1. Precise design of raw material ratio
(I) Chemical measurement control of basic raw materials
The modulus (M) of sodium silicate is defined as the ratio of the amount of silicon dioxide to sodium oxide (M = n (SiO₂)/n (Na₂O)), so the precise ratio of silicon source to sodium source in the raw material is the basis of modulus control. In production practice, liquid water glass is usually used as a precursor, and its initial modulus needs to be regulated by the reaction of sodium hydroxide and silica sand. Taking the HLNAP-1 powdered water glass produced by Hengli Chemical as an example, its target modulus is 2.0±0.1, and the molar ratio of SiO₂ to Na₂O in the sodium silicate solution needs to be strictly controlled during the preparation stage of liquid water glass.
In the specific operation, quartz sand (purity ≥ 95%, main component is SiO₂) can be used as silicon source, and industrial grade sodium hydroxide (NaOH content ≥ 99%) can be used as sodium source.
According to the definition of modulus, M = m/n, when the target modulus is 2.0, m/n = 2.0, that is, theoretically every 2 mol SiO₂ needs to react with 1 mol NaOH. However, in actual production, the conversion rate of silica sand (usually 85%-95%) and the loss of the reaction system need to be considered. Therefore, the concentration of SiO₂ and Na₂O in the reaction solution needs to be monitored in real time by titration, and the raw material input ratio needs to be adjusted dynamically. For example, when the initial solution modulus deviates from 2.0, it can be corrected by adding NaOH (lowering the modulus) or silica sol (increasing the modulus).
(II) Synergistic effect of additives
In order to improve the reaction kinetics and product structure, a small amount of additives can be introduced. For example, adding 0.1%-0.5% sodium sulfate (Na₂SO₄) during the preparation of liquid water glass can inhibit the excessive polymerization of silicon-oxygen bonds by adjusting the ionic strength and avoid modulus fluctuations; at the same time, adding about 0.2% sodium polyacrylate as a dispersant can improve the dispersibility of silica sand in alkaline solution and promote the uniformity of reaction, thereby ensuring the stability of the modulus. In addition, for products in special application scenarios, such as powdered sodium silicate for high-temperature resistant binders that require high modulus stability, trace amounts of lithium salts (such as Li₂CO₃, added in an amount of 0.05%-0.1%) can be introduced to use the strong polarization ability of lithium ions to regulate the silicate network structure and enhance the modulus control accuracy.
2. Key control links of the production process
(I) Preparation process of liquid water glass
Reaction temperature and pressure
The reaction of silica sand and sodium hydroxide is a solid-liquid heterogeneous reaction, and temperature and pressure directly affect the reaction rate and silica sand conversion rate. In the process system of Hengli Chemical, liquid water glass is prepared by high-pressure reactor, with reaction temperature controlled at 120-150℃ and pressure 1.0-1.5MPa. Under this condition, the dissolution rate of silica sand can reach 1.2-1.5g/(min・L), and the conversion rate can be stabilized at more than 92%. Too low temperature will lead to incomplete reaction, low modulus and large fluctuation; too high temperature may cause excessive polymerization, resulting in modulus measurement deviation. The PID temperature control system is used to control temperature fluctuation at ±2℃ and pressure fluctuation at ±0.05MPa to ensure the stability of the reaction process.
Stirring rate and reaction time
The stirring rate needs to be maintained at 150-200r/min to ensure full contact between the solid and liquid phases. The reaction time is usually 4-6 hours, which needs to be adjusted according to the silica sand particle size (when the silica sand particle size is ≤0.1mm, the reaction time can be shortened to 3 hours). The viscosity change of the reaction liquid is monitored by an online viscometer. When the viscosity reaches 15-20mPa・s, the reaction endpoint is determined. At this time, the solution modulus is close to the target value of 2.0.
(II) Optimization of spray drying process parameters
When liquid water glass is converted into a powdered product by spray drying, the heat transfer and mass transfer characteristics of the drying process will affect the microstructure of the product, and then have an indirect impact on the modulus. The key process parameters include:
Inlet temperature and outlet temperature
The inlet temperature is controlled at 300-350℃, and the outlet temperature is 120-140℃. High-temperature hot air can instantly dehydrate the droplets (drying time <5s), avoiding secondary polymerization or decomposition of the silicate structure due to long-term heating. If the inlet temperature is lower than 280℃, it may cause residual moisture (water content> 5%), affecting the accuracy of modulus measurement; if the temperature is higher than 380℃, it may cause local overheating, causing Na₂O to volatilize, making the measured modulus higher.
