Production and applications of calcium hydroxide with high specific surface area

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1 Introduction to Calcium Hydroxide with High Specific Surface Area. Calcium hydroxide (slaked lime, hydrated lime) refers to a powdered substance obtained by the reaction of quicklime with a controlled excess of water, generally containing less than 1 percent free water. Below 350°C: CaO + H2O → Ca(OH)2 + 276 (Kcal/KgCaO). In the 1980s, calcium hydroxide with a high specific surface area was primarily used to remove acidic gases from exhaust gases. It is clear that increasing the surface area of calcium hydroxide helps to improve efficiency; therefore, efforts were made to develop calcium hydroxide with a high specific surface area. It is widely used in the synthesis of pharmaceuticals and food additives, the production of high-tech biomaterials such as HA, the synthesis of VC phospholipids as feed additives, the production of food-grade konjac, calcium compounds, as well as calcium naphthenate, calcium lactate, calcium citrate, and additives for cola, alongside the synthesis of other high-quality organic chemicals. Surface area characterization methods: (1) BET method for measuring specific surface area ； (2) Measure the total pore volume and pore size distribution ； (3) Measure particle size and distribution ； (4) Electron microscopy observation of particle morphology. There are currently two methods for producing calcium hydroxide with a high specific surface area: one involves drying and grinding lime milk, resulting in calcium hydroxide with a specific surface area of up to 30 m2/g (referred to as the wet method) ； Secondly, high-specific-area calcium hydroxide can be obtained through various methods (referred to as the dry method), such as: (1) digestion using water or alcohol solutions ； (2) Sucrose aqueous solution ； (3) Add sulfonated lignin ； (4) Derivatives of amines, etc. The parameters of the digestion process also affect the surface area, such as water temperature, the degree of calcination of quicklime, the particle size of the quicklime fed in, the specific surface area of quicklime, the residence time of the material in the digester, and its maximum temperature, etc. For details, see Tables 1 and 2. Table 1: Effect of different calcination degrees on the specific surface area of calcium hydroxide. SB (soft-burned lime), HB (hard-burned lime). Specific surface area of quicklime: 1.18, 0.55 m2/g; specific surface area of calcium hydroxide: 241, 4.6 m2/g. Table 2: Effect of digestion conditions on the specific surface area of calcium hydroxide. Digestion conditions and specific surface area of calcium hydroxide (m2/g): Adding water at 5°C – 6.67; sucrose solution – 37–46.3; alcohol solution – 36–45.6. Figure 1a: Calcium hydroxide with high specific surface area; Figure 1b: Calcium hydroxide with low specific surface area. Figure 1: Electron micrographs of calcium hydroxide with different specific surface areas. In fact, among the physical properties of calcium hydroxide, the surface area is the most important one. For example, two microscopic particles of calcium hydroxide have different shapes: one is a three-dimensional particle as shown in the micrograph (Figure 1b), while the other is a two-dimensional particle in the form of a film, as shown in Figure 1a. The surface area of the latter is several times larger than that of the former, and its physicochemical properties (chemical reactivity, settling rate, plasticity, yield of lime slurry, etc.) also differ significantly. 