Potassium Silicate（HLKL-3）

How to reduce the insoluble content in liquid potassium silicate with modulus (M): 3.10-3.40?

1. Characteristics of liquid potassium silicate and analysis of insoluble sources

As one of the important products of Tongxiang Hengli Chemical Co., Ltd., liquid potassium silicate (modulus 3.10-3.40) is widely used in inorganic water-based coatings, floor curing agents, welding rod adhesives and other fields due to its excellent performance (such as high appearance transparency and strong alkalinity). However, if there are insolubles in the product, it will not only affect its appearance quality, but also may have a negative impact on the performance of downstream applications, such as clogging the paint nozzle and reducing the uniformity of the adhesive. Therefore, reducing the insoluble content is a key link in improving product quality.
From the perspective of chemical composition and production process, the insolubles in liquid potassium silicate with modulus (M): 3.10-3.40 mainly come from the following aspects:
Raw material impurities: The main raw materials for the production of potassium silicate are quartz sand (containing SiO₂), potassium hydroxide (KOH), etc. If the quartz sand contains impurity minerals such as Fe₂O₃, Al₂O₃, CaO (such as feldspar, mica, etc.), or potassium hydroxide contains impurities such as carbonates and sulfates, these impurities may not be able to fully participate in the reaction during high-temperature melting or liquid phase reaction, forming insoluble residues.
Incomplete reaction products: Potassium silicate is usually prepared by melting quartz sand and potassium hydroxide at high temperature (dry method) or liquid phase reaction under pressurized conditions (wet method). If the process parameters such as reaction temperature, pressure, and time are not properly controlled, the quartz sand may not be completely dissolved, forming unreacted SiO₂ particles.
Production process pollution: Corrosion products (such as iron oxides) on the inner wall of production equipment (such as reactors and pipelines), mechanical impurities (such as dust and metal debris) mixed in during transportation, and pollutants in the production environment may introduce insoluble substances.
Changes in storage and transportation: During storage, if liquid potassium silicate comes into contact with CO₂ in the air, carbonation may occur to generate K₂CO₃ and SiO₂ precipitates; in addition, if the storage container material reacts chemically with the product, insoluble matter may also be produced.

