| Spontaneous combustion of sulfide minerals is one of the most serious disasters in the mining of metal mines. Once a fire is initiated in stored sulfide ores and concentrates, the disaster will lead to a series of environmental and safety problems, and losing large quantities of mineral resources. When sulfide ore deposit exploited and exposed to air for a period of time, sulfide minerals will begin chemisorbing oxygen and oxidizing, releasing lots of heat. If the rate of heat generation in sulfide mineral stockpiles exceeds that of heat removal from the boundaries, the heat accumulates and their temperature rises gradually, and the oxidizing reaction will accelerate, up to the auto-ignition point.This thesis has been supported financially by several important projects, such as the National Science and Technology Pillar Program during the 11th Five-Year Plan Period of China (Study on the key technologies of spontaneous combustion tendency appraisal and the fire forecasting,2006BAK04B03) and the National Natural Science Foundation of China (Study on the safety assessment system of spontaneous combustion of sulfide ores in metal mines,51074181), etc. By referring to and analyzing many former research fruits about spontaneous combustion hazard in mines, a lot of study work has been made by combining theoretical analysis with practical application, aiming at the oxidation behavior of sulfide ores under ambient environment, the mechanism of spontaneous combustion, the test method and standard for evaluating spontaneous combustion tendency of sulfide minerals, mathematical models and simulations, non-contact measuring of temperature in metal mines. The main study contents and conclusions of this thesis are as follows:(1) Based on several famous databases over the world, about 300 papers toward spontaneous combustion in mines were consulted and analyzed. The current mechanisms on spontaneous combustion of sulfide minerals were discussed systematically, including the physical oxygen adsorption theory, chemical thermodynamics, electrochemistry mechanism, and bio-oxidation theory. Also, the test methods for spontaneous combustion tendency appraisal of sulfide minerals were set forth, including oxygen-adsorption velocity test, traditional crossing-point temperature test, adiabatic oxidation test, comprehensive test, new wire-mesh basket method, temperature programmed oxidation method, etc; and these methods were compared and classified by their intrinsic characteristics.(2) Several representative sulfide ore samples were obtained from typical metal mines. The oxidation process of each sample at low temperature was simulated in the laboratory. The colors, agglomeration properties, microstructures, chemical compositions, and mineralogical analysis of each sample before and after the oxidation were compared by X-ray diffraction analysis (XRD), scanning electron microscope (SEM), Energy Dispersive Spectrometry (EDAX), and Fourier Transform Infrared Spectroscopy (FTIR). The weight increment rate of each sample and the contents of water soluble iron and sulfate ions at different time were also measured. Furthermore, the main factors affecting the oxidation of sulfide ores were identified, including the crystal structure, chemical compositions, trace metal content, environmental temperature, oxygen concentration, air moisture, particle size, environmental pH value, ferric iron, bacteria, geological conditions, etc.(3) A new theory of mechanical activation for explaining spontaneous combustion of sulfide minerals was put forward, in which the chemical reaction activation of sulfide ores was considered to be heightened by all kinds of mechanical force during the mining. The apparent appearances, microstructures, paricle sizes, specific surface areas, and heat behaviors of activated samples were characterized by advanced apparatuses. It is found that sulfide ores after mechanical activation show many evident changes with decreased particle sizes, increased specific surface areas, agglomeration phenomenon, defect and deformation of lattice structure, and lower temperatures for the initial heat release and self-igniton points. At ambient environment, the activated samples are more susceptible to being oxidized.(4) The oxidation kinetics test method for spontaneous combustion tendency of sulfide minerals was advanced. Based on the new wire-mesh basket crossing-point temperature (CPT) method, an experimental system was assembled to gain oxidation and self-heating properties of three different sulfide minerals, and corresponding oxidation kinetics parameters were calculated. The combination of thermogravimetry (TG) with differential scanning calorimeter (DSC) was applied to test sulfide minerals, and the TG-DTG-DSC curves for each sample at different heating rates were gained. By the peak temperatures on DTG curves, the whole reaction process of each sample was divided into different stages, and the corresponding apparent activation energies were calculated using the Ozawa-Flynn-Wall method. Furthermore, activation energy value was considered to be used as the index for evaluating spontaneous combustion tendency. A new appraisal system for assessing spontaneous combustion tendency of sulfide minerals was built, and the concrete test procedure was also regulated.(5) Sulfide ore stockpiles in stope were seen as porous medium, and the mathematical models for describing the dynamic process of spontaneous combustion of sulfide minerals were deduced by the theories of heat and mass transfer theory, Darcy Law, conservation of mass and energy laws, etc. Also, the mathematical formulas for calculating spontaneous combustion period of sulfide minerals were improved. The concept of critical accumulative thickness for spontaneous combustion of sulfide concentrates in storage was put forward; the corresponding critical values for each sample under different environmental temperatures were gained by Frank-Kamenetskii's model of spontaneous combustion. Furthermore, considering the main factors that influence spontaneous combustion of sulfide ores in stope, the uncertainty measurement theory was applied to evaluate the spontaneous combustion hazard, which can solve the uncertainty problems in spontaneous combustion assessment of sulfide ores, and it can analyze the problems quantitatively.(6) The combination of experimental test and theoretical analysis was used to obtain several important parameters in the mathematical models of spontaneous combustion, including the heat release intensity, heat conductivity, oxygen consumption rate, porosity of ore piles, osmosis coefficient, etc. Combining with the locale boundary and initial conditions, FLUENT and ANSYS softwares were applied to simulate the air flow field, SO2 and O2 concentration fields, and dynamic temperature fields in typical metal mines that have serious spontaneous combustion phenomenon, and the simulation results were used to direct the fire control work effectively(7) The surface temperatures of diverse sulfide minerals were measured simultaneously by Raytek infrared thermometer and Center tangent thermometer. The relationship between the sensing data of the infrared thermometer and the actual surface temperature with various measuring distances, measuring angles, mineral types, and environmental conditions were investigated. Also, how the measuring errors come in bad conditions during practical applications were analyzed systematically. The improved instruments adopted in the laboratory were utilized to measure the temperature of sulfide ores and concentrates on locale, and showed good effects. |
PDF Full Text Request