Research On Characteristics Of The Newly Developed Thickening Gels For Prevention And Control Of Coal Spontaneous Combustion

Grouting is a common measure for prevention and control of coal spontaneous combustion in coal mining industry of our country. Considering grouting could consume a lot of water and usually lead to pipe blocking during transportation, we developed a new kind of thickening gels which could stabilize the loess suspension for long term in lower water-solid ratio and possess shear thinning characteristics that is beneficial for pipeline transportation. This paper presents a systematical study on this newly developed thickening gels using experimental study, theoretical analysis and calculation. The following conclusions are drawn:(1) Thickening gels are prepared by mixing the composite gels and loess, and the former is the solution of natural polysaccharide DG and cellulose ether HD.Experiments on loess suspending characteristics of thickening gels were conducted,and the results showed: When the concentration of composite gels exceeds 0.4%, the systems did not generate sediment after standing for 24 hours, and the zeta potential of loess particles was maintained near -30mV, which showed the thickening gels own the best suspending characteristics at critical concentration of composite gels of 0.4%,under which the thickening gels could suspend loess with any water-solid ratio larger than unity, and they own the best suspending characteristics at pH value of 7.5.Thickening gels could stabilize the loess for long term for the following reasons, on the one hand, the component of DG could chelate with calcium ions to form three dimensional networks while HD component could offer synergistic viscosity enhancement, and thus loess particles could be bound firmly in the networks; on the other, the adsorption of DG and HD molecules on surfaces of loess particles could increase the inter-particle repulsive potential and decrease the inter-particle attractive potential of loess, which cause the thermal motion of particles could not overcome the potential barrier and inter-particle repulsive forces to aggregate together.(2) Experiments on rheological characteristics of thickening gels were conducted using rheometer, and the results showed: when the mass ratio between natural polysaccharide DG and cellulose ether HD is 5:5, the composite gels own the largest viscosity, and thus the gelling materials' ratio of thickening gels is determined as 5:5;After the analysis of rheological characteristic curves of the thickening gels, we found they belong to pseudoplastic fluid with yield stress and the rheological constitutive model under concentration of 0.4% is r = 0.078+ 2.740?·0.427 ; Systematic tests on the viscoelasticity of the composite gels were carried out, and the results showed thickening gels belong to viscoelastic solid and are near to elastomer; Thickening gels own broader linear viscoelastic region (LVR) than DG gels, and thus the former possess stronger structure stability which means their structure could not be destroyed easily by shearing action; Once the structure of thickening gels is destroyed by shearing action, it could arrive at structure recovery ratio of 43.3% in 5 seconds, and the final structure recovery ratio is 76.0%; The curves for shear stress of thickening gels with different water-loess ratio all first decline and then go up; The viscosity and yield stress of thickening gels are increased with the increase of loess proportion; The rheological constitutive models for thickening gels with water-loess ratio of 3:1 and 4:1 are z=25.15 + 2.377?·0.663 , and ? = 14.02 + 2.054?·0.583 , respectively; The viscosity prediction model for thickening gels was constructed as ? ? 10-8 9358+8.157?+8.6781?00.0551 using continuous phase viscosity and dispersion phase volume fraction as the parameters, and the prediction values is very near the true values; When the thickening gels flow in DN100 and DN125 steel pipes at a flow rate of 30m3/h, the one kilometer pipe pressure drop are 1.203Mpa and 0.778Mpa, respectively.(3) Experiments on characteristics of thickening gels in fire prevention and control were conducted, and the results showed: The negative pressure of vacuum chamber of air sealing testing device could be maintained at -14.6kPa after using thickening gels, and the effects of thickening gels were better than coal gangue filling and adhesive tape sealing; After the cover of thickening gels, the temperature of hidden high-temperature fire source could change from 446.5? to 44.5? in 20 seconds, and temperature of sites 3cm over the fire source could change from 185.2?to 25.4? in 20 seconds; The cooling ability and cooling stability of thickening gels are better than yellow mud since the former possess better water-retaining property;During the oxidation and temperature increasing of coal, compared with the untreated coal, the thickening gels treated coal emit notably lower CO quantity, and the crossing point temperature improved by more than 50?, which shows thickening gels possess good retardant performance on the tested coal samples; Compared with the untreated coal at room temperature, the content of functional groups on thickening gels treated coal varied with different species in which the content of -OH and -COOH improved by 3% and 2.2%, that of C=O, C-O, and substitute benzene decreased by 0.6%, 1.5%,and 2.5%, the content of aliphatic and aromatic hydrocarbon changed by within 0.2%;During the oxidation and temperature increasing of coal, compared with the untreated coal, the change rate of functional groups on thickening gels treated coal varied with different species in which change rate of aliphatic hydrocarbon, C-O, -COOH and C=O decreased in different degrees, that of -OH increased, and change rate of -COO showed no significant change.