Large-scale and continuous production equipment and method of lactic acid powder

Abstract

The present application relates to a kind of lactic acid powder scale, continuous production equipment and production method.Lactic acid and calcium lactate are used as raw materials, first, both are input simultaneously according to the set ratio into first hollow paddle dryer, then input into second hollow paddle dryer to complete mixing, drying, lactic acid dehydration in raw material lactic acid digestion and its eutectic with calcium lactate etc.Process, then the mixture is input into pulverizer to be crushed, finally, the target lactic acid powder product is collected and screened by cyclone separator and matching screen.The hollow paddle dryer and its matching equipment used in the present application not only ensure the full mixing of raw materials, but also have heating and drying functions, which can realize the hydrolysis and conversion of lactic acid anhydride, lactylate, poly-lactylate in various specifications of lactic acid raw materials and its eutectic with calcium lactate, drying one-step continuous completion while mixing the feed, and finally shorten the production cycle of the usual process about 4-30 days to the present application process on the same day or shift.

Description

Technical Field

[0001] This invention relates to the field of food additives technology, specifically to a large-scale, continuous production equipment and method for lactic acid powder. Background Technology

[0002] Lactic acid powder is a powdered product of lactic acid, an acidity regulator in food. It is a mixture of lactic acid and calcium lactate, and objectively a eutectic of lactic acid and calcium lactate. The raw material, L-lactic acid, is produced through sugar fermentation, while calcium lactate is produced primarily from L-lactic acid. The proportions of lactic acid and calcium lactate in lactic acid powder products can be freely adjusted within a wide range, giving it good pH buffering properties to meet the diverse functional and usage requirements of the food industry. Lactic acid powder can be used as an acidity regulator and leavening agent in confectionery and baked goods. It can also enhance the cheesy and milky aromas of food or dairy products, making it an ideal additive for many dairy products. It is widely used in flavored yogurt, fermented cheese, ice cream, yogurt chips, fruit milk chips, and many other foods. As an antioxidant, lactic acid powder has significant preservation and antiseptic functions in food. When used in pickling vegetables, it can maintain the original crispness of the vegetables while minimizing the formation of nitrites during the pickling process. Lactic acid powder also has a flavor-enhancing effect and can be used as a raw material for various solid seasonings such as chicken essence powder (granules) and hot pot base. Lactic acid powder is widely used in baked goods, not only for emulsification, preservation, and solidification (shaping), but also for nutritional fortification. Under normal usage conditions, the solid powder form of lactic acid powder is easier to use than liquid lactic acid products, making it an ideal raw material for solid beverages, solid seasonings, instant salad dressings, onion-flavored cheese dressings, dry dairy products, and snack foods.

[0003] The lactic acid powder product provided by this invention contains almost all lactic acid in a free, directly titrable form. For example, when the labeled lactic acid content is as high as 62%, the degree of free lactic acid is 100%. This characteristic is another essential difference compared to commercial lactic acid (liquid at room temperature) which presents an advantage in dosage form (powder) in the usage environment! This is because, under normal environmental conditions, the free lactic acid in a lactic acid aqueous solution or commercial lactic acid with a labeled content greater than 30%–32% determined by the saponification method will not exceed 30%–32%, which is an inherent characteristic of lactic acid. The higher the labeled content of the lactic acid aqueous solution or commercial lactic acid, the higher the content of lactic anhydride, lactoyllactic acid, and polylactic acid. These lactic acids (labeled content greater than 30%–32%) not only fail to exhibit high acidity during use but also produce a significant bitter taste and unpleasant flavor. These problems are caused by the lactic anhydride, lactoyllactic acid, and polylactic acid, etc., dehydrated lactic acid products contained in high-content (greater than 30%–32%) lactic acid solutions.

