Mannheim Process for Potassium Sulfate (K₂SO₄) Production

Main Production Methods of Potassium Sulfate

Mannheim Process is industrial process for the production of K2SO4, a decomposition reaction between 98% sulfuric acid and potassium chloride at high temperatures with as a byproduct hydrochloric acid . The specific steps include mixing potassium chloride and sulfuric acid and reacting them at high temperatures to form potassium sulfate and hydrochloric acid.

Crystallization separation produces potassium sulfate through roasting of alkali like tung seed shell and plant ash, then followed by leaching, filtering, concentrating, centrifugal separation and drying to obtain potassium sulfate.

Reaction of Potassium Chloride and Sulfuric Acid at specific temperatures in a specific ratio is another method to get potassium sulfate. The specific steps include dissolving potassium chloride in warm water, adding sulfuric acid for the reaction, and then crystallizing at 100–140°C, followed by separation, neutralization, and drying to produce potassium sulfate.

Advantages of Mannheim Potassium Sulfate

Mennheim process is the primary method of potassium sulfate production overseas. The reliable and sophisticated method produces concentrated potassium sulfate with superior water solubility. The weak acid solution is suitable for alkaline soil.

Production Principles

Reaction Process:

1. Sulfuric acid and potassium chloride are proportionally metered and evenly fed into the reaction chamber of the Mannheim furnace, where they react to produce potassium sulfate and hydrogen chloride.

2. The reaction occurs in two steps:

i. The first step is exothermic and occurs at a lower temperature.

ii. The second step involves the conversion of potassium bisulfate to potassium sulfate, which is strongly endothermic.

Temperature Control:

1. The reaction must occur at temperatures above 268°C, with the optimal range being 500-600°C to ensure efficiency without excessive sulfuric acid decomposition.

2. In actual production, the reaction temperature is typically controlled between 510-530°C for stability and efficiency.

Heat Utilization:

1. The reaction is highly endothermic, requiring consistent heat supply from natural gas combustion.

2. Around 44% of the furnace’s heat is lost through the walls, 40% is carried away by exhaust gases, and only 16% is utilized for the actual reaction.

Key Aspects of the Mannheim Process

Furnace diameter is the decisive factor of production capacity. The largest furnaces globally have a diameter of 6 meters. At the same time, reliable driving system is the guarantee of continuous and stable reaction. Refractory materials must withstand high temperatures, strong acids, and offer good heat transfer. Materials for the stirring mechanisms must be resistant to heat, corrosion, and wear.

Hydrogen Chloride Gas Quality:

1.Maintaining a slight vacuum in the reaction chamber ensures that air and flue gases do not dilute the hydrogen chloride.

2.Proper sealing and operation can achieve HCl concentrations of 50% or higher.

Raw Material Specifications:

1.Potassium Chloride: Must meet specific moisture, particle size, and potassium oxide content requirements for optimal reaction efficiency.

2.Sulfuric Acid: Requires a concentration of 99% for purity and consistent reaction.

Temperature Control:

1.Reaction Chamber (510-530°C): Ensures complete reaction.

2.Combustion Chamber: Balances natural gas input for efficient combustion.

3.Tail Gas Temperature: Controlled to prevent exhaust blockages and ensure effective gas absorption.

Process Workflow

• Reaction: Potassium chloride and sulfuric acid are continuously fed into the reaction chamber. The resulting potassium sulfate is discharged, cooled, screened, and neutralized with calcium oxide before packaging.
• By-product Handling:
  • High-temperature hydrogen chloride gas is cooled and purified through a series of scrubbers and absorption towers to produce industrial-grade hydrochloric acid (31-37% HCl).
  • Tail gas emissions are treated to meet environmental standards.

Challenges and Improvements

1. Heat Loss: Significant heat is lost through exhaust gases and furnace walls, highlighting the need for improved heat recovery systems.
2. Equipment Corrosion: The process operates under high temperatures and acidic conditions, leading to wear and maintenance challenges.
3. Hydrochloric Acid By-product Utilization: The market for hydrochloric acid can be saturated, necessitating research into alternative uses or methods to minimize by-product output.

The Mannheim potassium sulfate production process involves two types of waste gas emissions: combustion exhaust from natural gas and byproduct hydrogen chloride gas.

Combustion Exhaust:

The temperature of the combustion exhaust is generally around 450°C. This heat is transferred through a recuperator before being discharged. However, even after heat exchange, the exhaust gas temperature remains at approximately 160°C, and this residual heat is released into the atmosphere.

Byproduct Hydrogen Chloride Gas:

The hydrogen chloride gas undergoes scrubbing in a sulfuric acid washing tower, absorption in a falling-film absorber, and purification in an exhaust gas purification tower before being discharged. This process generates 31% hydrochloric acid, in which higher concentration can lead to emissions not up to standards and causing a “tail drag” phenomenon in the exhaust. Therefore, real time hydrochloric acid concentration measurement turns important in production.

Following measures could be taken for better effects:

Reduce Acid Concentration: Lower the acid concentration during the absorption process with inline density meter for accurate monitoring.

Increase Circulating Water Volume: Enhance the water circulation in the falling-film absorber to improve absorption efficiency.

Reduce the Load on the Exhaust Gas Purification Tower: Optimize operations to minimize the burden on the purification system.

Through these adjustments and proper operation over time, the tail drag phenomenon can be eliminated, ensuring that emissions meet the required standards.