2 Methods for producing calcium hydroxide with high specific surface area 2.1 Dry method for producing calcium hydroxide with high specific surface area In the dry method, lime is mixed with water or steam in an equimolar ratio to carry out a digestion reaction (the actual amount of water in moles is slightly greater than that of calcium oxide), followed by air classification to remove impurities. Once the particles reach the desired size requirements, they are packaged for sale. Currently, dry processing is generally used in China, and its process flow is shown in Figures 2 and 3. Figure 2: Process flow diagram for the dry production of calcium oxide with high specific surface area. 1. Roof dust collector; 2. Raw material bin; 3. Manual rod valve; 4. Belt scale; 5. Water tank; 6. Pipeline pump; 7. Flow meter; 8. Pre-digester; 9. Digester; 10. Dust collector; 11. Fan; 12. Air lock valve; 13. Elevator; 14. Air lock valve; 15. High-efficiency separator; 16. Air lock valve; 17. Screw conveyor; 18. Elevator; 19. Coarse powder bin; 20. Roof dust collector; 21. Manual gate valve; 22. Collector; 23. Fan; 24. Screw conveyor; 25. Discharge valve; 26. Elevator; 27. Roof dust collector; 28. Finished product bin; 29. Manual gate valve; 30. Packaging machine. Figure 3: Diagram of the three-stage digester. a. Water; b. Quicklime; c. Pre-digester; d. Digester; e. Steam leading to the dust collector; f. Final-stage digester; g. Slaked lime. Mixer paddles: These are used to increase the specific surface area of calcium hydroxide through methods such as controlling the particle size of the feed, the digestion temperature, the residence time of the material, and the addition of other chemical additives. The changes in specific surface area are shown in Table 1 and Table 2. 2 Wet-process production of calcium hydroxide with high specific surface area: In the wet-process method, a slaking reaction is carried out at a mass ratio of lime to water greater than 4, followed by filtration to remove residues, dehydration, drying, and grading. Its reaction is complete, which facilitates impurity removal; the product has a high purity and excellent quality, but the equipment investment is substantial. Abroad, wet production is generally used. The process flow is as follows: Figure 4 shows the process flow diagram for high specific surface area calcium hydroxide (wet method). Figure 3 illustrates the applications of high specific surface area calcium hydroxide. Calcium hydroxide has a wide range of applications, but it is necessary to design the production process accordingly based on the specific application scenario – in other words, to produce \"specialized calcium hydroxide\" that yields optimal results. For medical use, for example, high-purity calcium hydroxide is required, with the levels of harmful heavy metals such as lead, arsenic, and mercury meeting the standards specified in pharmacopoeias ； Another example is the removal of acidic gases, where calcium hydroxide with a high specific surface area is used to achieve high efficiency. Main application areas: ● Environmental protection (production of bleaching powder, water softeners, treatment of acidic water, sewage treatment, desulfurization and dechlorination of flue gas from waste incineration, etc. ) ● Chemical production (various calcium salts, etc.) ● Lubricant production (lubricant additives such as calcium sulfonate with high base number, calcium-based greases, etc.) ● Food and pharmaceutical applications (food additives like calcium lactate; medical products such as calcium hydroxide paste, etc.) ● Other applications Below is the implementation plan for producing 15,000 tons per year of wet-processed calcium hydroxide with a high specific surface area: 3.1 Process and main equipment plan This plan uses lime as raw material (CaO ≥ 90%) ； SiO2 acid-insoluble residue ≤0.1% ； For a particle size of 8–10 mm, a process and equipment scheme for producing calcium hydroxide with a high specific surface area via wet methods is adopted. High-quality lime, after being digested in a spiral trough, screened by a vibrating screen, and degassed using a hydrocyclone, undergoes solid-liquid separation, drying, classification, and packaging to produce powdered calcium hydroxide products. The process flow is shown in Figure 4, and the main equipment is listed in Table 3. Table 3: List of Main Equipment for the Production of 15,000 tons/year of Calcium Hydroxide with High Specific Surface Area
