Effect Of Four Calcium Aluminate Cements On The Sinterability And Abrasion Resistance Of High-alumina Abrasion-resistant Coroplasts

In recent years, circulating fluidized bed boiler (CFB) as a major "clean combustion" device in the new or rebuilt thermal (thermal) power plant project has been widely used, and led to the CFB boiler supporting the continuous improvement and development of refractory materials. As we all know, CFB boiler lining structure is complex, there are a large number of high-concentration, high-speed circulation of coal particles and flue gas in the combustion chamber, which requires CFB boiler lining refractory not only to have good construction performance, but also to have a high strength and good wear-resistant performance. High-alumina wear-resistant plastic is mainly used in CFB boiler lining structure is more complex, can not be cast in a vertical mold, but can be prefabricated or manually pounded, coated parts, such as water-cooled wall, a reclaim area and cyclone separator and other parts. High-alumina wear-resistant plastic is generally made of aggregate, powder, binding agent and coagulant promoter according to a certain proportion of mixing, with a certain viscosity and plasticity of the refractory material. The selection of coagulant promoter in high alumina wear-resistant coroplasts and the study of coagulant promotion mechanism have an important impact on the construction performance of coroplasts and the service life of CFB circulating sulfurized bed boilers. Therefore, in this work, high alumina wear-resistant plastisols were prepared by using homogenized high alumina bauxite, silica micropowder and alumina micropowder as raw materials, and phosphate as binding agent, focusing on investigating the effects of four calcium aluminate cement coagulants on the coagulation mechanism of high alumina wear-resistant plastisols, as well as the effects of sintering and wear resistance of the plastisols after firing.

1.1 Raw Materials

The test homogenized high alumina bauxite aggregate (particle size of 3~1 and ≤1mm) and fine powder (≤0.074mm) were produced in Yangquan, Shanxi, SiO2 micropowder and α-Al2O3 micropowder were commercially available products, and the binding agent was a mixture of aluminum dihydrogen phosphate and phosphoric acid, in which the density of aluminum dihydrogen phosphate was 1.5g-cm-3, and the phosphoric acid mass fraction of phosphoric acid in the phosphoric acid solution was 45%. The coagulant promoter is commercially available 60, 65, 70 and 75 calcium aluminate cement. The chemical composition of raw materials and coagulants used in the test is shown in Table 1.

1.2 Preparation

The test base formulation (w) was: 45% homogenized high alumina bauxite aggregate, 20% homogenized high alumina bauxite fines, 12% α-Al2O3 micronized powder, 5% SiO2 micronized powder and 13% aluminum dihydrogen phosphate and phosphoric acid (mixed in a mass ratio of 11) binding agent, and to the base formulation were added 5% (w) of 60 cement, 65 cement, 70 cement, and 75 cement, respectively, with specimen no. corresponding to 1#, 2#, 3# and 4#.

The weighed material is added to the mixing barrel mixing 2min, using pounding mode molding for 40mm × 40mm × 160mm conventional specimens and 100mm × 100mm × 30mm wear-resistant specimens, room temperature maintenance 48h after demolding, continue to room temperature maintenance for 24h, and then 110 ℃ heat preservation 24h drying. After drying, some specimens were heat-treated at 1100℃ for 3h, and then cooled in the furnace for use.

1.3 Performance Test

Record the initial solidification time during the maintenance of the specimens after molding; in accordance with the relevant standards (YB/T5200-1993, GB/T3001-2007, GB/T5072-2008, GB/T5988-2007, GB/T18301-2001) for bulk density, room temperature flexural strength, room temperature compressive strength, post-burning line change rate, and abrasion resistance of specimens after drying and heat treatment.

The physical phase composition of the heat-treated specimens was analyzed by X-ray diffractometer; the fracture morphology of the heat-treated specimens at 1100°C was observed by scanning electron microscope.

2.1 Normal temperature performance

110 ℃ drying and 1100 ℃ heat-treated specimens at room temperature performance is shown in Figure 1. can be seen: 110 ℃ drying specimen bulk density with the alumina content of alumina cement increased in a gradual decrease in the trend, and the 1100 ℃ heat-treated specimen bulk density is generally smaller than the 110 ℃ drying after the specimen. Analysis: the specimen after heat treatment at 1100 ℃, the coagulant promoter aluminate cement hydrate will decompose and dehydrate, the bonding agent in the aluminum dihydrogen phosphate will generate low water or anhydrous phosphate, therefore, leading to a reduction in the bulk density of the specimen after heat treatment. It can also be seen from the trend of changes in the room temperature compressive strength and room temperature flexural strength in the figure that the room temperature mechanical strength of the specimen after heat treatment at 1100 ℃ is significantly smaller than that after drying at 110 ℃, indicating that changes in the physical phase of the phosphates of the binding agent and the high-temperature decomposition of the coagulant promoter alumina cement hydride are unfavorable for the mid-temperature strength of the plastics. This is similar to the trend of the specimen bulk density, the drying and heat treatment of the specimen room temperature flexural strength and room temperature compressive strength are with the increase of alumina content in the cement and gradually reduce. It is analyzed that the strength of specimens after drying mainly originates from the binding strength of the binding agent and the effect of coagulant promoter on the material, and the types and quantities of the binding agent are the same in the materials of all formulations, so the main factor causing the change of the strength of the plastics should be the coagulant promoter.

