Method for producing multi-granularity feed-grade calcium phosphate salt by concentrated acid method and device thereof

Abstract

The application discloses a production method and device of feed calcium with multiple particle sizes by concentrated acid method, and relates to the technical field of feed-grade calcium phosphate preparation. Carbon dioxide tail gas is sent into a washing tower, and washing liquid and washed carbon dioxide are obtained after the carbon dioxide tail gas is washed by process water; the washing liquid is sent into a slurry preparation tank, calcium carbonate is added for slurry preparation, and the washed carbon dioxide is sent into the slurry preparation tank; slurry after reaction in the slurry preparation tank is sent into a reaction tank, phosphoric acid is added into the reaction tank, and carbon dioxide-containing gas from the slurry preparation tank is blown into the reaction tank; slurry after reaction in the reaction tank is continuously reacted in a reactor, when the slurry is in a semi-dry state, the slurry is sent into a granulator by a spiral feeder; after granulation, drying, screening, crushing and cooling, feed calcium phosphate salt with multiple particle sizes is obtained. The carbon dioxide tail gas is used to prepare calcium carbonate emulsion containing calcium bicarbonate, the carbon dioxide is introduced into the reaction tank to improve the reaction efficiency of phosphoric acid and calcium carbonate, and a non-return material granulation process is adopted to increase the production efficiency of the feed calcium.

Description

Technical Field

[0001] This invention relates to the field of feed-grade calcium phosphate preparation technology, specifically to a method and apparatus for producing multi-particle-grade feed-grade calcium phosphate salts using concentrated acid. Background Technology

[0002] Feed-grade calcium phosphate (referred to as feed calcium) contains essential mineral nutrients necessary for animal nutrition, making it an excellent phosphorus and calcium supplement in poultry and livestock feed. The main varieties of feed calcium phosphate are: dihydrogen phosphate (MCP), dicalcium phosphate (DCP), mono- and dicalcium phosphate (MDCP), and tricalcium phosphate (TCP or DFP). In my country, feed calcium products mainly consist of DCP, MCP, and MDCP. Feed calcium phosphate is the second largest phosphorus chemical product after phosphate fertilizers. The production processes for feed calcium mainly include the dilute acid method and the concentrated acid method. The dilute acid method has lower requirements for the quality of phosphate rock and phosphoric acid, but can only produce DCP; while the concentrated acid method, although requiring higher quality phosphate rock and phosphoric acid, has a shorter process and can produce multiple types of phosphates such as DCP, MCP, and MDCP. MDCP has higher water-soluble phosphorus, higher absorption rate, lower addition amount, and is more environmentally friendly than DCP, making it more widely applicable and having higher market value. However, the traditional concentrated acid method suffers from problems such as insufficient reaction between phosphoric acid and calcium carbonate, resulting in high free acid content (3%–5%), high residual calcium carbonate content (3%–6%), easy agglomeration, easy moisture absorption, and decreased phosphorus content in the product. Furthermore, the production of granular calcium feed also suffers from high feed return rates and low production efficiency. Summary of the Invention

[0003] The purpose of this invention is to provide a production method and apparatus for producing multi-granular feed calcium using concentrated acid method, which solves the problems of unsatisfactory product quality and low production efficiency caused by large amount of recycled materials in existing concentrated acid methods.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a method for producing multi-granular feed calcium using concentrated acid method, characterized by comprising the following steps:

[0005] S1. The carbon dioxide tail gas generated in the concentrated acid method for calcium feeding is sent to a scrubbing tower, and after being washed with process water, a scrubbing liquid and washed carbon dioxide are obtained.

[0006] S2. The washing liquid is sent into the mixing tank, calcium carbonate is added to adjust the slurry, and the carbon dioxide after washing is sent into the mixing tank at the same time.

[0007] S3. The reacted slurry in the mixing tank is sent into the reaction tank, and phosphoric acid is added to the reaction tank. The carbon dioxide gas coming out of the mixing tank is blown into the reaction tank, and the slurry stays in the reaction tank for 1 to 2 minutes.

[0008] S4. The slurry after reaction in the reaction tank enters the reactor to continue the reaction for 10-30 minutes. When the slurry is in a semi-dry state, it is fed into the granulator by a screw feeder.

