How to increase the yield of alum in industrial production?

In the industrial production of alum (KAl(SO ₄)₂ ·12H ₂ O)), increasing yield requires optimization from multiple aspects, including raw material utilization, reaction efficiency, and crystallization processes. The following are specific technical measures and principle analyses:

I. Raw Material Pretreatment and Optimization

1. Enhancing Aluminum Source Activity

Bauxite Pulverization and Activation:

Pulverize bauxite to a finer particle size (e.g., ≤0.1mm) to increase the contact area with sulfuric acid and accelerate the acid dissolution reaction rate.

High-temperature calcination of bauxite (600-800℃) destroys its crystal structure, making it easier to react with sulfuric acid (e.g., converting diaspore to higher-activity amorphous aluminum oxide).

Waste Aluminum Scrap Pretreatment:

Thoroughly remove the surface oxide film (Al ₂ O ₃ ) with sodium hydroxide solution to prevent the oxide layer from hindering the reaction between aluminum and sulfuric acid: (Al ₂ O ₃ + 2NaOH → 2NaAlO ₂ + H ₂ O)

2. Raw Material Ratio Optimization

Sulfuric Acid Excess Control:

During acid dissolution, a slight excess of sulfuric acid (1.1-1.2 times the theoretical amount) ensures that the (Al ₂ O ₃ ) in bauxite completely reacts, but excessive amounts will increase subsequent neutralization costs.

Potassium Source Selection and Ratio:

Use potassium sulfate (K ₂ SO ₄ ) or potassium chloride (KCl) to react with sulfuric acid to produce potassium sulfate (consider byproduct treatment). Accurately proportion according to the molar ratio of (Al ₂ (SO ₄)₃ ): K ₂ SO ₄ = 1:1) to avoid insufficient potassium source leading to incomplete double salt synthesis.

II. Reaction Process Enhancement

1. Acid Dissolution Stage Efficiency Improvement

Heating and Stirring:

Increase the reaction temperature to 90-100℃ (near the boiling point of sulfuric acid) to accelerate Al dissolution (the reaction rate increases by approximately 2 times for every 10℃ increase in temperature); simultaneously use mechanical stirring or aeration stirring to reduce solid-liquid mass transfer resistance. ₂ O ₃ Pressure Assistance:

Use a pressurized reactor (0.5-1MPa) to increase the boiling point of sulfuric acid and react at a higher temperature (e.g., 120-150℃), shortening the acid dissolution time (traditional atmospheric pressure requires 4-6 hours, while pressure can shorten it to 2-3 hours).

2. Double Salt Synthesis Optimization

Solution Concentration Control:

The concentration of the mixed solution of aluminum sulfate and potassium sulfate should be controlled near the supersaturation critical point (e.g., solubility is approximately 50g/100g water at 60℃). Too low a concentration will reduce crystallization yield, while too high a concentration will easily lead to co-precipitation of impurities.

pH Adjustment:

Add dilute sulfuric acid to maintain the solution pH at 2-3 to inhibit the hydrolysis of Al³⁺ to form Al(OH)

) precipitate (hydrolysis is significant when pH>4), avoiding aluminum element loss. ₃ III. Crystallization Process Improvement

1. Cooling Crystallization Optimization

Gradient Temperature Control:

Use segmented cooling: first rapidly cool to 40-50℃ (to promote nucleation), then slowly cool to 20-25℃ (to promote crystal growth), avoiding excessively rapid cooling that leads to small crystals and increased impurity encapsulation.

Seed Crystal Addition:

Add pure alum seed crystals (particle size 0.5-1mm) to the solution to provide crystallization cores, reduce the number of spontaneous nuclei, promote the growth of large crystals, and improve the crystallization rate (addition amount is 0.5%-1% of the solution mass).

2. Evaporation Crystallization Application

For high-concentration solutions, vacuum evaporation can be used (reducing the boiling point and energy consumption) to evaporate and concentrate to supersaturation at 50-60℃, then cool and crystallize. Compared with simple cooling crystallization, this can increase the yield by 10%-15%.

IV. Impurity Removal and Recycling

1. High-efficiency Impurity Removal Process

Stepwise Impurity Removal:

After acid dissolution, first add potassium permanganate to oxidize Fe²⁺ to Fe³⁺, adjust the pH to 4.5-5.0 to allow Fe³⁺ and Mn²⁺ to form hydroxide precipitates; then add activated carbon to adsorb pigments and organic impurities. Control the filtration accuracy to below 5μm to prevent impurities from blocking crystallization sites.

Mother Liquor Treatment:

The mother liquor after crystallization contains approximately 10%-15% alum solute and can be recycled for acid dissolution or double salt synthesis to reduce the loss of aluminum and potassium elements (purification is required after 3-5 cycles to prevent impurity accumulation).

2. Byproduct Recovery

2. 副产物回收

The hydrogen gas produced during the acid dissolution process (e.g., waste aluminum method) can be collected and utilized, reducing safety hazards; the filtered residue (e.g., silicon dioxide) can be used to prepare refractory materials, improving the comprehensive utilization rate of raw materials.

V. Equipment and Process Control

1. Continuous Production Equipment

Using continuous reactors to replace batch operations, realizing a production line operation of acid dissolution, impurity removal, and salt synthesis, shortening the production cycle (traditional batch operations require 8-10 hours/batch, continuous operation can be reduced to 4-5 hours), and increasing the unit time output.

2. Automated Control

Through the PLC control system, real-time monitoring of reaction temperature, pH, concentration, and other parameters, precisely adjusting the amount of sulfuric acid added and the cooling rate, avoiding errors in manual operation that lead to fluctuations in yield (e.g., temperature control accuracy ±1℃, pH control ±0.2).