2) concentrating the fermentation clear liquid to obtain a fermentation clear liquid concentrated solution, adding concentrated sulfuric acid into the obtained fermentation clear liquid concentrated solution to perform double decomposition reaction to obtain an acidolysis solution, wherein the pH value of the acidolysis solution is controlled to be 1.5-2.0, and the acidolysis solution contains L-lactic acid and calcium sulfate; then carrying out solid-liquid separation on the acidolysis solution to obtain calcium sulfate solid and acidolysis filter clear solution, and drying the calcium sulfate solid at high temperature to obtain gypsum for buildings;
(3) performing primary decolorization on the acidolysis filtered clear liquid by using an activated carbon decolorizing column to obtain primary decolorized liquid, allowing the primary decolorized liquid to enter a first ion exchange column in a continuous ion exchange system for primary ion exchange, wherein the first ion exchange column is formed by connecting a cation column and an anion column in series, and the primary decolorized liquid is subjected to cation exchange and anion exchange in sequence to obtain primary ion exchange liquid; filtering the primary ion exchange liquid by a nanofiltration membrane and then pre-concentrating to obtain an L-lactic acid pre-concentrated solution; the L-lactic acid preconcentrated solution enters a second ion exchange column in a continuous ion exchange system for secondary ion exchange, the second ion exchange column is formed by connecting a cation column and an anion column in series, and the L-lactic acid preconcentrated solution is subjected to cation exchange and anion exchange in sequence to obtain secondary ion exchange solution; and (3) carrying out secondary decolorization on the secondary ion exchange liquid by using an activated carbon decolorization column, and then carrying out concentration treatment to obtain a lactic acid concentrated solution with the L-lactic acid content of 80-90%, wherein the lactic acid concentrated solution is food-grade L-lactic acid.
3. The method for purifying and refining the L-lactic acid as claimed in claim 2, wherein the food grade L-lactic acid is refined by short-path distillation to obtain high purity lactic acid with the L-lactic acid content of 98-100% for producing polylactic acid or being used as high purity food grade lactic acid.
4. The method for purifying and refining L-lactic acid as claimed in claim 2 or 3, wherein the temperature of the fermentation broth concentrate in step (2) is 75-80 ℃, and the concentration of L-lactic acid is 20-25% by mass/volume.
5. The L-lactic acid purification and refining method as claimed in claim 4, wherein in the step (3), the acidolysis filtered clear solution is subjected to primary decolorization by using an activated carbon decolorization column, granular activated carbon is filled in the activated carbon decolorization column, the feeding mode is low-in and high-out, the retention time of the acidolysis filtered clear solution in the activated carbon decolorization column is 6-10h, and the chroma of the obtained primary decolorized solution is less than 30 hazen; wherein, the filled granular activated carbon can be regenerated, and the regeneration mode adopts thermal regeneration.
6. The method for purifying and refining L-lactic acid according to claim 2 or 5, wherein the continuous ion exchange system comprises a first ion exchange column and a second ion exchange column, the first ion exchange column and the second ion exchange column are independently arranged, each of the first ion exchange column and the second ion exchange column is composed of a cation column and an anion column in series, the cation column adopts strong acid cation exchange resin as a filler, and the anion column adopts weak base anion exchange resin as a filler.
7. The method of claim 6, wherein the concentration of calcium ions in the primary ion exchange solution is 2ppm or less, the concentration of iron ions in the primary ion exchange solution is 1ppm or less, the concentration of sulfate ions in the primary ion exchange solution is 2ppm or less, and the concentration of chloride ions in the primary ion exchange solution is 2ppm or less;
the concentration of calcium ions in the secondary ion exchange liquid is less than or equal to 2ppm, the concentration of iron ions is less than or equal to 1ppm, the concentration of sulfate ions is less than or equal to 2ppm, and the concentration of chloride ions is less than or equal to 2 ppm.
8. The method for purifying and refining the L-lactic acid as claimed in any one of claims 2 to 7, wherein the nanofiltration membrane is used for filtering, the concentration multiple of the nanofiltration membrane is 10 to 15 times, the addition amount of the dialysis water is 15 to 25 percent, and the content of the L-lactic acid in the nanofiltration membrane filtrate is 12 to 16 percent.
9. The method for purifying and refining L-lactic acid according to claim 8, wherein the content of L-lactic acid in the pre-concentrated L-lactic acid solution obtained by pre-concentration is 35 to 45%.
10. The method for purifying and refining the L-lactic acid as claimed in claim 9, wherein the activated carbon column used for the second decolorization of the secondary ion exchange solution is filled with granular activated carbon in a manner of low-inlet and high-outlet, the retention time of the secondary ion exchange solution in the activated carbon column is 6-10h, and the chroma of the decolorized solution is less than 30 hazen.
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