[0009] S5. After granulation, the product is dried, sieved, crushed, and cooled to obtain multi-sized feed calcium phosphate.

[0010] A further technical solution is that the P2O5 content in the phosphoric acid is ≥50%.

[0011] A further technical solution is that the solid content of the slurry after reaction in the mixing tank is ≥65%, the reaction temperature in the reaction tank is 80~90℃, and the calcium-phosphorus ratio of the slurry after reaction in the reaction tank is 0.65~0.80.

[0012] A further technical solution is to control the amount of carbon dioxide-containing gas injected in step S3 so that the material height in the reaction tank is no more than 85% of the total height of the reaction tank.

[0013] A further technical solution is that the gas-liquid volumetric flow rate ratio in the scrubbing tower in step S1 is 3 to 1:1.

[0014] A further technical solution is that the calcium carbonate in step S2 has a purity of ≥98% and a particle size of ≤75μm.

[0015] A further technical solution is that the production apparatus used in the production method includes a washing tower, a mixing tank, a reaction tank, a reactor, a screw feeder, a granulator, a dryer, and a screening device connected in sequence. The tail gas outlet of the reaction tank is connected to the air inlet of the washing tower, the exhaust port of the washing tower is connected to the mixing tank, and the exhaust port of the mixing tank is connected to the reaction tank.

[0016] Reaction mechanism:

[0017] The basic process of the concentrated acid method involves reacting purified phosphoric acid and calcium carbonate in a mixing reaction tank, followed by granulation, drying, sieving, crushing, and packaging to obtain a qualified product. The main reactions are shown in equations (1) and (2).

[0018] CaCO3+2H3PO4= Ca(H2PO4)2·H2O+CO2↑ (1)

[0019] CaCO3+H3PO4+H2O = CaHPO4·2H2O+CO2↑ (2)

[0020] The concentrated acid method for preparing feed calcium is characterized by a low liquid-to-solid ratio, which easily leads to the formation of calcium carbonate encapsulated in the product, resulting in incomplete reaction between calcium carbonate and phosphoric acid (high free acid and high residual calcium carbonate). At the same time, due to the low liquid-to-solid ratio, the reaction is violent in the early stage, producing a large number of fine crystals that encapsulate free water. Moreover, some free water is required when generating dicalcium phosphate (see formula (2)), which causes the fluidity of the system to deteriorate in a short time. The diffusion rate of hydrogen ions in the system decreases rapidly, the reaction rate decreases accordingly, the material conversion rate decreases, and thus the product has a high level of free acid and residual calcium carbonate.

[0021] To overcome the problems of low conversion rate and low reaction rate caused by reduced encapsulation and hydrogen ion diffusion, resulting in high free acid and high residual calcium carbonate in the product, the present invention employs the following method:

[0022] The exhaust gas produced in the concentrated acid process is washed to absorb the phosphoric acid carried in the exhaust gas, reduce the temperature of the exhaust gas, and increase the concentration of carbon dioxide in the exhaust gas.

[0023] Carbon dioxide-containing exhaust gas is added to a calcium carbonate emulsion prepared by washing liquid and calcium carbonate. The carbon dioxide reacts with the calcium carbonate emulsion according to equation (3) to obtain a calcium carbonate emulsion containing relatively soluble calcium bicarbonate, i.e.

[0024] CaCO3+CO2+H2O=Ca(HCO3)2 (3)

[0025] 3. A calcium carbonate emulsion containing highly soluble calcium bicarbonate is introduced into the reaction tank, followed by the addition of phosphoric acid, and a reaction occurs.

[0026] Ca(HCO3)2 +2H3PO4= Ca(H2PO4)2·H2O+H2O+2CO2↑ (4)

[0027] Ca(HCO3)2 +H3PO4 = CaHPO4·2H2O+2CO2↑ (5)

[0028] 4. In the second step of the reaction, unreacted carbon dioxide is bubbled into the third step of the reaction between calcium carbonate emulsion and phosphoric acid to increase the fluidity of the system.