It can also be seen from the figure:1# specimen heat-treated line shrinkage is the largest, 2#-4# specimens heat-treated line shrinkage gradually decreases.

The mechanism of calcium aluminate cement in plastics is the interaction of calcium ions in aluminate cement with phosphate ions and hydrogen and oxygen ions in the binding agent, and the hardening reaction equation is:

Ca₂++PO3-4+5H+2O₂→CaHPO₄•2H₂O.

Binding agent in aluminum dihydrogen phosphate at room temperature to the adhesion of the main role, binding agent in the phosphoric acid is more with the material in the reaction of alumina to generate aluminum phosphate salt. Although the binding agent aluminum dihydrogen phosphate has binding properties, but the plastic hardening speed is slow, in order to accelerate its room temperature hardening, must be added to react with the acid phosphate alkaline metal compounds. Calcium ions exist in aluminate cement, while aluminate cement also plays a part of the role of binding agent for high-alumina wear-resistant plastics.

2.2 Hardening and wear resistance

Figure 2 shows the effect of cement type on the initial setting time of the specimens and the abrasion resistance after heat treatment at 1100℃. It can be seen that:1# and 2# specimens can realize hardening at 4.5h, while the initial setting time of 2#-4# specimens increases gradually, and 4# specimens realize hardening after 5.5h of maintenance. The results show that calcium ions in aluminate cement can indeed play a role in promoting coagulation, and the increase of alumina content in calcium aluminate cement as well as the decrease of calcium oxide content will weaken the effect of calcium aluminate cement on the coagulation of high-alumina abrasion-resistant coroplasts. However, in the process of plastic construction, if the speed of plastic coagulation is too fast, it will directly affect the construction time and construction quality of the material, therefore, when using calcium aluminate cement as the coagulant of high-alumina abrasion-resistant plastic, it is necessary to choose the type and quantity of coagulant reasonably according to the specific construction time and construction conditions.

It can also be seen from Figure 2:1#-4# specimens wear amount gradually increased, wear resistance is gradually decreasing trend. It is analyzed that the abrasion resistance of the plastic mainly depends on the factors such as bulk density of the plastic, apparent porosity, and the degree of combination of the aggregate and matrix.

Analyzing the results of Figures 1 and 2, it can be seen that the bulk density, room temperature mechanical properties and shrinkage of specimen #1# are the largest, indicating that it is the best sintered. Therefore, 1# specimen has the best abrasion resistance, and the gradual decrease in the sintering properties of 2#-4# specimens eventually weakened the abrasion resistance of the heat-treated coroplasts.

2.3 Physical composition

Figure 3 shows the XRD patterns of 1#, 2#, 3# and 4# specimens after heat treatment at 1100℃. It can be found that: the main physical phase composition of each specimen after heat treatment is mullite phase and corundum phase, and different kinds of aluminate cement do not have much effect on its physical phase composition; however, from 1# specimen to 4# specimen, there is a tendency for the intensity of the main diffraction peak of the corundum phase in the composition of the physical phase after heat treatment to increase, and there is a tendency for the intensity of the main diffraction peak of the mullite phase to decrease. This result is related to the procoagulant aluminate cement. The theoretical mineral compositions of each aluminate cement in each specimen were different, in which the theoretical minerals in 60 cement were dodeca heptaaluminate and monocalcium aluminate, while the theoretical mineral compositions of 65, 70 and 75 cements were monocalcium aluminate and dicalcium aluminate, and the content of dicalcium aluminate was higher in 75 cement, which was close to the theoretical composition of pure dicalcium aluminate. Therefore, the enhancement of the characteristic peaks of corundum phase in the heat-treated specimens with increasing alumina content in the coagulant promoter in the high-alumina plastisol formulations is in accordance with the theoretical analysis.

2.4 Microstructure

Figure 4 shows the fracture morphology of the specimen after heat treatment at 1100 ℃ at a magnification of 1000 times. It can be found that:1# specimen matrix is more dense, high degree of direct bonding, sintering is good.2# specimen matrix is relatively loose, a small number of tiny pores in the matrix, but the size of the pores is uniform, stellar distribution, and are not interconnected with each other.

The matrix of specimen #3 shows localized sintering, with large pores in the structure, which are continuous through each other, and the structure is obviously inhomogeneous. 4# specimens have pore sizes in the range of a few micrometers within the structure, and the degree of direct bonding is weak, which to some extent indicates that the room-temperature mechanical strength of the plastisol is low.

(1) Calcium ions in calcium aluminate cement were found to play a major role in promoting coagulation of high-alumina wear-resistant plastic, and the initial setting time of the specimens gradually increased with the gradual increase of calcium oxide content in calcium aluminate cement, and hardening was achieved only after 5.5h in specimen 4#.

(2) Aluminate cement promoter directly affects the room temperature mechanical properties of plastisols after drying at 110°C and heat treatment at 1100°C. With the decrease of calcium oxide content in the aluminate cement of 1#-4# specimens, bulk density and room temperature strength of plastisols after drying and heat treatment gradually decrease.

(3)After heat treatment, the phase composition of high-alumina wear-resistant plastics is corundum phase and mullite phase, and the increase of alumina content in the procoagulant aluminate cement will strengthen the crystallization characteristics of corundum phase in plastics.

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