Pollution of organic compounds and heavy metals in a coal gangue dump of the Gequan Coal Mine, China

Samples around a coal gangue dump of the Gequan Coal Mine were collected in April 2009. GC (gas chromatography) and GC/MS (gas chromatography/mass spectrometry) were employed to analyze the composition of organic matter in the samples. ICP-MS (inductively coupled plasma mass spectrometry) was used to determine the concentrations of heavy metals. The contents of organic extracts are within the range of 140-750 mg/kg. Alkand aro-ratios are relatively high. Compared to those of the background sample (GQ13 ), the contents of saturated hydrocarbon compounds in all the samples are relatively high. The contents of polycyclic aromatic hydrocarbons (PAHs) are relatively high with the distance getting closer to the coal gangue dump. These indicate that organic matter in the samples is from coal particles of the coal gangue dump. The distributions of heavy metals are very similar: the contents decrease with distance from the dump, which indicates that the harmful heavy metals from the coal gangue dump have polluted as thick as at least 500 m.

Effects of a proline solution cover on the geochemical and mineralogical characteristics of high-sulfur coal gangue

Batch experiments tively evaluate the inhibition were conducted to compara- effects and mechanisms of a low-concentration （1%） proline solution cover on the release of pollutants from high-sulfur coal gangue. High- sulfur coal gangue was continuously immersed in a proline solution and in deionized water （as a control treatment） for 540 days. The results showed that the coal gangue in the control treatment was oxidized and generated leachate with poor water qualities, i.e., the leachate exhibited lower pH values, higher redox potential values, higher pollutant concentrations （804^2-, Fe, Mn, Cu, and Zn）, and high levels of acidophilic sulfur-oxidizing bacteria. However, compared to the control treatment, the addition of the proline solution （1%） significantly improved the water quality of the leachate by significantly reducing the Eh values, the pollutant concentrations （804^2-, Fe^2＋, Fe, Mn, Cu, and Zn）, and the activity of acidophilic sulfur-oxidiz- ing bacteria and by significantly increasing the pH value to neutral. The proline treatment significantly inhibited the oxidation of coal gangue and the release of pollutants, mainly by inhibiting the activity of acidophilic sulfur-ox- idizing bacteria and by altering the heavy metal fractions and the mineralogical characteristics. Therefore, in engi- neering practice, workers should consider using an envi- ronmental friendly aqueous proline solution cover to achieve the in-situ control of pollutant releases from coal gangue dumps.

Coal/Gangue Volume Estimation with Convolutional Neural Network and Separation Based on Predicted Volume and Weight

In the coal mining industry,the gangue separation phase imposes a key challenge due to the high visual similaritybetween coal and gangue.Recently,separation methods have become more intelligent and efficient,using newtechnologies and applying different features for recognition.One such method exploits the difference in substancedensity,leading to excellent coal/gangue recognition.Therefore,this study uses density differences to distinguishcoal from gangue by performing volume prediction on the samples.Our training samples maintain a record of3-side images as input,volume,and weight as the ground truth for the classification.The prediction process relieson a Convolutional neural network(CGVP-CNN)model that receives an input of a 3-side image and then extractsthe needed features to estimate an approximation for the volume.The classification was comparatively performedvia ten different classifiers,namely,K-Nearest Neighbors(KNN),Linear Support Vector Machines(Linear SVM),Radial Basis Function(RBF)SVM,Gaussian Process,Decision Tree,Random Forest,Multi-Layer Perceptron(MLP),Adaptive Boosting(AdaBosst),Naive Bayes,and Quadratic Discriminant Analysis(QDA).After severalexperiments on testing and training data,results yield a classification accuracy of 100%,92%,95%,96%,100%,100%,100%,96%,81%,and 92%,respectively.The test reveals the best timing with KNN,which maintained anaccuracy level of 100%.Assessing themodel generalization capability to newdata is essential to ensure the efficiencyof the model,so by applying a cross-validation experiment,the model generalization was measured.The useddataset was isolated based on the volume values to ensure the model generalization not only on new images of thesame volume but with a volume outside the trained range.Then,the predicted volume values were passed to theclassifiers group,where classification reported accuracy was found to be(100%,100%,100%,98%,88%,87%,100%,87%,97%,100%),respectively.Although obtaining a classification with high accuracy is the main motive,this workhas a remarkable reduction in the data preprocessing time compared to related works.The CGVP-CNN modelmanaged to reduce the data preprocessing time of previous works to 0.017 s while maintaining high classificationaccuracy using the estimated volume value.

