What is alumina clinker?

The principal minerals in calcined bauxite are diaspore and boehmite, with the diaspore content increasing as the Al₂O₃/SiO₂ ratio rises. Secondary minerals include rutile, goethite, and others; occasionally, minor amounts of gibbsite and dickite may also be present. Based on the aluminum oxide content, bauxites are typically classified into three grades and five sub‑grades, with their respective Al₂O₃ levels and colors as follows:

      First-class, extra-grade: 85% or more, earthy gray or dark gray.

Grade I, Level 1: 75–85% earthy gray or dark gray

Second-class, second-grade, 65–75% white-gray

Third class, third grade, 50–65%, bluish-gray

Grayish-blue, below 50% for Level 4

Alumina clinker belongs to sedimentary deposits and is classified into alluvial deposits and vein deposits. Alluvial deposits are overlain by a hard red clay layer, accompanied by thick strata of limestone; these are commonly referred to as “grain ginger stone.” The ore bodies occur in layered forms, with extensive lateral extents that may stretch for several li along strike, typically 3–4 meters thick, though thicker sections can exceed 7–9 meters. The material is pure, with a hard, dense texture. Vein deposits are covered by limestone, have smaller areas, and occur in pocket‑like clusters, usually ranging from tens to several dozen meters per pocket; they are sometimes intergrown with limestone, with a thin layer of fine red clay separating them. The material is coarser and less pure.

Alumina‑rich bauxite clinker must undergo calcination to become fully sintered, after which it can be used to produce high‑alumina bricks. At present, most enterprises employ either rotary kilns or shaft kilns for the calcination of high‑alumina bauxite.

Compared with traditional calcination methods, rotary kilns have the following characteristics:

1. Low energy consumption: Compared with conventional dome‑top converters, coal consumption per unit of steel can be reduced by 40%. The product quality is excellent, stability is high, and it enjoys a strong price competitiveness in the market.

2. Simple and reliable operation, requiring minimal personnel and ensuring stable process performance.

3. Minimal environmental pollution, low maintenance requirements, and high operational availability.

4. It is applicable to a variety of fuels, including coal gas, pulverized coal, and fuel oil.

5. It can be upgraded to automated control, making the system even more refined and stable.

Shaft kilns are well suited for small and medium-sized enterprises. Their key features include a simple structure, ease of implementation, low capital investment and construction costs, reduced consumption of raw materials and fuel per unit of product, lower production costs, a small footprint, high space utilization, the absence of mechanical feeding systems, convenient operation, straightforward process control, and the ability to operate even in areas with no or limited electricity. For these reasons, they are particularly appropriate for small and medium-sized enterprises.