Production technology of paper faced gypsum board made of phosphogypsum

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Production technology of paper faced gypsum board made of phosphogypsum
Gypsum plasterboard is a kind of building decoration material that consumes a large amount of gypsum, and is widely used in indoor decoration such as suspended ceiling and partition wall. As the quality of phosphate rock decreases gradually, the content of impurities in phosphogypsum, a by-product of ammonium phosphate production, is relatively high. When it is used in the production of gypsum plasterboard, there will be a problem that the gypsum plasterboard does not stick to paper. When producing paper faced gypsum board with phosphogypsum containing high potassium and sodium, the phenomenon of non sticking paper appears. By using independently developed inhibitors, potassium and sodium can form insoluble salts with low solubility; By selecting appropriate production process, adjusting the temperature of primary dehydration and secondary dehydration, adding inhibitor and other effective control methods, qualified gypsum plasterboard can be produced from phosphogypsum.
Shijiazhuang Jingjin Mechanical Equipment Installation Co., Ltd. Zhou Yanhui 13833456715
1. Preface
Phosphogypsum is an industrial by-product in the production of wet process phosphoric acid. Its main component is calcium sulfate dihydrate. Phosphogypsum as a raw material for building materials has been developed in many aspects, among which the production of gypsum plasterboard with phosphogypsum has become an effective way to use phosphogypsum because of its large consumption and wide use.
2. Production process of paper faced gypsum board made of phosphogypsum
Paper faced gypsum board is a building board made of phosphogypsum as the main raw material, mixed with appropriate fiber reinforced materials and additives, mixed with water, poured between two layers of face protection paper, and formed, solidified, cut, dried and cut. The production of gypsum plasterboard with phosphogypsum mainly includes two processes: powder making and board making. See Figure 1 for the process flow.
(1) In the pulverizing process, phosphogypsum with w (H2O) of 15%~20% is sent to the rotary dryer by belt conveyor for pre drying, and free water and part of crystal water are removed after drying, and then it enters the calciner for dehydration to obtain hemihydrate gypsum powder. The rotary dryer drying process with direct contact heat exchange method is adopted for pre drying, and the fluidized bed furnace process with indirect contact heat exchange method is adopted for calcination. Waste gas is discharged after being treated by cyclone dust remover and electrostatic precipitator.
(2) The plate making process is divided into three parts: batching system, plate making system and heat source system. The semi hydrated gypsum powder processed in the pulverizing process is mixed with a proper amount of fiber reinforced materials, admixtures and water to form a uniform slurry. The slurry is compounded by the molding machine and the gypsum board face paper in the molding station to form a board type. The gypsum board gradually forms a wet gypsum board with certain strength and regular shape through natural solidification on the solidification belt. The wet gypsum board is accurately cut into veneer according to the set size by the precisely and automatically controlled cutting system, After drying in the drying kiln and cutting into regular sizes, the finished gypsum plasterboard can be obtained.
3. Key points of production process control
3.1 Temperature control of phosphogypsum dehydration
Phosphogypsum is dewatered in two steps. Firstly, the free water in phosphogypsum is removed. In this step, the phosphogypsum is directly contacted with the hot gas from the coal-fired fluidized bed furnace. After the temperature of phosphogypsum material rises to 80~120 ℃, the free water is removed; Then enter the heat transfer oil fluidized bed calciner to dehydrate the dihydrate gypsum into hemihydrate gypsum. In this step, the indirect contact heat exchange method is used to control the outlet material temperature of phosphogypsum in the fluidized bed furnace at 160~180 ℃ to obtain hemihydrate gypsum powder. The temperature should be adjusted according to the setting time of the produced phosphogypsum powder. The initial setting time of phosphogypsum powder is controlled at 4-5 min, and the final setting time is controlled at 7-8 min. The setting time of phosphogypsum powder can be obtained through setting test. If the setting time of phosphogypsum powder is too fast, it can be controlled by increasing the outlet material temperature of fluidized bed calciner; If the setting time of phosphogypsum powder is too long, it can be controlled and adjusted by reducing the material temperature at the outlet of fluidized bed calciner.
