Gypsum serves as a core fundamental raw material in building materials, chemical industry and infrastructure sectors. According to raw material sources, it can be classified into two categories: natural gypsum and industrial by-product gypsum. Its processing technology mainly consists of two core procedures: drying to remove free moisture and calcination to eliminate crystal water. The two procedures differ drastically in process principles, control parameters, equipment selection and quality control standards, which directly determine the finished gypsum quality, energy consumption cost and service performance. Combining current industrial standards, this paper systematically elaborates the process characteristics, equipment selection and production control essentials of gypsum drying and calcination, and clarifies the technical boundary and production keys of the two procedures.
I. Classification of Gypsum Raw Materials and Definition of Industrial Standards for Moisture Content
As defined in the national standard GB/T 5483-2024 Natural Gypsum, natural gypsum refers to naturally occurring non-metallic ores mainly composed of calcium sulfate dihydrate (CaSO₄·2H₂O). Based on mineral components, it is divided into three types: gypsum (G), anhydrite (A) and mixed gypsum (M), acting as the primary raw material for gypsum processing. Industrial by-product gypsum such as desulfurization gypsum and phosphogypsum are secondary gypsum raw materials generated during industrial production, and also widely adopted in gypsum building material manufacturing.
Moisture in gypsum raw materials falls into two types: free moisture (adhesion moisture) and crystal water. In accordance with GB/T 5484-2012 Methods for Chemical Analysis of Gypsum and building material testing standard JC/T 1021.8-2007, free moisture refers to external water adsorbed on the surface and pores of gypsum particles, removable via low-temperature heating; crystal water is structural water chemically bound to calcium sulfate minerals, which can only be removed through high-temperature calcination. The dehydration processes for these two types of water are completely independent.
Naturally occurring moisture content of different gypsum raw materials falls within fixed ranges, serving as basic production parameters for the industry:
· Natural gypsum ore: Raw ore features hard texture and dense structure with low free moisture. Its conventional industrial moisture range is 3%～8%. Per requirements of GB/T 5483-2024, qualified natural gypsum raw materials for building materials shall contain no more than 3.0% adhesion moisture. Raw materials exceeding this limit require pre-drying pretreatment.
· Industrial by-product gypsum (desulfurization gypsum, phosphogypsum): After power plant desulfurization, chemical reaction and filter-press dewatering, materials exist as wet filter cakes with high porosity and strong water retention capacity. Their conventional moisture content ranges from 18%～25%, far higher than natural gypsum ore, making them the primary feedstock for the drying process.
Direct storage, transportation and processing of high-moisture gypsum raw materials bring prominent drawbacks: high-moisture materials tend to agglomerate and adhere to equipment walls, greatly increasing transportation unit consumption. During storage, they are prone to damp deterioration, caking and hardening. This not only raises costs for transportation, storage and feeding, but also causes material loss and uneven feeding, severely impairing the stability of subsequent calcination. Therefore, drying pretreatment is an essential upstream procedure for large-scale gypsum production.
II. Process Principle, Equipment Selection and Technical Essentials of Gypsum Drying
Gypsum drying is a low-temperature physical dehydration process. Its core objective is to fully remove free moisture from raw materials without damaging gypsum mineral structure or stripping crystal water, ultimately producing stable dihydrate gypsum (CaSO₄·2H₂O). The golden rule for drying is "avoid over-drying". This process features lower technical difficulty and better controllable parameters. As required by national testing standards, free moisture of dried finished products shall be controlled within 1.0% to meet standards for subsequent storage, conveying and calcination processing.
Compared with drying of ordinary materials, gypsum drying faces unique technical challenges: natural gypsum ore has uneven hardness and irregular lump size; industrial by-product gypsum filter cake possesses strong viscosity, prone to agglomeration and poor air permeability. General-purpose dryers commonly suffer material accumulation, uneven drying, wall adhesion, blockage and low drying efficiency. Hence, gypsum drying equipment requires targeted structural optimization. Currently mainstream applicable equipment and application characteristics in the industry are listed below:
Optimized Rotary Dryer: It is widely used for gypsum drying. Conventional rotary dryers cannot tackle filter cake agglomeration, so forced crushing, dispersing and material lifting devices shall be installed inside the cylinder. Dynamic dispersing structures break agglomerated filter cakes and large ore lumps, expand contact area between materials and hot airflow, and thoroughly eliminate wall adhesion and drying dead zones. It is universally applicable for drying natural gypsum ore and desulfurization gypsum filter cake, featuring wide production capacity adaptability and low operation & maintenance cost. Nevertheless, unstable finished product quality restricts its application mostly to small and medium-sized factories producing low value-added goods.
Air-Swept Calcining Mill (drying-specific configuration): It integrates dispersing, drying and preliminary grinding functions, serving as a preferred high-efficiency and energy-saving device. It can directly handle high-moisture viscous gypsum filter cakes; incoming materials are crushed and dispersed at high speed instantly, while hot air circulation realizes instant low-temperature drying. During equipment ordering, dedicated low-temperature drying air ducts, temperature control modules and low-wind-speed energy-saving configurations shall be selected to precisely regulate drying temperature and hot air flow, preventing crystal water loss caused by local overheating. While ensuring qualified drying and pure dihydrate gypsum output, it drastically cuts unit energy consumption, suitable for medium-to-high-end large-scale gypsum production lines.