Fabrication of pollution-free coal gangue-based catalytic material utilizing ferrous chloride as activator for efficient peroxymonosulfate activation

Novel coal gangue-based persulfate catalyst(CG-FeCl_(2))was successfully synthesized by the means of calcinating under nitrogen atmosphere with the addition of ferrous chloride tetrahydrate(FeCl_(2)·_(4)H_(2)O).The phase transformation of the prepared materials and gas products during the heating process are thoroughly investigated.It is suggested that ferrous chloride participated in the phase transformation and formed Si-O-Fe bonds.And the main gaseous products are H_(2)O,H_(2),and HCl during the heating process.Besides,the ability of CG-FeCl_(2) to activate peroxymonosulfate(PMS)for catalytic degradation of polycyclic aromatic hydrocarbons(PAHs)and phenol was deeply studied.More than 95%of naphthyl,phenanthrene and phenol were removed under optimizied conditions.In addition,1O_(2),·OH,and SO_(4)·−were involved in the CG-FeCl_(2)/PMS system from the free radical scavenging experiment,where 1O_(2) played a major role during the oxidation process.Furthermore,CG-FeCl_(2)/PMS system exhibited superior stability in a relatively wide pH range and the presence of common anion from related degradation experiments.Overall,the novel CG-FeCl_(2) is an efficient and environmentally friendly catalyst,displaying potential application prospect in the field of PAHs and phenol-contaminated wastewater treatment.

Failure mechanisms and destruction characteristics of cemented coal gangue backfill under compression effect of non-uniform load

Backfill mining is one of the most important technical means for controlling strata movement and reducing surface subsidence and environmental damage during exploitation of underground coal resources. Ensuring the stability of the backfill bodies is the primary prerequisite for maintaining the safety of the backfilling working face, and the loading characteristics of backfill are closely related to the deformation and subsidence of the roof. Elastic thin plate model was used to explore the non-uniform subsidence law of the roof, and then the non-uniform distribution characteristics of backfill bodies’ load were revealed. Through a self-developed non-uniform loading device combined with acoustic emission (AE) and digital image correlation (DIC) monitoring technology, the synergistic dynamic evolution law of the bearing capacity, apparent crack, and internal fracture of cemented coal gangue backfills (CCGBs) under loads with different degrees of non-uniformity was deeply explored. The results showed that: 1) The uniaxial compressive strength (UCS) of CCGB increased and then decreased with an increase in the degree of non-uniformity of load (DNL). About 40% of DNL was the inflection point of DNL-UCS curve and when DNL exceeded 40%, the strength decreased in a cliff-like manner;2) A positive correlation was observed between the AE ringing count and UCS during the loading process of the specimen, which was manifested by a higher AE ringing count of the high-strength specimen. 3) Shear cracks gradually increased and failure mode of specimens gradually changed from “X” type dominated by tension cracks to inverted “Y” type dominated by shear cracks with an increase in DNL, and the crack opening displacement at the peak stress decreased and then increased. The crack opening displacement at 40% of the DNL was the smallest. This was consistent with the judgment of crack size based on the AE b-value, i. e., it showed the typical characteristics of “small b-value-large crack and large b-value-small crack”. The research results are of significance for preventing the instability and failure of backfill.

Direct-impact of sieving coal and gangue

Gangue from underground separation of coal can directly be used for filling mined out areas, saving transport capacity and reducing the amount of waste polluting the environment above the ground. We introduced a structure and operating principle of an underground direct-impact sieving device by which a separation experiment was carried out. By means of high speed conveyer belts, coal and gangue impacted the breaking board at high speeds ranging from 6 to 14 m/s. Given the differences of hardness between coal and gangue, after selective crushing, the gangue with the higher hardness was crushed less and coal with lower hardness crushed more, which could be separated by a 50 mm sieving plate. The material above the sieving plate was disposed of as gangue and the material below as coal. The results indicate that the crush ratio below the 50 mm sieving plate increases linearly with an increase in impact velocity and decays exponentially with an increase in hardness. Employing this equipment to separate coal and gangue, the hardness of coal f should be <2. This separation device provides relatively good effect in separating coal and gangue with a relatively wide difference of hardness.

Particle size distribution of coal and gangue after impact-crush separation

Based on the separation and backfilling system of coal and gangue, the mineral material impact experiments were conducted utilizing the hardness difference between coal and gangue according to the uniaxial compression experiments. The broken coal and gangue particles were collected and screened by different size meshes. The particle size distributions of coal and gangue under different impact velocities were researched according to the Rosin-Rammler distribution. The relationships between separation indicators and impact velocities were discussed. It is found from experiments that there is a fully broken boundary of coal material. The experimental results indicate that the Rosin-Rammler distribution could accurately describe the particle size distribution of broken coal and gangue under different impact velocities, and there is a minimum overlap region when the impact velocity is 12.10 m/s which leads to the minimum mixed degree of coal and gangue, and consequently the benefit of coal and gangue separation.