[0029] The reaction tail gas has a relatively high temperature and is mainly composed of carbon dioxide gas, carrying water vapor and a small amount of phosphoric acid. After being washed with process water, the tail gas temperature decreases, and the water vapor and phosphoric acid enter the process water. The tail gas then becomes a low-temperature, high-concentration carbon dioxide gas. The low temperature and high concentration are conducive to the reaction of carbon dioxide with calcium carbonate to produce calcium bicarbonate.

[0030] Compared to pure calcium carbonate emulsion, calcium carbonate emulsion containing calcium bicarbonate exhibits superior mixing at the molecular level during its reaction with phosphoric acid in the reaction tank. This results in a more homogeneous and rapid reaction. The reaction of calcium bicarbonate with phosphoric acid requires no additional free water, and the formation of calcium dihydrogen phosphate releases some water, which enhances the system's fluidity and hydrogen ion diffusion rate. Furthermore, compared to the reaction of calcium carbonate with phosphoric acid, the reaction of calcium bicarbonate with phosphoric acid releases more carbon dioxide, further increasing fluidity, promoting more uniform mixing of reactants, reducing entanglement, and ensuring a more complete reaction. Experiments have shown that the fluidity of the calcium carbonate-phosphoric acid reaction system is directly proportional to the liquid-to-solid ratio; a higher liquid-to-solid ratio results in better fluidity, as does higher temperature and greater foam production. Pumping some unreacted carbon dioxide from the mixing tank into the reaction tank to increase foam production can further enhance system fluidity and promote the reaction between calcium carbonate and phosphoric acid.

[0031] Meanwhile, the experiment found that as reactions (1), (2), (4), and (5) proceeded, the reactants gradually lost their plasticity and became semi-dry materials. These semi-dry materials could be directly fed into the granulator via a screw feeder for granulation. The granulation process eliminated the need for material return, significantly improving granulation and production efficiency. After granulation, drying, sieving, and crushing yielded multi-grade calcium feed products, with at least three different graded calcium feed products.

[0032] Compared with the prior art, the beneficial effects of this invention are as follows: Phosphoric acid is used as the phosphorus source; calcium carbonate emulsion containing calcium bicarbonate is prepared using the carbon dioxide tail gas generated during the concentrated acid method of calcium feed production as the calcium source for the concentrated acid method of calcium feed; excess carbon dioxide during the slurry preparation process is introduced into the reaction tank to increase the mixing efficiency and hydrogen ion diffusion efficiency of the reaction system, thereby improving the reaction efficiency of phosphoric acid and calcium carbonate, and thus increasing the conversion rate of phosphoric acid and calcium carbonate in the materials; and the process of using a non-returning granulation process to increase the calcium feed production efficiency by utilizing the gradual loss of fluidity of the reaction system in the later stages; after granulation, drying, screening, and crushing, multi-size finished feed calcium phosphate salts with good product consistency are obtained. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0035] The production apparatus used in this invention includes a washing tower, a slurry mixing tank, a reaction tank, a reactor, a screw feeder, a granulator, a dryer, and a screening device connected in sequence. The tail gas outlet of the reaction tank is connected to the air inlet of the washing tower, the exhaust port of the washing tower is connected to the slurry mixing tank, and the exhaust port of the slurry mixing tank is connected to the reaction tank. The reactor is a tank reactor with a stirring device. Example

[0036] Phosphoric acid (P2O5 content 50.02%) and calcium carbonate emulsion containing calcium bicarbonate (CaCO3 content 65.00%) were added to the reaction tank. The reaction temperature was 80 °C, and the calcium-to-phosphorus ratio of the material at the reaction tank outlet was controlled at 0.65. Simultaneously, the reflux of CO2 was controlled to ensure that the material height in the reaction tank did not exceed 85% of the total height of the tank to prevent overflow. The tail gas from the reaction tank entered a scrubbing tower, where it was scrubbed countercurrently with process water at room temperature at a gas-to-liquid volumetric flow rate ratio of 3:1. After scrubbing, the process water was used as the scrubbing liquid and entered the slurry mixing tank. Calcium carbonate (purity 98.45%, 95% particle size ≤75μm) was added simultaneously. Then, the scrubbed carbon dioxide-containing gas was introduced into the slurry mixing tank, where the solid content was controlled at 65%, resulting in a calcium carbonate emulsion containing calcium bicarbonate, which was then fed into the reaction tank. Unreacted carbon dioxide gas was bubbled into the reaction tank to increase the fluidity of the reactants. The reaction slurry remained in the reaction tank for 1 minute.

