Also known as Sintered Pre-melted Refining Slag, Sintered Calcium Aluminate is a calcium-aluminum composite inorganic clinker mineral produced by proportioning, mixing and high-temperature solid-phase sintering of limestone (calcium oxide source) and bauxite (aluminum oxide source). Its main mineral phases are calcium aluminate (CA) and dodecacalcium heptaluminate (C₁₂A₇), with a small amount of dicalcium silicate. It differs from fused calcium aluminate manufactured via electric arc melting.

It is generally supplied in lump or granular forms. Adopting rotary kiln high-temperature solid-phase sintering technology, this product features lower production cost, excellent reactivity, uniform and stable chemical composition, as well as good moisture resistance and anti-pulverization performance. It is mainly divided into two categories: refining slag for steelmaking and calcium aluminate powder for water treatment, and is widely applied in iron and steel metallurgy, sewage treatment, refractory and building material industries.

• Production Process

1. Raw Material Crushing and Batching

The main raw materials are bauxite (Al₂O₃ content: 45–75%) and limestone containing calcium carbonate. Secondary aluminum ash from the electrolytic aluminum industry is also used as an aluminum source by some manufacturers. Ores are crushed and ground into fine powder, then accurately batched according to the target ratio of CaO and Al₂O₃. Products for steelmaking adopt a higher calcium content, while those for water treatment contain more alumina. The contents of impurities including silicon dioxide and ferric oxide are strictly controlled.

2. Mixing and Pelletizing

Powders are fully blended by ball milling, then mixed with water to form green pellets under a molding pressure of 8–12 MPa. This process prevents material agglomeration and kiln ring formation during sintering, and ensures good air permeability.

3. Gradient High-temperature Sintering (Core Process)

Materials are fed into a rotary kiln for staged heating: free water and crystal water are removed at 200–600 ℃; calcium carbonate decomposes into calcium oxide at 600–900 ℃. Sintering is carried out at 1250–1350 ℃ for 1 to 1.5 hours, where calcium oxide reacts with alumina in solid phase to form calcium aluminate minerals. Insufficient reaction occurs below 1250 ℃, and over-sintering above 1400 ℃ will drastically reduce product activity.

4. Quenching Treatment

The discharged clinker is rapidly air-cooled to inhibit the transformation of active high-temperature minerals into inert crystals, so as to retain its chemical reactivity.

5. Crushing, Screening and Finishing

The cooled clinker is crushed and screened. Granular products are prepared for steelmaking use, while fine powder is produced for water treatment. The finished products are finally dedusted and packaged for storage and delivery.

• Physical & Chemical Properties

1. Appearance: Sintered clinker is grayish-brown lumps or granules; water-treatment calcium aluminate is off-white fine powder.

2. Melting point: 1320–1380 ℃. It melts rapidly and the molten slag presents excellent fluidity and spreadability.

3. Stability: Resists water absorption and pulverization at room temperature; stable during long-term storage and transportation with negligible composition change.

4. Bulk density: 1.8–2.4 g/cm³ with moderate porosity.

5. Reactivity: Higher activity than fused refining slag. It undergoes hydration in water and releases aluminum ions quickly when reacting with acid.

6. At high temperatures, calcium oxide enables strong desulfurization and deoxidization, combining with sulfur and oxygen in molten steel to form floating slag.

7. It hydrates in water and releases aluminum ions, which is the key property for producing polyaluminum chloride water purifiers.

8. As an alkaline mineral, its aqueous solution can precipitate fluoride ions, phosphate ions and heavy metal ions in water.

9. Chemically stable at room temperature, non-toxic, and generates less dust than bulk quicklime with superior environmental performance.

• Main Applications

1. Iron and Steel Metallurgy (Main Application)

It serves as pre-melted refining slag as well as desulfurizer and deoxidizer for LF ladle refining. It accelerates slag formation and removes sulfur and oxygen from molten steel with a desulfurization efficiency of 85%–90%. Featuring good slag fluidity and low dust emission, it shortens refining time and cuts power consumption. It is applicable to carbon steel, alloy steel and special steel production. The granular form facilitates feeding and greatly reduces dust pollution compared with quicklime powder.

2. Water Treatment and Environmental Protection

Used as raw material to produce polyaluminum chloride (PAC) by reacting calcium aluminate powder with hydrochloric acid. This process is simple and cost-effective.

Applied for phosphorus and fluoride removal in industrial wastewater. It hydrolyzes to form aluminum hydroxide flocs which adsorb and precipitate fluoride, phosphate and heavy metal pollutants in electroplating wastewater, mine wastewater and domestic sewage.

3. Refractory and Building Materials

Raw material for aluminate cement, which features rapid setting and hardening and is suitable for emergency repair projects and concrete construction at low temperatures.

Basic raw material and binder for refractory castables, improving the medium and low temperature strength of refractory products.

Concrete admixture to enhance the early strength of concrete.

4. Solid Waste Recycling

Secondary aluminum ash from the aluminum industry can be used as raw material for sintered calcium aluminate production. This approach realizes resource recovery of hazardous aluminum waste and reduces waste disposal costs.