2. Technical paths to reduce the content of insoluble matter

(I) Raw material optimization and pretreatment
Select high-purity raw materials
Quartz sand: Select high-purity quartz sand with a SiO₂ content of ≥99% to reduce the content of impurities such as Fe₂O₃ (≤0.01%) and Al₂O₃ (≤0.05%). For example, remove ferromagnetic impurities in quartz sand through magnetic separation, or use pickling (such as hydrofluoric acid treatment) to remove metal oxides attached to the surface.
Potassium hydroxide: Use industrial grade one (purity ≥ 85%), and strictly control its carbonate (≤ 1.0% in terms of K₂CO₃) and sulfate (≤ 0.1% in terms of K₂SO₄). Potassium hydroxide can be further purified by recrystallization process to reduce the introduction of impurities.
Raw material pretreatment process
Quartz sand crushing and classification: crush the quartz sand to a suitable particle size (such as D90 ≤ 50μm) to increase the reaction contact area. At the same time, remove coarse particles and impurity minerals by screening or airflow classification to ensure the uniformity of raw material particle size.
Potassium hydroxide dissolution optimization: When dissolving potassium hydroxide, use deionized water and control the dissolution temperature (such as 60-80℃) and stirring speed (such as 200-300r/min) to ensure complete dissolution and avoid residual undissolved particles.
(II) Optimization of production process parameters
Optimization of wet process (taking liquid phase method as an example)
Reaction temperature and pressure: Potassium silicate with a modulus of 3.10-3.40 is usually prepared by pressurized liquid phase reaction. Studies have shown that when the reaction temperature increases from 120℃ to 150℃ and the pressure increases from 0.3MPa to 0.6MPa, the dissolution rate of quartz sand can be increased by 30%-50%, significantly reducing unreacted SiO₂ particles. It is recommended to control the reaction temperature at 140-150℃, maintain the pressure at 0.5-0.6MPa, and extend the reaction time to 4-6 hours to ensure that the quartz sand is fully dissolved.
Material ratio: Strictly control the molar ratio (modulus) of KOH and SiO₂. For products with a target modulus of 3.10-3.40, the theoretical molar ratio (K₂O:SiO₂) is 1:3.10-1:3.40. In actual production, the proportion of KOH can be appropriately increased (such as 5%-10% excess) to promote the dissolution of SiO₂, but excessive KOH should be avoided to cause the product to be too alkaline and increase costs.
Stirring intensity and method: A combination of an anchor stirrer and a turbine stirrer is used. In the early stage of the reaction (0-2 hours), a high speed (such as 400r/min) is used to enhance mass transfer. In the later stage (2-6 hours), the speed is reduced to 200r/min to avoid excessive stirring, which leads to increased energy consumption and equipment wear and impurities.
Dry process optimization (melting method)
Melting temperature and time: The dry reaction requires quartz sand and potassium hydroxide to be melted at high temperature (usually ≥300℃). Increasing the melting temperature to 350-400℃ and extending the insulation time to 2-3 hours can make the reaction more complete. For example, at 380℃ for 2.5 hours, the conversion rate of quartz sand can reach more than 98%, significantly reducing the insoluble content.
Melting equipment selection: Use a melting furnace lined with corundum or quartz to reduce the chemical reaction between the equipment material and the reactants (such as the dissolution of iron). At the same time, regularly clean the attachments on the furnace wall to avoid the accumulation of impurities.
(III) Purification and separation technology
Filtration process
Multi-stage filtration combination:
Preliminary filtration: After the reaction liquid is cooled, a plate and frame filter (filter cloth material is polypropylene, pore size 20-50μm) is used to remove larger particle impurities (such as unreacted quartz sand, equipment corrosion products).
Fine filtration: Fine filtration is performed through membrane filtration technology (such as ceramic membrane or organic membrane). Ceramic membrane (pore size 0.1-0.5μm) can retain more than 99% of insoluble matter, and is resistant to high temperature and has good chemical stability. It is suitable for highly alkaline potassium silicate solution. For example, using a ceramic membrane with a pore size of 0.2μm and filtering at a pressure of 0.2-0.3MPa can effectively remove micron-sized insoluble particles.
Application of filter aids: Add an appropriate amount of filter aids (such as diatomaceous earth and perlite) before filtration. Its porous structure can absorb tiny particles and improve filtration efficiency and clarity. The amount of filter aid added is usually 0.5%-1.0% of the mass of the feed liquid, and the specific parameters need to be optimized through experiments.
Centrifugal separation: For potassium silicate solutions with low viscosity (such as dilute solutions in the range of 34.0-37.0 degrees Baume), a disc separator can be used for centrifugal separation. The centrifugal speed is controlled at 3000-5000r/min, and the centrifugal time is 10-20 minutes, which can effectively separate insoluble particles with higher density (such as iron filings and mud).
Ion exchange and adsorption:
If the insoluble matter contains metal ions (such as Fe³+, Al³+), it can be removed by ion exchange resin. For example, the use of strong acid cation exchange resin (such as styrene sulfonic acid resin) can adsorb cations such as Fe³+ and Al³+ in the solution, reduce the content of metal impurities, and reduce the precipitation of hydroxides caused by the hydrolysis of metal ions.
Activated carbon adsorption: Add 0.1%-0.3% activated carbon (specific surface area ≥1000m²/g) to the solution, stir and adsorb for 30-60 minutes at 50-60℃, which can remove pigments, organic matter and some metal ions and improve the transparency of the solution.
(IV) Equipment and production environment control
Equipment material upgrade: Equipment that contacts materials, such as reactors, pipelines, storage containers, etc., are made of stainless steel (such as 316L), glass lining or polytetrafluoroethylene to avoid the generation of impurities such as Fe²+ and Fe³+ due to corrosion of ordinary carbon steel. For example, the corrosion rate of stainless steel is only 1/100 of that of carbon steel, which can significantly reduce the insoluble matter introduced by equipment wear.
Production environment cleanliness control: Dust-proof facilities (such as air purification systems) are set up in the processes of batching, reaction, filtration, etc., and epoxy resin coating is used on the workshop floor to reduce dust pollution. Operators need to wear dust-free work clothes and gloves to avoid the introduction of impurities by humans.
Equipment cleaning and maintenance: Establish strict equipment cleaning procedures. After each production, rinse the reactor and pipelines with deionized water to ensure that there is no material residue. Regularly perform chemical cleaning (such as using dilute alkali solution or citric acid solution) on the filtration equipment (such as membrane components) to restore the filtration performance and avoid impurities blocking the filter holes.
(V) Storage and transportation process control
Storage container selection: Use sealed plastic barrels (such as HDPE barrels) or stainless steel tanks to store liquid potassium silicate, and avoid using corrosive containers such as iron barrels. The storage environment should be cool and dry, away from acidic gases (such as CO₂, SO₂) to prevent product carbonation.
Transportation process protection: The transportation vehicle must be clean and dry to avoid mixing with other chemicals. Take shading measures during transportation in summer to prevent high temperature from causing product volatilization or deterioration; pay attention to heat preservation in winter to prevent the solution from freezing and causing structural damage and precipitation.
Storage period management: The product storage period is usually no more than 6 months, and the insoluble content needs to be re-tested after the period. If precipitation is found, it can be filtered or reheated to dissolve (such as heating to 60-80℃ and stirring) before use.