Core quality control bottom line for gypsum drying: only free moisture shall be removed in the whole process. The complete crystal structure of dihydrate gypsum must be preserved, and premature dehydration generating impurities such as hemihydrate gypsum and anhydrite shall be avoided, so as to guarantee stable physicochemical properties of raw materials and lay a uniform and stable material foundation for subsequent calcination.
III. Gypsum Calcination Principle, Classification and Refined Control Essentials
Gypsum calcination is a high-temperature chemical modification process and the core procedure forming finished gypsum products, with far higher technical complexity and stricter parameter control precision than drying. Its core principle is to accurately heat dried dihydrate gypsum to remove partial crystal water and produce engineering-usable hemihydrate gypsum, which is the key preparation process for building gypsum and gypsum products. In accordance with GB/T 9776-2022 Calcined Gypsum, building gypsum mainly consists of β-hemihydrate gypsum; subcategories also include α high-strength hemihydrate gypsum. The two products differ significantly in performance and calcination technology.
3.1 Core Phase Transformation Principle of Calcination
Precisely controlled calcination of dihydrate gypsum (CaSO₄·2H₂O) removes three-quarters of crystal water to generate hemihydrate gypsum (CaSO₄·½H₂O), which qualifies as finished gypsum. Uncontrolled temperature, over-calcination or prolonged holding time will further strip residual crystal water and produce ineffective impurities including anhydrite, which drastically degrades core properties of finished products such as strength and setting time. Therefore, calcination determines the quality of final gypsum products.
3.2 Mainstream Calcination Processes and Equipment
Two mainstream calcination processes (one-step and two-step) are adopted in the industry, matching different production capacities and product demands with mature supporting dedicated equipment systems:
· One-step process: Drying and calcination are completed integrally. It features concise workflow and low investment cost, fit for small and medium-sized production lines. Core equipment includes air-swept calcining mill (Taeda Mill) or gypsum rotary kiln. Raw materials enter the equipment once to finish dehydration and phase transformation, with high automation and easy operation.
· Two-step process: Free moisture is removed via drying first, followed by staged high-temperature calcination. It delivers high process control precision and excellent product uniformity, suitable for manufacturing high-end α-hemihydrate gypsum and high-strength building gypsum. Core equipment includes steam calciner, externally heated kettle, sectional calcining kiln, Taeda Mill, etc. Precise zoned temperature control can effectively avoid mixed-phase issues.
3.3 Key Production Control Points for Calcination
The primary challenges of gypsum calcination lie in precise temperature control, elimination of mixed phases and uniform phase transformation. Specific control essentials are as follows:
Accurate temperature control: Operate strictly within the phase transformation temperature range for hemihydrate gypsum to prevent under-calcination at low temperature and over-calcination at high temperature. Under-calcination leaves large amounts of untransformed dihydrate gypsum, leading to slow setting and low strength of finished products; over-calcination generates anhydrous hard gypsum and results in product failure and poor stability.
Prevention of multi-phase mixing: Strictly control calcination uniformity to avoid coexistence of dihydrate gypsum, hemihydrate gypsum and anhydrite in finished materials. Multi-phase mixing directly triggers unstable setting time, severe strength fluctuation and poor workability of gypsum products, failing to meet requirements of GB/T 9776-2022 for calcined gypsum.
Standardized post-treatment workflow: High-temperature gypsum materials after calcination require rapid forced cooling to terminate thermal phase transformation and avoid continuous over-calcination by waste heat. After cooling, materials undergo fine grinding to unify particle fineness, then are conveyed to gypsum homogenizing silos for ageing treatment. Ageing stabilizes the hydration activity of hemihydrate gypsum, balances physicochemical indicators of materials and eliminates batch differences, acting as a critical final procedure to guarantee consistent finished product quality.
IV. Value Summary of Full Gypsum Processing Flow
If crude gypsum raw materials are compared to raw cotton, hemihydrate gypsum finished products applied in construction, decoration and industrial sectors are refined garments manufactured via elaborate processing. Seemingly simple gypsum powder undergoes full-chain refined processing including raw material screening, moisture pretreatment, low-temperature drying, high-temperature phase transformation, rapid cooling, fine grinding and homogenizing ageing. Parameter control, equipment matching and standard implementation of every procedure are analogous to textile processing steps like spinning, dyeing, cutting and sewing, which collectively decide the quality, performance and application value of end products. This also verifies the core industrial logic of gypsum processing: Drying consolidates the foundation, while calcination determines quality.
References
GB/T 5483-2024 Natural Gypsum (raw material definition, classification and limit of adhesion moisture)
GB/T 5484-2012 Methods for Chemical Analysis of Gypsum (testing methods for free moisture and crystal water)
GB/T 9776-2022 Calcined Gypsum (finished product quality and hemihydrate gypsum performance standards)
JC/T 1021.8-2007 Methods for Chemical Analysis of Gypsum Ores for Building Materials — Part 8: Determination of Calcium Sulfate, Calcium Sulfate Hemihydrate, Crystal Water and Adhesion Water in Gypsum Ores (special industrial testing standard)
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