Serial Number | Equipment Name | Specifications & Performance | Quantity | Remarks
1 | Ring Hammer Crusher | 1 | | |
2 | Bucket Elevator | 1 | | |
3 | Electromagnetic Vibration Feeder | 1 | | |
4 | Silo | 1 | | |
5 | Double-Blade Slurry Mixing Machine | Stainless Steel | 1 | |
6 | Slag Discharger | 1 | | |
7 | Hot Water Tank | 1 | | |
8 | Vibrating Screen | Stainless Steel | 1 | |
9 | Rough Slurry Tank | 2 | | |
10 | Rough Slurry Pump | Stainless Steel | 1 | |
11 | Hydrocyclone | 2 | Stainless Steel | |
12 | Hydrocyclone | 2 | Stainless Steel | |
13 | Hydrocyclone | 2 | Stainless Steel | |
14 | Hydrocyclone Pump | 2 | Stainless Steel | |
15 | Slag Liquid Tank | 1 | | |
16 | Slag Liquid Separator | Stainless Steel | 1 | |
17 | Slag Liquid Pump | 1 | | |
18 | Fine Slurry Tank | 4 | | |
19 | Fine Slurry Pump | Stainless Steel | 2 | |
20 | Filter Press | 2 | Rubber/Plastic | |
21 | Filtrate Tank | 2 | | |
22 | Filtrate Pump | 1 | | |
23 | Grinder | Stainless Steel | 1 | |
24 | Twin-Screw Dryer | Stainless Steel | 1 | |
25 | Elevator | Stainless Steel | 1 | |
26 | Silo | Stainless Steel | 1 | |
27 | Electronic Belt Scale | 1 | | |
28 | Disk Dryer | Stainless Steel | 1 | |
29 | Disperser | Stainless Steel | 1 | |
30 | Cyclone Separator | 1 | | |
31 | Bag Filter | 1 | | |
32 | Packaging Machine | Stainless Steel | 1 | |
33 | Induced Draft Fan | 1 | | |
34 | Exhaust Pipe | 1 | | |
35 | Condensate Tank | 1 | | |
36 | Condensate Tank | 1 | | |
Note: The material of all equipment (1) is stainless steel ； (2) After filtration, everything is engineering plastics and stainless steel. 3.2 Non-process conditions (1) Civil engineering conditions: The main buildings are listed in Table 4. Table 4: List of Major Buildings
No. | Building Name | Length × Width × Height (m) | Area (m²) | Structural Requirements | Remarks
1 | Digestion and Refining | 36×21×67 | 56 | Full system sealing required |
2 | Pressure Filtration, Drying, and Packaging | 36×21×67 | 56 | Full system sealing required |
3 | Product Storage Facility | 18×18×63 | 24 | |
4 | Office Building | 600 | | |
5 | Canteen, Dormitories, and Gatehouse | 600 | | |
Total | | 3036 | | |

(2) Water supply rate: 4.0–5.0 m³/h
(3) Installed capacity: Approximately 300 kW
(4) Steam: Saturated steam at 0.4 Mpa, approximately 4 t/h. (Or natural gas, 150 m3/h) 3.3 Investment estimate: See Table 5 for details. Table 5: Investment Estimation Table
No. | Item | Amount | Proportion | Remarks
1 | Production equipment | 6.6 million yuan
2 | Equipment installation fees | 0.6 million yuan
3 | Factory building | 3.3 million yuan, 300㎡
4 | Analytical instruments | 1 million yuan
5 | Other auxiliary facilities | 3.7 million yuan
6 | Design fees | 0.5 million yuan
7 | Contingency funds | 0.5 million yuan
8 | Total fixed assets | 16.2 million yuan
9 | Working capital | 3 million yuan
Total: 19.2 million yuan

3.4 Economic Analysis
(1) Consumption quotas are detailed in Table 6. Table 6: Consumption quotas
Item, specification, unit, quantity
Lime, CaO ≥ 90%: 0.93 t
Natural gas, >8000 kcal/m³: 366.7 m³
Raw water: 3 m³
Electricity, 380/220V: 100.0 kWh
Packaging bags: 20 pcs

(2) The average wage per person and the prices of raw materials and auxiliary materials are shown in Table 7. Table 7: Average wage and prices of raw materials and auxiliary materials. Item Name, Unit Price, Tax Rate: Average wage, yuan/year – 18,000; Lime, yuan/ton – 2,800, 17%; Water, yuan/m3 – 2.26%, 17%; Electricity, yuan/kW·h – 0.55, 17%; Natural gas, yuan/m3 – 2.21, 17%. (3) Cost-benefit analysis for 15,000 tons of pharmaceutical and food-grade calcium hydroxide: Total project investment: 16.2 million yuan; Production cost: 718 yuan/ton; Selling price of the product: 2,500 yuan/ton; Value-added tax to be paid: 1.03 million yuan/year; Profit before taxes: 22.06 million yuan/year; Profit after taxes: 14.78 million yuan/year; Payback period for investment: 1.82 years. The production process for this project is mature, and the product quality is stable.