Weining Xie

Removing inorganics from nonmetal fraction of waste printed circuit boards by triboelectric separation.

Metals recycling from WPCBs has been studied for a long time, which results in the appearance of many proven techniques. However, the nonmetal fraction in WPCBs has not been fully recycled due to hybridpropertiesof inorganic and organic composition. In order to improve the usability of the nonmetal fraction from WPCBs, nonmetal materials separation by using a laboratory triboelectric separation system was carried out to improve the reuse efficiency of WPCBs nonmetal fraction. The optimum tribocharger material was investigated by using the charge-mass ratio measurement system, and PMMA is the optimum tribocharger material compared with PVC, PPFT, PPR, SS. The effects of airflow, voltage and feed rate on triboelectric separation were investigated. The product LOI of positive plate is up to 77.26% with recovery rate of 25.49%, while the product LOI of negative plate is down to 47.35% with recovery rate of 35.37%, and the remove rate of inorganics is up to 43.02% by triboelectric separation. The analysis results of X-ray diffraction indicate that the main inorganic materials mixed in nonmetal fraction are SiO2, Al2O3, CaO, Cu, Fe, Sn. The X-ray fluorescence analysis shows that the triboelectric separation can effectively remove the content of SiO2 and Al2O3. The scanning electron microscope images show that inorganics tribocharge positively and distribute in product collection grooves that close to negative plate.

Energy-size reduction of coals in the Hardgrove machine

To simulate the grinding process in an industrial vertical spindle mill, the Hardgrove mill was fitted with a power-measuring meter to investigate the breakage characterizations of different narrow size fractions of coal. Power consumption in the grinding process was precisely recorded at the sampling frequency of 1 second with the help of sampling software for power. The experimental results indicated that the energy input for grinding decreased with increases in time for each sample. Changes in the weight percentage in the top-size interval showed that the first-order law was not satisfied to describe the total grinding process. The new produced fine coal had a negative effect on the breakage behavior of coarse particles. But removal of −0.09 mm size can help to increase the energy input for grinding and the generating rate of −0.09 mm also accelerated in turn. The relationship between t10 and energy input can be described by the classical breakage function. Specific energy consumed by raw coal with a small size was higher when subjected to the same value of t10, due to the relatively higher specific surface area of ground coal at the same breakage ratio of 10.

Improving the efficiency of coal triboelectric separation by chemical conditioning

ABSTRACT A study of fine coal beneficiation in a laboratory triboelectric separation system was carried out to provide a fundamental understanding of improving the separation efficiency of thermal coal powder by chemical conditioning. Separation results of untreated and chemically treated fine coal were investigated. The experimental results demonstrated that the charge–mass ratios of clean coal and minerals increased greatly with ethanol serving as the modification chemical. The optimal deash efficiency of 85.46% was obtained when the coal was treated by 2.0 kg/t ethanol. However, excessive dosage of chemical agents had a bad effect on the separation results.

Effect of tribocharger material on the triboelectric characteristics of coal and mineral particles

ABSTRACT Dry triboelectric separation of coal depends on tribocharge difference of the coal and gangue minerals. A suitable tribocharger material which makes the coal and minerals tribocharged of opposite polarity is of great importance for triboelectric separation. In this paper, the composition of coal was analyzed by x-ray diffraction (XRD), and the triboelectric characteristics of coal and other ash-forming minerals were tested using stainless steel (SS), polyvinyl chloride (PVC), pentatricopeptide repeats (PPR), polyfluortetraethylene (PPFT), and polymethyl methacrylate (PMMA) tribochargers in lab triboelectric unit. The charge−mass ratio of coal and mineral particles were presented and evaluated. Infrared spectroscopy was adopted to analyze the different tribocharge properties between clean coal and minerals. The results show that the gangue minerals in coal are mainly pyrite, kaolin, calcite, dolomite, and quartz. The conductive mineral had the lowest chargeability, especially tribocharged with conductive material. Compared with the inorganic surface, the organic surface is easy to lose electrons and charge positively. According to the triboelectric characteristics of coal and minerals, PMMA is more suitable as tribocharger material for triboelectric separation of pulverized coal compared with PVC, PPFT, PPR, and SS. The better separation efficiency of Tai Xi anthracite coal is obtained by using PMMA tribocharger.

Comparison of flotation behavior of middling coal ground by wet-milling with different media

ABSTRACT Breakage could promote liberation of minerals and make recovery of coal with low ash content possible by flotation technology. In the research presented in this paper, middling coal was ground by wet-milling with different media, namely wet-milling iron ball (WMIB) and wet-milling zirconia ceramics (WMZC). As the size-reduction of middling coal is a physical process, Fourier transform infrared spectroscopy (FTIR) analyses of ground products indicate no difference in organic functional groups. Exposure of mineral and cover of hydrolysis reaction products of iron ion on the generated surface of products of WMIB increase the hydrophilicity and worsen the flotation selectivity when compared with products of WMZC. Float-sink tests show that ground products of WMIB have a relatively higher cumulative yield at low density. However, the ash content of flotation concentrate in each size fraction of WMIB is higher than that of WMZC, which is mainly caused by the decrease of hydrophobicity and worse floating selectivity of coal ground by WMIB. Combustible matter recovery (CR) shows an increase with the decrease of particle size, but CR in flotation concentrating on each sized products is similar to products ground by different media.