[0004] Lactic acid powder can be produced using lactic acid and lactate (calcium) as raw materials, or using lactic acid and calcium carbonate as raw materials. These lactic acid powder products, compared to single raw materials (lactic acid or calcium lactate), offer advantages in both performance and dosage form, leading to expanded application ranges, improved flavor, and enhanced efficacy. Besides possessing the general properties of organic acids and hydroxy acids (fruit acids), lactic acid's typical characteristic is that as its concentration in aqueous solution increases, it readily undergoes intermolecular dehydration to form lactyl lactic acid and intramolecular dehydration to form lactic anhydride. Generally, lactic acid aqueous solutions with an actual lactic acid content greater than 30% (m / m) will generate lactyl lactic acid at room temperature. Furthermore, the higher the concentration, the longer the time, and the higher the temperature, the faster and greater the amount of lactyl lactic acid formed. Typically, commercial lactic acid with a specification ≥80% contains ≤30% (m / m) free lactic acid. Further dehydration of this commercial lactic acid to a low moisture content yields an aqueous solution that is essentially a mixture of free lactic acid, lactic anhydride, lactyllactic acid, polylactic acid, and water, with a low free lactic acid content. For this low-moisture mixture, heating without a catalyst at atmospheric or reduced pressure at a temperature below 190°C for an appropriate time can completely dehydrate the lactic acid, yielding a mixture of lactic anhydride, lactyllactic acid, polylactic acid, and lactide. The content of this mixture, based on lactic acid units, can reach approximately 125% of the theoretical value. Regardless of the lactic acid content, if sufficient water is added until the lactic acid solute content is ≤30% (m / m), and then stirred at an appropriate temperature for a sufficient time, the lactic anhydride, lactyllactic acid, polylactic acid, and lactide in the system can be completely hydrolyzed into an aqueous solution of free lactic acid; that is, the above process is reversible. Further experimental studies have shown that significant amounts of alkali metal or alkaline earth metal ions have a complete catalytic effect on the hydrolysis and release of lactic anhydride, lactyl lactic acid, polylactic acid, and lactide. In addition to the general properties of edible organic calcium acids, calcium lactate's most typical characteristic is that it is more readily soluble in water at room temperature than similar organic calcium acid salts, exhibiting the highest solubility. Especially under heating conditions, it can dissolve in water indefinitely, a property unique among organic calcium acid salts.

[0005] Currently, the production methods for lactic acid powder products can be broadly categorized into the following two types:

[0006] Category A – Microencapsulation processes that introduce significant amounts of non-lactic acid and lactate foreign matter, such as CN1674797A, CN1232411A, and CN1232411A. Products using microencapsulation processes that introduce significant amounts of non-lactic acid and lactate foreign matter are similar to the products described in this invention only in product category; they differ significantly in raw materials and applications, and are therefore not comparable.

[0007] Category B – Processes involving the mixing of lower-content commercial lactic acid and commercial lactate (calcium lactate), such as CN101292770A, CN201519598U, CN105263534A, and CN105249399A. The lactic acid powder product obtained by this process is the same as that obtained by the process of this invention, but it has the following three drawbacks:

[0008] (1) The mixing of raw materials lactic acid and calcium lactate is carried out by traditional manual or semi-mechanized methods, which generally has problems such as poor mixing effect, high labor intensity and low efficiency; (2) The moisture content of the compounded mixture is usually much higher than the quality technical indicators of the target product (lactic acid powder). Therefore, it is necessary to match the independent drying and crushing process. During the drying process, there is obvious lactic acid evaporation loss (about 6%-8% of the total raw material), which pollutes the environment and increases production costs; (3) After the raw materials lactic acid and calcium lactate are mixed, they need to be placed for crystal rearrangement so that the lactic anhydride, lactyl lactic acid and polylactic acid contained in the lactic acid raw materials can be fully digested. Under normal conditions, this process generally takes 20-30 days. Therefore, the production cycle from raw materials to products is generally more than 20 days, which is not conducive to product quality control (especially microorganisms) and seriously affects the overall production efficiency.

[0009] A search revealed a twin-screw extrusion mixing and drying-free preparation method for compound food additive lactic acid powder (CN108157932A). Although this method can provide lactic acid powder products that are the same as or similar to those of the present invention, and can better solve the mixing problem while avoiding the drying problem, it still cannot solve the problem of long production cycle caused by "placement for crystallization rearrangement". Although the one-step production equipment and production method for acidity regulator lactic acid powder (CN114177855A) can solve the problems and defects pointed out in (2)-(3) above, the raw materials are fundamentally different from those of the present invention. Its process involves obvious chemical reactions and requires solving the problem of lactic acid collection that accompanies the drying process. It has disadvantages such as complex process, large equipment investment, and difficulty in maintaining the production environment.