Atomization pressure and nozzle aperture
A pressure atomization nozzle is used, with an atomization pressure of 6-8MPa and a nozzle aperture of 1.0-1.2mm. Under this parameter, the average droplet size can be controlled at 50-80μm, ensuring uniform distribution of powder particle size after drying (100 mesh pass rate ≥95%, such as HLNAP-1 type products). Too low atomization pressure will result in too large droplet size, forming large particle agglomerates after drying, and there may be residual liquid components that are not completely dried inside, affecting the modulus uniformity; too high pressure may produce too much fine powder (<200 mesh particles account for >10%), increase dust loss, and may change the bulk density of the product (target value 0.6Kg/L), indirectly affecting the sampling representativeness during modulus testing.
(III) Aging and homogenization treatment
The dried powdered product needs to be aged in a sealed warehouse for 24-48 hours, with the aging temperature controlled at 40-50℃ and humidity <30% RH. During the aging process, the moisture distribution and microstructure inside the powder are further balanced, which can reduce the modulus fluctuation range by ±0.03. For batch-produced products, air flow homogenization equipment is used for mixing (homogenization time 1-2 hours, air flow speed 15-20m/s) to ensure the modulus uniformity of each batch of products (modulus deviation between batches ≤±0.05).
3. Analysis of factors affecting modulus control and countermeasures
(I) Raw material quality fluctuations
Silica sand purity and particle size
If the content of impurities such as Fe₂O₃ and Al₂O₃ in silica sand exceeds 1.0%, it will react with NaOH to generate corresponding sodium salts, consume sodium sources, and cause the actual modulus to be too high. Countermeasures: Use magnetic separation + pickling process (10% hydrochloric acid soaking for 2 hours) to remove impurities and increase the purity of silica sand to more than 98%. Uneven distribution of silica sand particle size (such as particle size span > 0.3mm) will lead to inconsistent reaction rates, and the local modulus deviation can reach ±0.2. Solution: Use vibration screening to achieve particle size classification, and use silica sand with a particle size of 0.05-0.1mm as raw material.
Sodium hydroxide deliquesce problem
Industrial-grade sodium hydroxide is easy to absorb moisture during storage, resulting in a decrease in the effective NaOH content (the measured content may be less than 95%), which leads to deviations in the ratio calculation. Countermeasures: Purchase sodium hydroxide in sealed barrels, recalibrate the concentration by acid-base titration before use, and adjust the feed amount according to the measured value.
(II) Process parameter fluctuations
Changes in the heat transfer efficiency of the reactor
After long-term use, the inner wall of the reactor may be scaled (the main component is calcium silicate), resulting in a decrease in the heat transfer coefficient and a lag in the reaction temperature. Solution: Perform chemical cleaning regularly (once a quarter) (use 5% hydrofluoric acid solution for 2 hours of circulation cleaning) to restore the heat transfer efficiency to more than 90% of the initial value.
Material accumulation phenomenon in spray drying tower
If excessive powder accumulates on the inner wall of the drying tower (residence time > 24 hours), it may deliquesce due to moisture absorption, forming high-viscosity agglomerates, affecting the stability of the subsequent atomization drying process. Countermeasures: Install an automatic vibration device (vibration 5-10 times per hour, amplitude 5-8mm), and clean the inner wall after each shift to control the thickness of the accumulated material to ≤1mm.
(III) Systematic error of the detection method
The modulus detection usually uses acid-base titration, but the details of the operation process may introduce errors. For example, if the water temperature exceeds 60℃ when the sample is dissolved, it will accelerate the hydrolysis of silicate, resulting in a low SiO₂ measurement value and a small modulus calculation value. Improvement method: Use deionized water at 30℃±2℃ when dissolving the sample (such as the dissolution rate of HLNAP-1 type product ≤60s/30℃), and use a magnetic stirrer for rapid stirring (speed 300r/min) to ensure complete dissolution within 2 minutes and reduce hydrolysis losses. In addition, the choice of indicator (such as the difference in the color change range of methyl orange and phenolphthalein) will also affect the determination of the titration endpoint. It is recommended to use potentiometric titration (end point determination error < 0.1mL) instead of the traditional indicator method to improve the accuracy of analog-to-digital detection (repeated measurement deviation ≤ ±0.02).
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