Development And Properties Of β Desulphurzation Gypsum Power For Ceramic Mould

With the gradual enhancement of the environmental awareness and the tightening of the energy saving and emission reduction policies in China,the limited mining of the natural minerals resource and the effective disposal of the solid waste have become the focus.Flue gas desulfurization（FGD）gypsum comes from coal power plants which is a kind of solid waste after the desulfurization of coal-burning flue gas.It can replace of a part of natural gypsum in some application fields,such as in the field of buiding materials.However,the natural gypsum is still the main raw materials in the field of gypsum mould for ceramics.There are some advantages when using gypsum to prepare the mould for ceramics,such as simple processing,good water absorption and clear copying and so on,so it is used widely in the production of daily-use ceramics in China currently.However,some shortage of the gypsum mould like the poor corrosion resistance and the low strength affect its use life.,which need to be improved.Additionally,the protective mining policy for natural gypsum ore bring higher cost for the ceramic production enterprise.This paper use FGD gypsum to prepare the ceramic mould instead of natural gypsum.First,the FGD gypsum is calcined intoβsemi-water FGD gypsum which is conducted as the raw of the mould for ceramics.By studying the influence of water temperature,stirring speed and stirring time on the semi-water FGD gypsum,the optimal moulding process is determined.By adding carboxymethyl cellulose,sand and Portland cement to the semi-water FGD gypsum,the strength of gypsum is improved and the optimal prescription is determined.By studying the influence of diatomaceous earth and water paste ratio on the water absorption of gypsum,the water absorption performance of the gypsum is raised.Mould gypsum which is prepared according to the optimal formula is molded into block specimens according to the optimal molding process and water paste ratio.The actual use process of these block specimens are simulated by dry-wet circle experiments.The durability properties of these test blocks in the dry-wet cyclic process are explored.The study found that three factors including the stirring time,the stirring rate and the molding water temperature will affect the performance of the gypsum mould.When the stirring time is short,the gypsum slurry will be uneven,and if the stirring time is too long,the crystal grains of gypsum will be broken.If the stirring rate is too fast,air will be introduced into the gypsum slurry,and when the molding water temperature is too low,the hydration reaction is too slow,which can affect the development of gypsum crystals.However,the water temperature is too high is not good,which will cause the initial coagulation time to be too fast and the formation of coarse grains.The above conditions will reduce the strength of the gypsum mould.The final optimum forming process was as follows:stirring time is 90s,stirring rate is 300r/min,molding water temperature is 15℃.According to the experimental study on the improvement of the strength of semi-water FGD gypsum,it was found that the-COO-contained in carboxymethyl cellulose（CMC）bonded with the Ca²⁺in gypsum,which can adsorb on the surface of gypsum,which would prolong the initial and final setting time of gypsum.When the incorporation amount was0.15%of CMC,the highest fracture strength,compressive strength were 6.35MPa and23.19MPa.The incorporation of sand can improve the wear resistance of mould gypsum,and325 mesh is the proper.But the sand itself does not have the gelatinous,and its incorporation can not be too much.which is the best adding amount is 5%when its dry fracture strength can reach 6.31MPa.When portland cement is mixed,ettringite and hydrated calcium silicate will be produced which can improve the strength of the gypsum.The optimum adding amount of portland cement is 8%,when its dry fracture strength and compressive strength are 6.43MPa and 24.32MPa.The three raw materials were mixed with CMC（0.15%）,sand（5%）and portland cemen（8%）.The experimental results showed that the dry flexural strength and compressive strength were 7.18MPa and 25.53MPa.Studies on the improvement of the water absorption rate of semi-water FGD gypsum show that both adding diatomite and increasing the W/G ratio can increase the water absorption rate of gypsum,but both can reduce the strength of gypsum.By comprehensive comparison,the method of increasing the W/G ratio is more simple and cheaper.When the W/G ratio is increased to 0.72,the properites such as water absorption rate,initial and final setting time and strength of the test block all meet the QB/T 1639-2014 standards.The ordinary semi-water FGD gypsum powder without any modification is prepared into test specimens according to the 0.72 W/G ratio and an unoptimized molding process,meanwhile,the molud gypsum powder was prepared according to the optimal mixture amount,and their test blocks were prepared under the 0.72 W/G ratio and optimal molding process.The two kinds of test blocks were subjected to dry-wet cyclic experiments in pure water and electrolyte solution.It was found that the water absorption rate and the dissolution rate increased continuously with the increase of the cyclic numbers in pure water,and the strength decreased continuously,which indicated that the water dissolve the gypsum crystal grains and the size and number of the pores between gypsum crystal grains increase resulting in the deterioration of gypsum mould.In electrolyte solution,with the increase of cyclic number,water absorption rate and dissolution rate increase firstly,and then decrease.This is because the electrolytes can react with CaSO₄ to form refractory material blocking the pores in gypsum,but less in the beginning of the cyclic experiment,so the water absorption and dissolution rate is increased.However,the amount of insoluble substances significantly increase at the later stage of the cyclic experiment and the pores in gypsum mould are seriously filled in.Therefore,in the later stage of the cyclic process,the water absorption rate and the dissolution rate decrease,even show negative value.In electrolyte solution,the reaction between the electrolyte solution and the gypsum as well as the generation of the insoluble substances results in the decline of the strength of the sample block.Moreover,the strength of sample blocks in electrolyte solution become worse than that in water at the same experimental conditons.Whether in the water environment or in the electrolyte solution environment,the test blocks（B₂）made of mould gypsum with the optimal content of the mixture has a better durability than that（B₁）made of the unmodified semi-water FGD gypsum.The typical result is that the fracture strength of the sample blocks B₂ was 55%higher than that of the sample blocks B₁ after 80 circles of dry-wet cyclic experiments in the electrolyte solution.