[0010] In summary, existing lactic acid powder production processes generally suffer from problems such as difficulty in controlling the production environment, high labor intensity, long production cycle, low production efficiency, and high production costs. More importantly, there is a general lack of evaluation and understanding of key indicators (free lactic acid) for the final product of lactic acid powder, making it impossible to fundamentally control product quality. Summary of the Invention

[0011] One of the objectives of this invention is to provide a set of equipment for the large-scale and continuous production of lactic acid powder. This set of equipment includes a primary dryer, a secondary dryer, a pulverizer, a cyclone separator, and a pulse dust collector. The inlet and outlet of the primary dryer are respectively connected to the feeding device (a feeder for conveying calcium lactate and a feed pipe for conveying lactic acid) and the inlet of the secondary dryer. The outlet of the secondary dryer is connected to the feeder of the pulverizer. The outlet of the pulverizer is connected to the cyclone separator and the pulse dust collector.

[0012] Furthermore, both the primary and secondary dryers are hollow paddle dryers, and both are equipped with steam heating pipelines.

[0013] Furthermore, a cooling system (spiral tube condenser) is installed at the evaporation port of the primary and secondary dryers. The circulating cooling water pipeline of this system is connected to the vacuum pump. The circulating cooling water is mainly used to indirectly cool the water-containing steam extracted from the evaporation chambers of the primary and secondary dryers, thereby recovering the free lactic acid that is azeotropically released with the steam during the heating and drying process of the primary and secondary dryers.

[0014] Furthermore, the feed inlet of the primary dryer is continuously and enclosed by a screw feeder, and the discharge outlet of the primary dryer is continuously output by a closed twin-screw extrusion conveyor, which then feeds the material into the secondary dryer in a closed state.

[0015] Furthermore, the discharge port of the secondary dryer uses the same closed twin-screw extrusion conveyor as the discharge port of the primary dryer to continuously output materials.

[0016] Furthermore, the feeder of the pulverizer is specifically a semi-open twin-screw feeder. After being cooled, hardened, and crushed by the semi-open twin-screw feeder, the material enters the pulverizer and is pulverized to the target fineness, ultimately producing qualified lactic acid powder products on a large scale.

[0017] Furthermore, the production equipment also includes an induced draft fan, which is connected to a pulse dust collector. A cyclone separator is located in front of the pulse dust collector, and the cyclone separator is connected to the air outlet at the top of the crusher, thereby achieving the suction effect of the induced draft fan on the crushed material within the crusher chamber.

[0018] The second objective of this invention is to provide a method for large-scale, continuous production of lactic acid powder using the aforementioned equipment. This method includes the following steps: (a) feeding raw materials lactic acid and calcium lactate into a primary dryer for mixing, where the lactic acid dehydrated products (mainly lactic anhydride, lactyl lactic acid, and polylactic acid) in the lactic acid raw materials are fully hydrolyzed, released, and dried under the combined action of heating and calcium lactate; (b) feeding the mixture from step (a) into a secondary dryer, where the lactic acid dehydrated products in the lactic acid raw materials continue to be fully hydrolyzed and released, and are fully dried under a heated and molten state; (c) feeding the mixture from step (b) into a pulverizer for pulverization, and then feeding it into a cyclone separator for separation. Most of the material is output from the bottom of the cyclone separator to obtain lactic acid powder product, while a small portion of the material enters a pulse dust collector located on its side from the top of the cyclone separator for recovery.

[0019] Furthermore, the heating temperature of the primary dryer is 90-110℃, and feeding begins when the temperature of the heating chamber of the primary dryer reaches 60℃. At the same time as feeding, the vacuum device is activated to maintain the vacuum degree in the evaporation chamber of the primary dryer greater than 0.01MPa.

[0020] Furthermore, the temperature of the mixture output from the primary dryer is not lower than 80°C.

[0021] Furthermore, the heating temperature of the secondary dryer is 105-125℃, the temperature of the molten mixture inside is controlled at 120±5℃, and the vacuum degree in the evaporation chamber of the secondary dryer is greater than 0.02MPa.

[0022] Furthermore, the temperature of the circulating cooling water shared by the primary and secondary dryers is controlled at 45±5℃. The circulating cooling water is connected to the vacuum pump for indirect cooling, mainly used to cool and recover the lactic acid that escapes during the heating process.

[0023] Furthermore, the lactic acid powder product obtained contains 50%-62% lactic acid by mass, and the degree of free lactic acid is not less than 99.5%.

