Desulfurization gypsum is also known as smoke desulfurization gypsum, sulfur gypsum or FGD gypsum, the main component is the same as natural gypsum, calcium sulfate dihydrate CaSO4·2H2O, content ≥ 93%. Desulphurized gypsum is a by-product of the FGD process, a technology that uses lime-limestone to recover sulfur dioxide from flue gases from coal or oil. The technology is to grind lime-limestone into a slurry, so that the SO2-containing flue gas after dust removal passes through the slurry scrubber to remove SO2. Lime slurry reacts with SO2 to generate calcium sulfate and calcium sulfite, calcium sulfite is oxidized into calcium sulfate, and industrial by-product gypsum, called desulfurization gypsum, is widely used in building materials and other industries.
Generation of desulfurized gypsum
The lime-limestone powder is added to water to make a slurry, as an absorbent with a pump into the absorption tower and full contact and mixing with the flue gas, sulfur dioxide in the flue gas and calcium hydroxide in the slurry and the air drummed in from the lower part of the tower for oxidation reaction to generate calcium sulfate, calcium sulfite, after reaching a certain saturation, discharge the absorption tower, and then concentrate and dehydrate to make its water content less than 10%, crystallize to form a mixture of gypsum dihydrate and calcium sulfite. It is then sent to the gypsum silo for stacking with a conveyor.
Basic Requirements for Desulfurization Gypsum Raw Materials (JC/T2074-2011)
| Ordinal | items | indicators | |
| Level 1 (A) | Level 2 (B) | ||
| 1 | Odor (wet-based) | No odor | No odor |
| 2 | Attached water content (%) (wet basis) | ≤10.00 | ≤12.00 |
| 3 | Calcium sulfate dihydrate (CaSO4.2H2O)(dry basis)(%) | ≥95.00 | ≥90.00 |
| 4 | Calcium sulfite hemihydrate (CaSO3.1/2H2O) (dry basis)(%) | ≤0.5 | ≤0.5 |
| 5 | Water-soluble magnesium salt (MgO) (dry basis)(%) | ≤0.1 | ≤0.2 |
| 6 | Water-soluble sodium salt Na2O (dry basis) (%) | ≤0.06 | ≤0.08 |
| 7 | pH (dry base) | 5-9 | 5-9 |
| 8 | Chloride ion (Cl-)(dry basis)(mg/kg) | ≤100 | ≤200 |
| 9 | Whiteness (dry basis) | Report measured values | Report measured values |
Features of desulfurized gypsum
1. The same points of desulfurized gypsum and natural gypsum
(1) The hydration kinetics and condensation characteristics are consistent;
(2) The physical and chemical properties of the main mineral phase, the five forms and seven variants after transformation are consistent, and desulfurized gypsum can completely replace natural gypsum for building materials and ceramic molds;
(3) Both are not radioactive and do not endanger health.
2. The difference between desulfurized gypsum and natural gypsum
(1) The original physical state is different: natural gypsum is a block bonded together, while desulfurized gypsum exists as a separate crystalline granule; The grindability between desulfurized gypsum impurities and gypsum is quite different, the coarse particles of natural gypsum after grinding are mostly impurities, while the particles of desulfurized gypsum are gypsum, and the fine particles are impurities, and its characteristics are the opposite of natural gypsum;
(2) Particle size and gradation: the particle size of flue gas desulfurization gypsum is relatively even, its distribution zone is very narrow, high fineness (more than 200 mesh), particles are mainly concentrated between 30-60μm, and the grading is far worse than the gypsum powder after grinding natural gypsum. Therefore, the existing calcination equipment and production process of slaked gypsum using natural gypsum as raw material cannot be completely copied, and the production process and equipment designed for chemical gypsum should be adopted; For example, desulfurization gypsum is not suitable for fluidized calcination equipment, because its particle size is too small, and it is easy to be blown out before burning; For example, if the autoclave is used to produce high-strength gypsum from natural gypsum, the desulfurized gypsum cannot be directly put into autoclaving; In addition, the particle size distribution of desulfurized gypsum is very narrow, and after processing mature gypsum powder, it must be ground and modified to produce grade difference to make it have better coagulation strength; In the grinding modification, the modification effect caused by the gradation of the crushing pressure is not good, while the effect of splitting force formation is the best, followed by the collision force; Therefore, the selection of modified mill pays attention to its mechanical characteristics.
(3) High water content, poor fluidity, only suitable for belt conveying.
(4) The difference in the composition of impurities leads to the difference in its dehydration characteristics, grindability and macroscopic characteristics such as mechanical properties and rheological properties of slaked gypsum powder after calcination.
(5) The output of desulfurized gypsum is relatively evenly distributed throughout the country, especially the output of desulfurized gypsum in the eastern developed areas where gypsum products are consumed in large quantities is also very large, and the grade of desulfurized gypsum is very high, generally more than 90%; This makes up for the major shortcomings of China's high-grade natural gypsum reserves are small, the production is low, and its products are far from the place of consumption.
Second, the utilization principle of desulfurization gypsum
1) Adhere to the principle of expanding utilization, efficient utilization, clean utilization and making the best use of materials, focus on promoting the recovery and recycling of desulfurization gypsum with a large amount and wide range and great resource potential, reasonably extend the industrial chain, develop high value-added comprehensive utilization products, reduce secondary pollution, improve resource utilization efficiency, and realize the organic unity of economic, social and environmental benefits.
(2) Adhere to the principle of market orientation, adapt measures to local conditions, produce and sell the right way, determine the priority development areas for the comprehensive utilization of desulfurization gypsum, improve resource utilization efficiency, reduce waste discharge important measures, and become the driving force for the healthy development of enterprises.
(3) Comprehensive consideration, the deep processing production line of desulfurization gypsum should be organically combined with the flue gas desulfurization system of the power plant and seamlessly connected; At the same time, the deep processing and production of desulfurized gypsum can not produce new pollution sources, and the waste heat of the power plant is used as a heat source as much as possible.
Uses of desulfurized gypsum
Gypsum powder is one of the five major gel materials, occupies an important position in the national economy, widely used in construction, building materials, industrial molds and art models, chemical industry and agriculture, food processing and medicine beauty and many other applications, is an important industrial raw materials. The difference between desulfurization gypsum powder and general gypsum powder lies in the difference in physical composition, desulfurization gypsum powder also contains silica, sodium oxide, calcium carbonate, calcium sulfite, limestone, calcium chloride, magnesium chloride, etc. Compared with other gypsum powders, desulfurized gypsum powder has the characteristics of renewable, small particle size, stable composition, low content of harmful impurities and high purity.