Utilization of Coal Gangue and Copper Tailings as Clay for Cement Clinker Calcinations

In order to avoid environmental pollution from Coal gangue （CG） and copper tailings （CT）, the utilization as cement clinker calcinations was experimentally investigated. Low-calcium limestone was also selected as another raw material. The clinker component and microstructure were analyzed by XRD and SEM. The experimental results showed that qualified cement clinker could be generated by substituting CG and CT compound for clay. While mixed with high-calcium limestone and low-calcium limestone, the calcinations temperature were 50 ℃ or 100 ℃ lower than that of clay. CT and CG contain oxygen-rich minerals and potential of geological rock energy. The energy of CG performs functions and drops down sintering temperature. The calcination time was shortened and the clinker sintering coal consumption reduced while substituting CG and CT for clay, and also served the reutilization of low-calcium limestone, CG and CT.

Effect of Coal Gangue with Different Kaolin Contents on Compressive Strength and Pore Size of Blended Cement Paste

The effects of activated coal gangue on compressive strength, porosity and pore size distribution of hardened cement pastes were investigated. Activated coal gangue with two different kaolin contents, one higher and one lower, were used to partially replace Portland cement at 0%, 10%, and 30% by weight. The water to binder ratio（w/b） of 0.5 was used for all the blended cement paste mixes. Experimental results indicate that the blended cement of activated coal gangue mortar with higher kaolin mineral content has a higher compressive strength than that with lower kaolin mineral content. The porosity and pore size of blended cement mortar were significantly affected by the replacement of activated coal gangue.

Structure and Pozzolanic Activity of Calcined Coal Gangue during the Process of Mechanical Activation

On the basis of analyzing coal gangue＇s chemical and mineral compositions, the structure change of coal gangue during the mechanical activation was investigated by XRD, FTIR, NMR, and the mechanical strength of the cement doped coal gangue with various specific surface area was tested. The experimental results indicate that, the lattice structure of metakaolin in coal gangue samples calcined at 700 ℃ disorganizes gradually and becomes disordered, and the lattice structure of α-quartz is distorted slightly. The pozzolanic activity of the coal gangue increases obviously with its structural disorganization.

Character of the Si and Al Phases in Coal Gangue and Its Ash

Analysis of the Si and AI phases in coal gangue fuel and its ash is important for use of coal gangue ashes. A comprehensive study by theoretical and experimental analyses with differential thermal analysis, X-ray diffraction and Infrared Spectroscopy has been made in the present article to explore the diagram of the Si and Al phases in coal gangue fuel and its ashes. It is found that kaolinite and quartz are the main phases in coal gangue fuel. The ratio of moles A1203 to SiO2 （i.e., Al2O3 （mole） / SiO2 （mole）） is usually no more than 0.5 in most coal gangue fuel and its ashes. The kaolinit at about 984℃ releases a large quantity of SiO2, which makes calcine coal gangue more active than coal gangue itself. The relationship between the ratio A1203 （mole）]SiO2（mole）and the components of coal gangue ash is analyzed, resulting in a formula to calculate the quantity of each phase. Applying the formula to the testing samples from an electric plant in north China supports the above conclusions.

Synthesis and Thermal Insulation Performance of Silica Aerogel from Recycled Coal Gangue by Means of Ambient Pressure Drying

Silica aerogel materials are well recognized for their superinsulation performance and are regarded as one of the hot candidates to revolutionize building insulation. To date, high production cost related to exorbitant precursors as well as cumbrous multi-step hydrophobization process has often narrowed the field of applications. In this work, granular silica aerogel materials were synthesized by extracting Si O2 from recycled rich silicon coal gangue, followed by one-step hydrophobization and ambient pressure drying. Lightweight(about 0.16 g/cm3) and nanostructural aerogels were obtained through this route. They exhibit a 3D open porous microstructure with around 600 cm2/g surface area and 20 nm of the average pore diameter, thermal conductivity of 4-5 mm packed granules is 20-25 m W/(m·K), which was proved by both guarded hot plate and hot-wire transient methods. This study offers a new facile route for the synthesis of silica aerogel from recycled solid waste coal gangue and suggests a method, which may lead to a cost reduction in terms of industrial production.