[0024] The large-scale, continuous production equipment and process for lactic acid powder provided by this invention aims to achieve the aforementioned definition of lactic acid powder products. It utilizes a two- or multi-stage hollow paddle dryer to achieve full contact and mixing of raw materials (lactic acid and calcium lactate), as well as simultaneous temperature-controlled digestion, eutectic formation, dehydration, and drying. This ensures that the conversion and digestion of lactic acid raw materials, eutectic formation of calcium lactate, and drying are completed continuously in the same step within the same equipment, while simultaneously recovering the identifiable amount of lactic acid that escapes with steam azeotropically during the drying process. This invention can use lactic acid and calcium lactate of any content and specification as raw materials, shortening the production cycle from 4-30 days in conventional methods to the same day or shift. It also significantly improves raw material utilization while ensuring large-scale production, continuous operation, and product quality, truly achieving a balance between energy saving and environmental friendliness. Attached Figure Description

[0025] Figure 1 This is a side view of the lactic acid powder production equipment described in this invention.

[0026] Figure 2 This is a top view schematic diagram of the lactic acid powder production equipment described in this invention.

[0027] Figure 3 This is a flow chart of the lactic acid powder production process of the present invention.

[0028] Figure 4 This is a functional diagram of the lactic acid powder production equipment of the present invention.

[0029] 1-Feeder, 2-First-stage hollow paddle dryer, 3-Condenser, 4-Cyclone separator, 5-Second-stage hollow paddle dryer, 6-Pulse dust collector, 7-Pulverizer feeder, 8-Pulverizer. Detailed Implementation

[0030] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following description is provided in conjunction with specific embodiments and accompanying drawings.

[0031] Lactic acid powder, as a compound food additive, is widely used in various foods such as baking, solid seasonings, solid beverages, and confectionery, with documented consumption exceeding 10,000 tons per year. Currently, about half of the lactic acid powder products are produced and supplied by the Dutch company PURAC, while the remaining half are produced and supplied by three domestic factories. PURAC's production method uses lactic acid and alkali metal carbonates (calcium carbonate, magnesium carbonate) as raw materials, utilizing production equipment similar to that used for producing alkali metal salts (calcium lactate, magnesium lactate) of lactic acid, and controlling corresponding target conditions. This method has advantages such as high product quality and short production cycles, but also presents challenges such as large equipment investment, site selection, and regulatory restrictions. Domestic manufacturers use lactic acid and calcium lactate as raw materials, which has advantages such as relatively small equipment investment and no site selection or regulatory restrictions. Furthermore, the raw material mixing methods vary among manufacturers (the degree of manual or mechanized processing), resulting in significant differences in overall process efficiency. Due to the generally lengthy crystallization rearrangement process (the digestion of dehydrated substances in the lactic acid raw materials), both manufacturers have a significant drawback: long production cycles. In addition, there are uncontrollable factors in the drying and pulverizing processes to varying degrees, resulting in problems such as discontinuous processes in each stage, long overall production cycle, low comprehensive efficiency, and uncertainty about how to guarantee product quality.

[0032] The lactic acid powder product produced by this invention is a mixture with lactic acid and calcium lactate as the main components. The lactic acid in the mixture exists in a free state that can be directly titrated. This is also the principle indicator for measuring and defining lactic acid powder products. It is not simply a matter of mixing raw materials lactic acid and calcium lactate in a target ratio to obtain lactic acid powder products. Only when the lactic acid in the mixture is completely hydrolyzed and freed to form a eutectic product of lactic acid and calcium lactate can the true lactic acid powder product described in this application be obtained. The inventors discovered that the lactic acid raw material (commercial lactic acid) used in the production of lactic acid powder is essentially a mixture of free lactic acid, lactic anhydride, lactyl lactic acid, and polylactic acid with water. Under conditions of thorough mixing and relatively high heating temperatures (60–125°C), even in the presence of a small amount of water (even bound water in the corresponding proportion of calcium lactate raw material), calcium lactate can convert the lactic anhydride, lactyl lactic acid, and polylactic acid in the corresponding proportion of lactic acid raw material into free lactic acid. The key conditions are: ① sufficient contact between the raw materials (lactic acid and calcium lactate); ② avoiding excessively high temperatures (<190°C) that would cause lactic acid decomposition. In existing lactic acid powder production methods, manual or semi-mechanized mixing methods cannot achieve sufficient contact between the raw materials (lactic acid and calcium lactate). While twin-screw extrusion, as reported in the literature, can mix the two uniformly, it suffers from drawbacks such as stringent requirements on raw material specifications, limitations imposed by the stroke (residence time), and difficulty in raising and maintaining the temperature to ensure stable conversion conditions for the lactic acid raw material. Furthermore, all existing processes rely on a relatively long period of storage to complete the conversion (hydrolysis, release) of raw lactic acid. Some processes also depend on independently equipped hot air drying units for product production, which does not conform to the principles of energy conservation and cost reduction. Since lactic acid constitutes a significant proportion of the azeotrope of lactic acid and water and there is no effective recovery method, there is objectively a waste of raw materials and environmental damage. In short, existing processes are fundamentally incapable of achieving continuous, large-scale production organization and design for lactic acid powder.

