Both Ammonium Sulfate - What is the Coking-Grade and Caprolactam-Grade Ammonium Sulfate?

Ammonium sulfate is an inorganic compound with the chemical formula (NH₄)₂SO₄. It is a product of the reaction between sulfuric acid and ammonia, containing 20–21% nitrogen and 24% sulfur. The pure form appears as colorless, transparent crystals in the orthorhombic system.

• Its aqueous solution is acidic, and it is insoluble in alcohol, acetone, and ammonia water.

• It has hygroscopic properties, which cause it to solidify into lumps when it absorbs moisture.

• It is mainly used as a fertilizer, suitable for various soils and crops. As a nitrogen fertilizer, it is cost-effective and acts as a fast-acting nitrogen source.

• It is generally suitable for crops such as wheat, corn, rice, cotton, sweet potatoes, hemp, fruit trees, and vegetables.

• For soils, it is most suitable for neutral and alkaline soils but not for acidic soils.

• It also has applications in the textile, leather, and pharmaceutical industries.

Production of Ammonium Sulfate

The production of ammonium sulfate primarily uses synthetic ammonia or by-products recovered from industrial processes such as coking, oil refining, and organic synthesis (e.g., iron and steel industry, chemical industry). These raw materials are then neutralized with sulfuric acid.

The chemical reaction equation is: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Main Sources of Ammonium Sulfate

Ammonium sulfate mainly comes from three sources:

1. Coking-grade ammonium sulfate: Recovered during coke production.

2. Caprolactam-grade ammonium sulfate: Recovered during the synthesis of caprolactam.

3. Ammonium sulfate from other industries: Including by-products from acrylonitrile production, methyl methacrylate production, and flue gas desulfurization in power plants.

Due to differences in production processes, the composition and quality of ammonium sulfate vary significantly.

1. Coking-Grade Ammonium Sulfate

Ammonium sulfate is one of the key chemical products mass-produced in coking plants. There are two main production methods: the saturator method and the non-saturator method. Most large-scale coking plants adopt the saturator method to produce ammonium sulfate and recover ammonia from coke oven gas.

• Ammonia in coke oven gas readily reacts with sulfuric acid to form ammonium sulfate. The absorption of ammonia by sulfuric acid is a fast, irreversible chemical reaction.

• In coking plants, this absorption reaction can occur either in a saturator or an absorption tower.

The saturator method for ammonium sulfate production is further divided into three subtypes:

1. Direct method: Hot coke oven gas is cooled by a gas condenser, purified by an electric tar precipitator, and then enters the saturator. Inside the saturator, ammonia in the gas combines with sulfuric acid to form ammonium sulfate.

   
   

2. Indirect method: Ammonia in the gas is absorbed with cold water in an ammonia scrubber. The resulting ammonia water is sent from a distillation column to the saturator, where it reacts with concentrated sulfuric acid to produce ammonium sulfate.

   
   

3. Semi-direct method: After cooling the coke oven gas, the condensed ammonia water is distilled in an ammonia distillation column to release ammonia. This ammonia, together with the ammonia in the raw gas, enters the saturator. It passes through the mother liquor layer and reacts with the sulfuric acid solution to form acid ammonium sulfate.

Product Characteristics

• Most coking-grade ammonium sulfate exists as small powdery crystals. Some products with low impurity content can form rhombic columnar crystals up to 3 mm in size.

• Even crystals larger than 2 mm have structural defects, making them brittle and prone to cracking or pulverization.

• Its appearance ranges from white to light gray, depending on the amount of coal tar mixed into the ammonium sulfate.

• Its nitrogen content is approximately 20–20.8%.

2. Caprolactam-Grade Ammonium Sulfate

This type of ammonium sulfate is a by-product of caprolactam production.

Production Process

In the caprolactam production process, liquid ammonia is neutralized with sulfuric acid during the by-product ammonium sulfate stage to obtain ammonium sulfate mother liquor. A separation device removes substances required for caprolactam production, and the remaining material is sent to a crystallization tower for reaction to form crystalline ammonium sulfate. Finally, the product is dried to get the final ammonium sulfate.

• Approximately 1.4 tons of ammonium sulfate are by-produced for every 1 ton of caprolactam.

• Key indicators: Nitrogen content ≥21%, moisture content ≤0.2%, free acid content ≤0.03%.

Product Characteristics

• It appears as white or pale yellow, transparent square or rhombic crystals with a particle size of 0.5–5.0 mm.

• Its nitrogen content is above 21%.

• Ammonium sulfate produced by this process has better quality and, accordingly, a higher price.

3. Ammonium Sulfate from Other Industrial By-Products

• Acrylamide-grade ammonium sulfate: It forms transparent white or translucent brown rhombic crystals with a particle size of 0.5–1.5 mm. Few crystals exceed 2 mm in size, and its nitrogen content ranges from 20.8–21%.

• Flue gas desulfurization-grade ammonium sulfate: Recovered from flue gas desulfurization in thermal power plants, it contains high levels of dust. It appears as gray powder with no large particles, and its nitrogen content is 17–20%.

Applications of Ammonium Sulfate

The application of ammonium sulfate is mainly determined by its particle size:

• Large particles (2–4 mm): Used in bulk blending (BB) fertilizers and single-component fertilizers.

• Medium particles (1–2 mm): Used in single-component fertilizers.

• Powdery products (particle size <1 mm): Mostly used as raw material for compound fertilizers or extruded granular fertilizers, but can also be directly applied as a single-component fertilizer.

• It has wide applicability, suitable for rice and various dry-land crops. It can be used as both base fertilizer and topdressing.

• Limitation: Its low nitrogen content makes it unsuitable for formulating high-nitrogen compound fertilizers or bulk blending fertilizers, which restricts its application scope.

• In soil chemistry, it is an acidic fertilizer. It can be used to improve saline-alkali soils and weakly alkaline soils by gradually reducing soil pH.

Precautions for Use

1. Do not store or apply it together with other alkaline fertilizers or alkaline substances, as this will reduce its fertilizer efficiency.

2. Avoid long-term application of ammonium sulfate on the same cultivated land, as it will acidify the soil and cause compaction.

3. It is not suitable for application on acidic soils.