Conversion of coal gangue into alumina, tobermorite and TiO_2-rich material

A large amount of coal gangue from coal mining and processing is regarded as waste and usually stockpiled directly. In order to recycle the valuable elements from the coal gangue, an integrated process is proposed. The process consists of three steps: 1concentrating alumina from the coal gangue via activation roasting followed by alkali leaching of Si O2 which produces alumina concentrate for alumina extraction by the Bayer process; 2) synthesizing tobermorite whiskers from the filtrated alkali liquo containing silicate via a hydrothermal method and reusing excess caustic liquor; and 3) enriching titanium component from the Baye process residue by sulfuric acid leaching. Alumina concentrate with 69.5% Al_2O_3 and mass ratio of alumina to silica(A/S) of 5.9pure 1.1 nm tobermorite whisker and TiO_2-rich material containing 33% TiO_2 are produced, respectively, with the optimal parameters Besides, the actual alumina digestion ratio of alumina concentrate reaches 80.4% at 270 oC for 40 min in the Bayer process.

Source apportionment of heavy metals in soils around a coal gangue heap with the APCS-MLR and PMF receptor models in Chongqing,southwest China

This study studied the characteristics and source apportionment of heavy metal pollution in the agricultural soil surrounding a gangue coal heap in Chongqing,China by using absolute principal component scores-multiple linear regression(APCSMLR)model and positive matrix factorization(PMF)model.The applicability of the models was compared in the assessment of source apportionment.The results showed that the average contents of Cd,Hg,As,Pb,Cr,Cu,Ni,and Zn in the surface soil were 0.46,0.14,9.66,31.2,127,95.6,76.0,and 158 mg/kg,respectively.Combined with the spatial distribution and correlation analyses,the results of source apportionment were consistent for both the APCSMLR and PMF models.Cd,Hg,As,and Pb were mainly affected by the gangue heap accumulation,with respective contributions of 74.6%,79.4%,69.1%,and 67.2%based on the APCS-MLR model and respective contributions of 69.7%,60.7%,57.4%,and 41.9%based on the PMF model.Ni and Zn were mainly affected by industrial and agricultural activities,while Cr and Cu were mainly affected by natural factors.The results of the source apportionment were approximately consistent between the APCS-MLR and PMF models.The combined application of the two receptor models can make the results of source apportionment more comprehensive,accurate,and reliable.

The Mechanical Properties and Hydration Characteristics of Cement Pastes Containing Added-calcium Coal Gangue

The mechanical properties of several kinds of coal gangue calcined with limestone were Researched so as to find the optimum way of calcinations with limestone. Microstructure and property of hydration process of cement pastes containing added-calcium coal gangue were analyzed by means of scanning electron microscope （SEM） and method of mercury in trusion poremeasurement （MIP）, etc. The experiment can approve those results： when proper amounst of gypsum and fluorite were taken as mineralizers in the course of calcinations of added-calcium coal gangue, activity of coal gangue can be effectively improved. The results of mechanical property and structural characteristic such as hydration process, hydration product and microstructure etc. of cement pastes containing added-calcium coal gangue are consistent.

Catalytic spectrophotometric determination of trace vanadium in fly ash and coal gangue by Triton X-100 enhancing effect

Trace Ⅴ(Ⅴ) catalyzes mightily the decolorization reaction of arsenazo Ⅲ(AsA Ⅲ) by oxidizing with H_2O_2 in a pH 4.0 HAc-NaAc buffer solution, and the addition of TritonX-100 can further increase the sensitivity of the reaction and its catalytic extent is linear withthe content of Ⅴ(Ⅴ). A catalytic spectrophotometric procedure for determining trace Ⅴ(Ⅴ)wasdeveloped. The results show that the maximun absorption of the color solution is at 560 nm and thedetection limit of the method for Ⅴ(Ⅴ) is 0.014 mg·L^(-1). Beer's law is obeyed for Ⅴ(Ⅴ) in therange of 0.00-0.20 mg·L^(-1). The recoveries are 99.0%-104.6%, and the relative standarddeviations (RSD) are 2.7%-3.7%. Combined with ion-exchange resin, the method has been applied to thedetermination of trace vanadium in fly ash and coal gangue with satisfactory results.