[0033] Therefore, this invention has developed a set of... Figure 1-2 The diagram shows a large-scale, continuous production line for lactic acid powder. This equipment includes a feeder, a primary hollow paddle dryer, a secondary hollow paddle dryer, a pulverizer feeder, a pulverizer, a cyclone separator, and a pulse dust collector, all connected in sequence. An induced draft fan is connected to the pulse dust collector, providing the function and effect of removing moisture-laden air. The primary and secondary hollow paddle dryers have similar structures, each containing a cavity to hold the material. An evaporation port is located at the top of the cavity and connected to a vacuum device or system. A steam heating jacket is installed on the outer wall and connected to a heating steam pipe. The vacuum device or system is also connected to circulating cooling water for cooling. The material enters through the feeder and feed pipe, flows through the primary and secondary hollow paddle dryers, and then through the pulverizer feeder into the pulverizer for crushing. It then enters the cyclone separator for separation. Qualified material is discharged from the bottom of the cyclone separator, while the remaining material is drawn into the pulse dust collector from the top of the cyclone separator for recovery.

[0034] Example 1

[0035] ① Calculate the material requisition quantity for this batch and send it to the production site based on the product specifications specified in the production order and the batch capacity (4000-4500kg per mixer).

[0036] ② Set the heating temperature of the primary hollow paddle dryer to 110℃. After the temperature of its heating chamber reaches 60℃, start the conveying pump of the primary hollow paddle dryer and the raw material lactic acid. At the same time, continuously, stably, and evenly feed the raw material calcium lactate into its drying chamber (cavity) from the high-level feeder through the feeder of the primary hollow paddle dryer. Simultaneously, start the vacuum device shared by the primary and secondary hollow paddle dryers to maintain the vacuum gauge reading of the evaporation chamber of the primary hollow paddle dryer greater than 0.010MPa. Adjust the heating steam to maintain the material temperature within the range of 90~110℃, and control the temperature of the circulating cooling water at a stable flow rate at 45±5℃ to ensure that the material temperature output from the primary hollow paddle dryer is not lower than 80℃.

[0037] ③ Simultaneously with the start-up of the primary hollow paddle dryer, set the heating temperature of the secondary hollow paddle dryer to 105–125℃. Material with a temperature not lower than 80℃ output from the primary hollow paddle dryer enters the secondary hollow paddle dryer, maintaining the material in the heating chamber of the secondary hollow paddle dryer in a molten state at approximately 120±5℃. Adjust the vacuum pumping rate to maintain a vacuum gauge reading greater than 0.02MPa at the evaporation port of the evaporation chamber of the secondary hollow paddle dryer. During this period, control the temperature of the circulating cooling water connected to the vacuum system and condenser to 45±5℃ to recover the small amount of lactic acid released during the azeotropic process and ensure that the temperature of the material output from the secondary hollow paddle dryer remains at 120±5℃.

[0038] ④ The mixed material enters the semi-open twin-screw feeder of the pulverizer below from the closed twin-screw extrusion conveyor at the outlet of the secondary hollow paddle dryer. During the subsequent conveying process, the material comes into contact with ambient air, causing its temperature to drop, harden, and be crushed before entering the pulverizer to be pulverized to the target fineness. Most of the material enters the cyclone separator and is synchronously screened by the vibrating screen and then synchronously conveyed by the pneumatic conveyor to the one-dimensional mixer (two alternating feeders can ensure continuous operation).

[0039] ⑤ After the materials in the mixer are mixed and inspected and qualified, a specified proportion of anti-caking agent silicon dioxide is added. After mixing evenly again according to the verified procedure, the mixture is packaged and stored for sale. There is no need for long-term storage and crystallization rearrangement treatment.