3.2 Control of paper core adhesion
The factors affecting the paper core bonding in the production process of gypsum plasterboard involve raw materials, auxiliary materials and process control, such as gypsum, face paper, various additives (starch, coagulant, foaming agent, paper fiber, etc.), gypsum slurry preparation, the setting speed of gypsum slurry, the incompatibility between drying speed and temperature, Factors such as uneven addition of foaming agent or poor foam stability of foaming agent will affect the paper core bonding of gypsum board. In most cases, it is the result of a combination of several factors. In production, raw and auxiliary materials suitable for the production process should be selected in combination with the actual production, the process control indicators should be adjusted in time, and careful operation can minimize the problem of poor paper core adhesion.
In the process of using phosphogypsum to produce gypsum plasterboard, attention should also be paid to the content of K+, Na+and other impurities in phosphogypsum. If the content of potassium and sodium impurities in phosphogypsum is too high, due to the migration of potassium and sodium ions, when the board is in a low temperature and high humidity environment, the board is very easy to absorb water, forming a layer of salt film at the junction of the paper core, that is, "frost" phenomenon, which makes the paper core detached. When the content of potassium and sodium impurities in phosphogypsum is too high, a certain amount of inhibitor (CaCO3 or CaO) should be added before the phosphogypsum is dehydrated. After mixing and aging, the required hemihydrated phosphogypsum powder can be prepared by conventional methods.
3.3 Speed regulation of wet plate solidification belt
When the gypsum plasterboard formed by the molding machine is transported on the solidification belt, the natural solidification gradually forms a wet gypsum board with certain strength and regular shape. The solidification of wet gypsum board can be controlled by adjusting the speed of the solidification belt, which is usually controlled at 16-24m/min. If the final setting of the wet gypsum board is not completed on the setting belt, the running speed of the setting belt can be lowered to facilitate the final setting of the wet gypsum board and enhance the strength of the wet gypsum board.
3.4 Drying speed and temperature control of wet gypsum board drying process
The drying kiln body of this drying procedure is tunnel kiln body, which is composed of conveying roller, chain, chain wheel, main air duct, return air duct, exhaust system and distributor. The drying kiln is equipped with 8-layer conveying roller table and heat pipe, which are indirectly heated by heat transfer oil. The drying kiln is divided into 5 sections. The drying temperature is controlled by sections, and the temperature of each section should be appropriately adjusted. Generally, the air temperature in the high temperature area of the drying kiln is controlled to be 150~180 ℃, and the drying speed (i.e. the rotation speed of the drying kiln chain and the traveling speed of the gypsum plasterboard) is 2~3m/min. If the drying speed and temperature are not controlled properly, it is easy to cause over burning or under burning of gypsum board. It should be adjusted in time according to the change of gypsum bonding performance. If the drying temperature is too high and the chain speed of the drying kiln is too slow, it is easy to cause the gypsum board to burn too much and the paper core of the gypsum board will bond and peel off; If the drying temperature is too low and the chain speed of the drying kiln is too fast, it is easy to cause under burning of gypsum board and paper rubbing on gypsum board paper surface.
3.5 pH control of phosphogypsum
The pH value of phosphogypsum shows the content of phosphorus and fluorine in phosphogypsum, and phosphorus element will affect the setting time and strength of gypsum. Therefore, the fluctuation of pH value of phosphogypsum will affect the setting time of slurry and the bonding performance of paper core, and will also have a negative impact on the strength of gypsum. The pH value of phosphogypsum should be above 4. For phosphogypsum with a pH value below 4, a regulator (lime) can be added for adjustment.
4. Project implementation effect
(1) With many advantages such as light weight, fire prevention, sound insulation, good thermal insulation, strong processability, and convenient construction, gypsum plasterboard has been rapidly promoted in building materials, and its output is growing. However, compared with developed countries, the per capita annual consumption of gypsum plasterboard in China is still very low, and the gypsum plasterboard industry in China still has huge market potential.
(2) The project of using phosphogypsum as raw material to produce gypsum plasterboard conforms to the national industrial policy, conforms to the development strategy of circular economy, and belongs to the "industrial solid waste recycling" project in the aspect of comprehensive utilization of resources. It not only makes full use of waste industrial by-products to replace natural resources for production, with low cost, strong market competitiveness, but also reduces environmental pollution and caters to the market demand. In the long run, It has broad prospects for development.
(3) The project of comprehensive utilization of phosphogypsum to make gypsum plasterboard has the advantages of less investment, simple process, easy implementation, significant economic, social and environmental benefits, and can be used as a key demonstration and promotion project of comprehensive utilization of phosphogypsum in the same industry in China.