[0037] After the fluidity of the reaction slurry decreases, it is transferred from the reaction tank to a trough reactor equipped with an agitator to continue the reaction. The reaction continues for 10 minutes, during which the slurry gradually loses its plasticity and becomes a semi-dry material. This semi-dry material is then conveyed by a screw conveyor into a granulator. The material continues to react and granulate in the granulator at a rate of 300 rpm, remaining for 5 minutes before entering a blower dryer. After drying, the outlet material temperature is 80℃, and the finished product moisture content is 4%.

[0038] After screening, multi-grade feed calcium can be produced with particle sizes of 0.5–0.15 mm, 2–0.5 mm, and ≤0.15 mm (obtained by crushing products larger than 2 mm using a crusher). The main indicators of the three types of products are shown in Table 1, and Table 1 shows good consistency among the three types of multi-grade feed calcium.

[0039] Table 1 Technical Indicators for Multi-Grain Calcium Feed

[0040]

[0041] Example 2

[0042] Phosphoric acid (P2O5 content 51.30%) and calcium carbonate emulsion containing calcium bicarbonate (CaCO3 content 67%) were added to the reaction tank. The reaction temperature was 90 °C, and the calcium-to-phosphorus ratio of the material at the reaction tank outlet was controlled at 0.80. Simultaneously, the reflux of CO2 was controlled to ensure that the material height in the reaction tank did not exceed 85% of the total height of the tank, preventing overflow. The tail gas from the reaction tank entered a scrubbing tower, where it was scrubbed countercurrently with process water at room temperature at a gas-liquid ratio of 1:1. After scrubbing, the process water was used as the scrubbing liquid and entered the slurry mixing tank. Calcium carbonate (purity 98.45%, 95% particle size ≤75μm) was added, and then the scrubbed carbon dioxide-containing gas was introduced into the slurry mixing tank. The solid content in the slurry mixing tank was controlled at 70%, resulting in a calcium carbonate emulsion containing calcium bicarbonate, which was then fed into the reaction tank. Unreacted carbon dioxide gas was bubbled into the reaction tank to increase the fluidity of the reactants. The reaction slurry remained in the reaction tank for 2 minutes.

[0043] After the fluidity of the reaction slurry decreases, the slurry in the reaction tank is fed into a trough reactor equipped with a stirrer to continue the reaction. After 20 minutes of reaction, the slurry gradually loses its plasticity and becomes a semi-dry material. The semi-dry material is conveyed into a granulator by a screw conveyor. The material continues to react and granulate in the granulator at a rate of 300 rpm, and after a residence time of 5 minutes, it enters a blower dryer. After drying, the outlet material temperature is 85℃, and the moisture content of the finished product is 3.6%.

[0044] After screening, multi-grade feed calcium can be produced with particle sizes of 0.5–0.15 mm, 2–0.5 mm, and ≤0.15 mm (obtained by crushing products larger than 2 mm using a crusher). The main indicators of the three types of products are shown in Table 2 after testing.

[0045] Table 2 Technical Indicators for Multi-Grain Calcium Feed

[0046]

[0047] Example 3

[0048] Phosphoric acid (P2O5 content 52.23%) and calcium carbonate emulsion containing calcium bicarbonate (CaCO3 content 65%) were added to the reaction tank. The reaction temperature was 90 °C, and the calcium-to-phosphorus ratio of the material at the reaction tank outlet was controlled at 0.75. Simultaneously, the reflux of CO2 was controlled to ensure that the material height in the reaction tank did not exceed 85% of the total height of the tank to prevent overflow. The tail gas from the reaction tank entered a scrubbing tower, where it was scrubbed countercurrently with process water at room temperature at a gas-liquid ratio of 2:1. After scrubbing, the process water was used as the scrubbing liquid and entered the slurry mixing tank. Calcium carbonate (purity ≥98%, 95% particle size ≤75μm) was added, and then the scrubbed carbon dioxide-containing gas was introduced into the slurry mixing tank. The solid content in the slurry mixing tank was controlled at 65%, resulting in a calcium carbonate emulsion containing calcium bicarbonate, which was then fed into the reaction tank. Unreacted carbon dioxide gas was bubbled into the reaction tank to increase the fluidity of the reactants. The reaction slurry remained in the reaction tank for 3 minutes.