[0040] Lactic acid powder products with lactic acid mass percentages of 55% and 60% were prepared according to the above method. The total lactic acid content W1 and free lactic acid content W2 were determined according to the method in GB1886.173—2016. Based on these results, the degree of lactic acid freeness W was calculated using the formula (W=W2 / W1×100%). The relevant test results are shown in the table below:

[0041]

[0042] Fifty batches of product were produced using the above process (20 batches of 55% lactic acid and 30 batches of 60% lactic acid). All batches of lactic acid powder fully met the expected content specifications and corresponding quality requirements. The actual yield from raw materials to finished product was approximately 99%. Considering that some lactic acid with a content of approximately 9% (m / m) was recovered from the bottom of the cooling water receiver at the bottom of the condenser, the actual primary conversion rate of the raw materials was greater than 99%. More importantly, the average production cycle from raw material input to packaging was 4-12 hours, which significantly improved production efficiency compared to the 4-30 day production cycle of all currently disclosed and used processes for producing lactic acid powder using lactic acid and calcium lactate as raw materials.

Claims

1. A method for producing lactic acid powder in a large scale and continuously by using a set of lactic acid powder production equipment in a large scale and continuously, characterized in that: The production equipment comprises a first-stage dryer, a second-stage dryer, a pulverizer, a cyclone separator and a pulse dust collector; the feeding inlet and the discharging outlet of the first-stage dryer are connected with a feeding device and the feeding inlet of the second-stage dryer respectively, the discharging outlet of the second-stage dryer is connected with a feeding device of the pulverizer, and the discharging outlet of the pulverizer is connected with the cyclone separator; the method comprises the following steps: (a) feeding raw materials of lactic acid and calcium lactate into the first-stage dryer in a proportion for mixing, and realizing sufficient hydrolysis, dissociation and dry water removal of lactic acid dehydrates in the lactic acid raw materials under the combined action of heating and calcium lactate; (b) feeding the mixed materials in step (a) into the second-stage dryer to continuously hydrolyze, dissociate and dry the lactic acid dehydrates in the lactic acid raw materials completely in a heating and melting state; (c) feeding the mixed materials in step (b) into the pulverizer for pulverization and then into the cyclone separator for separation, most of the materials being output from the bottom of the cyclone separator to obtain lactic acid powder products, and a small amount of materials being input from the top of the cyclone separator into the pulse dust collector arranged on the side thereof for recovery; the first-stage dryer and the second-stage dryer are both hollow paddle dryers, and steam heating pipelines are arranged on the first-stage dryer and the second-stage dryer; a circulating cooling water pipeline is arranged on the condenser at the evaporation opening of the first-stage dryer and the second-stage dryer, and the circulating cooling water pipeline is connected with a vacuumizing device for cooling the water vapor containing in the vacuumizing device extracted from the first-stage dryer and the second-stage dryer; the heating temperature of the first-stage dryer is 90-110 DEG C, and feeding is started when the temperature of the heating chamber of the first-stage dryer reaches 60 DEG C, the vacuumizing device is started at the same time to maintain the vacuum degree in the evaporation chamber of the first-stage dryer greater than 0.01 MPa, and the temperature of the mixed materials output from the first-stage dryer is not lower than 80 DEG C; the heating temperature of the second-stage dryer is 105-125 DEG C, the temperature of the molten mixed materials in the second-stage dryer is controlled to 120±5 DEG C, and the vacuum degree in the evaporation chamber of the second-stage dryer is greater than 0.02 MPa.

2. The method of claim 1, wherein: The feeding inlet of the first-stage dryer adopts a screw feeder for continuous closed feeding, the discharging outlet of the first-stage dryer adopts a closed double-screw extrusion conveyor for continuously outputting materials, and the materials are input into the second-stage dryer in a closed state; the discharging outlet of the second-stage dryer adopts the same closed double-screw extrusion conveyor as the discharging outlet of the first-stage dryer for continuously outputting materials.

3. The method of claim 1, wherein: The feeding device of the pulverizer is a semi-open double-screw feeding device.

4. The method of claim 1, wherein: The production equipment further comprises an air induction fan connected with the pulse dust collector.

5. The method of claim 1, wherein: The temperature of the circulating cooling water matched with the condenser at the evaporation opening of the first-stage dryer and the second-stage dryer is controlled to 45±5 DEG C.

6. The method of claim 1, wherein: The mass percentage content of lactic acid in the prepared lactic acid powder products is 50%-62%, and the lactic acid dissociation degree is not lower than 99.5%.

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