Impact of coal gangue on the level of main trace elements in the shallow groundwater of a mine reclamation area

Coal gangue is the most used filling material during reclamation of areas suffering subsidence from min- ing. Main trace element levels (F, As, Hg, and Pb) in shallow groundwater in the reclamation area may be affected by leaching from the gangue. This can has an impact on the application of the water for agricul- tural irrigation or use as drinking water. Therefore, it is of great significance to understand the effect coal gangue has on the shallow groundwater of a reclaimed area. We studied the effect of coal gangue on fluo- rine, arsenic, mercury, and lead levels in the shallow groundwater of a reclamation area by testing the water and the coal gangue. One well near the reclamation area was used as a control well and element levels in water from this well and from the soil next to the well were also measured. The results show that the levels of these elements are increasing in the reclamation area over time. The increase in fluorine, arsenic, mercury, and lead in monitor wells varies from 7.42% to 8.26%, from 7.13% to 7.90%, from 4.85% to 6.48%, and from 4.69% to 6.42%, respectively. Fluorine and arsenic levels are lower in monitor wells than in the control water. The other elements are found in greater concentration than in the control. The Nemerow index also indicates that the shallow groundwater in the reclamation area I is moderately affected by the back-filling coal gangue, while the shallow groundwater in the reclamation area II and III are slightly affected by the back-filling coal gangue. This shallow groundwater could be used for agri- cultural irrigation or for drinking.

Co-combustion of municipal solid waste and coal gangue in a circulating fluidized bed combustor

Mixed incineration of municipal solid waste (MSW) in existing coal gangue power plant is a potentially highefficiency and low-cost MSW disposal way. In this paper, the co-combustion and pollutants emission characteristic of MSW and coal gangue was investigated in a circulating fluidized bed (CFB) combustor. The effect of MSW blend ratio, bed temperature and excess air ratio was detailedly studied. The results show the NOX and HC1 emission increases with the increasing MSW blend ratio and the SO2 emission decreases. With the increase of bed temperature, the CO emission decreases while the NOX and SO2 emission increases. The HC1 emission is nearly stable in the temperature range of 850-950℃. The increase of excess air ratio gradually increases the NOX emission but has no significant effect on the SO2 emission. The HC1 emission firstly increases and then decreases with the increase of excess air ratio. For a typical CFB operating condition with excess air ratio of 1.4, bed temperature of 900℃ and MSW blend ratio of 10%, the original CO, NOX, SO2 and HC1 emissions are 52, 181, 3373 and 58 mg/Nm^3 respectively.

Modification of ash flow properties of coal rich in calcium and iron by coal gangue addition

Flow property of coal ash and slag is an important parameter for slag tapping of entrained flow gasifier.The viscosity of slag with high contents of calcium and iron exhibits the behavior of a crystalline slag,of which viscosity sharply increases when temperature is lowered than temperature of critical viscosity(TCV).The fluctuation in temperature near the TCVcan cause an accumulation of slag inside the gasifier.In order to prevent slag blockage,it is necessary to adjust the ash composition by additive to modify the flow property of coal rich in calcium and iron.Main components of coal gangue are Al_(2)O_(3) and SiO_(2),which is a potential additive to modify the ash flow properties of these coals.In this work,we investigated the ash flow properties of a typical coal rich in calcium and iron by adding coal gangue with different SiO_(2)/Al_(2)O_(3)ratio.The results showed that the ash fusion temperatures(AFTs)firstly decreased,and then increased with increasing amount of coal gangue addition.Chemical composition of coal ash rich in calcium and iron moved from gehlenite primary phase to anorthite,quartz and corundum primary phases.The slags with coal gangue addition behaved as a glassy slag,of which the viscosity gradually increased as temperature decreased.Besides,a high SiO_(2)/Al_(2)O_(3)ratio of coal gangue was beneficial to modify the slag viscosity behavior.Addition of coal gangue with a high SiO_(2)/Al_(2)O_(3)ratio impeded formation of crystalline phases during cooling.This work demonstrated that coal gangue addition was an effective way to improve the ash flow properties of the coal rich in calcium and iron for the entrained flow gasifier.

Activity and Structure of Calcined Coal Gangue

Coal gangue was activated by means of calcination in seven temperature ranges. Systematic research was made about activation mechanism and structural evolution. Glycerin-ethanol method, SEM, M1P and XRD were used to determine the variation of structure and activation of coal gangue during the calcination. The experimental results show that because of heat treatment in the range of calcination temperature, mineral composition and microstructure of coal gangue are changed. In addition, its activity is improved evidently. The amount of lime absorbed by the sample calcined at 700 ℃ is 2-4 times that by uncalcined coal gangue in the course of hydration. When NaOH is added to coal gangue-lime system, hydration reaction of the system is sped up and the microstructure of hydrating samples of coal gangue is improved.