Separation Performance of Different Particle Sizes Using an Industrial FGX Dry Separator

ABSTRACT Large-scale experiments were conducted using an FGX-3 dry separator to beneficiate a Chinese coal. The separation results indicated that the FGX separator provided ash reduction from 48.7% in the feed to 31.28% in the clean coal with a mass yield of 65.12%. The clean-coal yield increased as the size range decreased from 80-50 mm to 3-0 mm. Coal with particle sizes between 80 mm and 6 mm responded favorably, while the 6-3 mm size range could not be beneficiated effectively, and coal below 3 mm size did not respond to any separation. The calculation of separation efficiencies indicated that the coarser the particle size the lower the separation density and the lower the Ep value. The amount of 1.8 float material in the tailings was less, which meant lower loss of the low-density, low-ash content particles to the tailings.

Effect of Fine Particles on the Breakage Behavior of Coarse Coal in the Hardgrove Mill

abstract It is well known that fine particles have a negative influence on the energy size reduction of other materials in the grinding chamber. But a quantitative evaluation about it is relatively less. In this article, a Hardgrove mill was used to investigate the effect of fine coal on the grinding energy, specific rate of breakage of the top size, and the production of fine coal in -0.074 mm size fraction. A qualitative description of the friction coefficient between coal and grinding media demonstrates that this index drops with more fines added in the particle bed. The decreasing degree of grinding energy increases 2.5 times if added level of fine coal changes from 2.5 g to 10 g. Breakage of the top size at different added levels of fine coal still follows the first-order law. But due to the decrease of grinding energy and the cushion effect of added fine coal, the breakage rate of coarse particles decreases 2% to 25% if compared with the single breakage. Decreases of grinding energy and breakage rate finally result in a lower yield of fines (-0.074 mm). Hence, added fines decrease the grinding energy efficiency and make the breakage time consuming.

Impact of particle density on the classification efficiency of the static air classifier in Vertical Spindle Mill

In order to investigate the impact of density on the classification behavior of particles in the static classifier of Vertical Spindle Mill, the sensitivity of overflow yield to the increase of air amount for narrowly sized pyrite, carborundum, quartz and coal samples were compared in a lab-scale classifier, respectively. Response surface methodology is used to analyze the combined effect of size and density on the classification. Wide size classification was also conducted and results show that both the yield and R90 of overflow increase with the decreasing of density, and the growth of air amount would also lead them to rise. The Whiten’s model was applied to illustrate the influence of density on the sharpness of classification, corrected cut size and fishhook effect. Results show that material with a lower density would have a higher fishhook effect parameter, classification sharpness and corrected cut size. The increase of air amount would result in a more evident fishhook effect for the high density material. Based on the Whiten’s model, a new classification efficiency model with the addition of particle density in various forms was established. This new model could describe the classification efficiency of materials with different density in the identical experiment conditions.

Investigation of the energy-size reduction and mineral liberation of raw coal in the Hardgrove mill

ABSTRACT Raw coal was ground in a Hardgrove mill, and mineral liberation from coal was investigated by float-sink test and electron probe microanalyzer (EPMA). Based on EPMA, associated conditions of mineral were determined. Analyses of energy-size reduction show that energy efficiency decreases with time. For +0.074 mm progenies, yields of particles in −1.5 g.cm−3 decrease with the increase of size and time, and products in +1.8 g.cm−3 show the opposite trend. EPMA of 0.25–0.125 mm, 1.5–1.8 g.cm−3 products ground for 10 min indicates that only part of minerals are liberated. For −0.074 mm products, the yield of particles in −1.5 g.cm−3 decreases by 23.36% in last several minutes, but ash content of +1.8 g.cm−3 also decreases by 13.18%. Since breakage of raw coal in last 9 min does not reduce particle size obviously, and liberation degree cannot be improved dramatically, closed grinding of 10 min in Hardgrove mill is proper to balance the size reduction and mineral liberation.

Mechanism and Fine Coal Beneficiation of a Pulsating Airflow Classifier

ABSTRACT Traditional air classifiers produce binary splits based on the terminal velocity rather than the density of the feed. In this study, a pulsating air with a sine-like shape wave was introduced into a vertical column air classifier to achieve the density dominant separation on −6+3 mm fine coal. A lab-scale separation device was used to investigate the effects of pulsating characteristics (amplitude and frequency) and particles volumetric concentration on pulsating airflow separation in a batch process. The optimum operating conditions of 34.1 cm pulsation amplitude, 2.45 Hz frequency and 500 g feed amount achieved 48.08% separation efficiency, 68.22% combustible matter recovery and produced a clean coal with 17.98% ash (from a feed containing 42.02% ash), 47.73% yield and Ep value of 0.17. The results showed that the pulsating air classifier has a potential for fine coal dry beneficiation. The motion of particles in a pulsating airflow field under a hindered settling condition has been presented.