[0049] After the fluidity of the reaction slurry decreases, it is transferred from the reaction tank to a trough reactor equipped with an agitator to continue the reaction. After 30 minutes of reaction, the slurry gradually loses its plasticity and becomes a semi-dry material. The semi-dry material is then conveyed by a screw conveyor into a granulator. The material continues to react and granulate in the granulator at a rate of 300 rpm, remains for 5 minutes, and then enters a blower dryer. After drying, the outlet material temperature is 85℃, and the finished product moisture content is 3.5%.

[0050] After screening, multi-grade feed calcium can be produced with particle sizes of 0.5–0.15 mm, 1.2–0.5 mm, 1.2–2.5 mm, and ≤0.15 mm (obtained by crushing products larger than 2 mm using a crusher). The main indicators of the four types of products are shown in Table 3, and Table 3 shows good consistency among the four types of multi-grade feed calcium.

[0051] Table 3 Technical Indicators for Multi-Grain Calcium Feed

[0052]

[0053] Although the invention has been described herein with reference to several illustrative embodiments, it should be understood that many other modifications and implementations can be devised by those skilled in the art, which will fall within the scope of this disclosure. More specifically, various modifications and improvements can be made to the components or layouts within the scope of this disclosure, the drawings, and the claims. Besides modifications and improvements to the components or layouts, other uses will be apparent to those skilled in the art.

Claims

1. A method for producing multi-granular feed-grade calcium phosphate using concentrated acid, characterized in that, The production method includes the following steps: S1. The carbon dioxide tail gas generated in the concentrated acid process of feed-grade calcium phosphate is sent to a scrubbing tower, and after being washed with process water, a scrubbing liquid and washed carbon dioxide are obtained. S2. The washing liquid is sent into the mixing tank, calcium carbonate is added to adjust the slurry, and the carbon dioxide after washing is sent into the mixing tank at the same time. S3. The reacted slurry in the mixing tank is sent into the reaction tank, and phosphoric acid is added to the reaction tank. The carbon dioxide gas coming out of the mixing tank is blown into the reaction tank, and the slurry stays in the reaction tank for 1 to 2 minutes. S4. The slurry after reaction in the reaction tank enters the reactor to continue the reaction for 10-30 minutes. When the slurry is in a semi-dry state, it is fed into the granulator by a screw feeder. S5. After granulation, the product is dried, sieved, crushed and cooled to obtain multi-granule feed-grade calcium phosphate.

2. The method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in claim 1, characterized in that: The phosphoric acid contains ≥50% P2O5.

3. The method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in claim 1, characterized in that: The solid content of the slurry after reaction in the mixing tank is ≥65%, the reaction temperature in the reaction tank is 80~90℃, and the calcium-phosphorus ratio of the slurry after reaction in the reaction tank is 0.65~0.

80.

4. The method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in claim 1, characterized in that: In step S3, the amount of carbon dioxide-containing gas injected is controlled so that the material height in the reaction tank is not greater than 85% of the total height of the reaction tank.

5. The method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in claim 1, characterized in that: In step S1, the gas-liquid volumetric flow rate ratio in the scrubbing tower is 3 to 1:

1.

6. The method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in claim 1, characterized in that: In step S2, the calcium carbonate has a purity of ≥98% and a particle size of ≤75μm.

7. A method for producing multi-granular feed-grade calcium phosphate using concentrated acid as described in any one of claims 1 to 6, characterized in that: The production apparatus used in the production method includes a washing tower, a mixing tank, a reaction tank, a reactor, a screw feeder, a granulator, a dryer, and a screening device connected in sequence. The tail gas outlet of the reaction tank is connected to the air inlet of the washing tower, the exhaust port of the washing tower is connected to the mixing tank, and the exhaust port of the mixing tank is connected to the reaction tank.

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