3. Quality inspection and process monitoring

(I) Inspection methods and standards
Determination of insoluble content: Refer to GB/T 26524-2011 "Industrial Potassium Silicate" standard and use the weight method for determination. The specific steps are: take a certain amount of sample, filter it with quantitative filter paper, wash the residue with hot water until there is no potassium ion (test with sodium tetraphenylborate solution), dry it to constant weight, and calculate the mass fraction of insoluble matter. The goal is to control the insoluble content to ≤0.1% (mass fraction).
Other indicators related detection: Simultaneously monitor the product's Baume degree, density, silica content, potassium oxide content, modulus and other indicators to ensure that the main performance of the product is not affected while reducing the insoluble matter. For example, if the filtration process causes the SiO₂ content to decrease, it can be compensated by adjusting the ratio of the reaction materials.
(II) Process monitoring system
Inspection of raw materials entering the factory: When each batch of quartz sand and potassium hydroxide enters the factory, its impurity content (such as Fe₂O₃, Al₂O₃, carbonate, etc.) is tested. Unqualified raw materials are strictly prohibited from being put into production.
Online monitoring: pH sensors, temperature sensors, and pressure sensors are installed in the reactor to monitor the reaction process in real time. When the pH value or temperature deviates from the set range, an automatic alarm is issued and the process parameters are adjusted.
Intermediate product detection: After the reaction is completed, samples are taken before filtration to detect the insoluble content. If it exceeds the standard, it needs to be re-filtered or returned to the furnace for reaction. After filtration and before packaging, samples are taken again for testing to ensure that the finished product meets the quality requirements.

4. Practical basis and advantages

As an enterprise specializing in the production of inorganic silicon products, Tongxiang Hengli Chemical Co., Ltd has unique technical accumulation in the regulation of colloidal silica and silicate microstructure, which provides theoretical support for optimizing the production process of liquid potassium silicate. The company's existing production lines have high-efficiency production capacity and can quickly respond to process optimization needs, such as adjusting the stirring system of the reactor or introducing membrane filtration equipment to achieve precise control of the insoluble content.
In addition, the company focuses on customized product solutions. In the technical research and development of reducing the insoluble content, it can combine the application needs of different customers (such as the high requirements for transparency in the coating industry and the sensitivity of the foundry industry to impurities) to provide targeted process adjustment suggestions. At the same time, relying on a wide range of market application scenarios (covering electronics, clothing, papermaking and other fields), the company can continuously improve the production process through downstream feedback, forming a virtuous cycle of "R&D - production - application - optimization".