Yaqun He

Recovery of metals from waste printed circuit boards by a mechanical method using a water medium.

Research on the recycling of waste printed circuit boards (PCB) is at the forefront of environmental pollution prevention and resource recycling. To effectively crush waste PCB and to solve the problem of secondary pollution from fugitive odors and dust created during the crushing process, a wet impacting crusher was employed to achieve comminution liberation of the PCB in a water medium. The function of water in the crushing process was analyzed. When using slippery hammerheads, a rotation speed of 1470 rpm, a water flow of 6m(3)/h and a sieve plate aperture of 2.2mm, 95.87% of the crushed product was sized less than 1mm. 94.30% of the metal was in this grade of product. Using smashed material graded -1mm for further research, a Falcon concentrator was used to recover the metal from the waste PCB. Engineering considerations were the liberation degree, the distribution ratio of the metal and a way to simplify the technology. The separation mechanism for fine particles of different densities in a Falcon concentrator was analyzed in detail and the separation process in the segregation and separation zones was deduced. Also, the magnitude of centrifugal acceleration, the back flow water pressure and the feed slurry concentration, any of which might affect separation results, were studied. A recovery model was established using Design-Expert software. Separating waste PCB, crushed to -1mm, with the Falcon separator gave a concentrated product graded 92.36% metal with a recovery of 97.05%. To do this the reverse water pressure was 0.05 MPa, the speed transducer frequency was set at 30 Hz and the feed density was 20 g/l. A flow diagram illustrating the new technique of wet impact crushing followed by separation with a Falcon concentrator is provided. The technique will prevent environmental pollution from waste PCB and allow the effective recovery of resources. Water was used as the medium throughout the whole process.

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.

Fluidization Characteristics of the Dense Gas-Solid Fluidized Bed Separator Based on the Mixed-Medium Solids of Magnetite and Paigeite Powder

Fluidization characteristics of the mixed magnetite and paigeite powder were studied by the combination approach of numerical simulation and experimental verification to optimize the design of the medium solids used in the dense gas-solid fluidized bed separator (DGSFBS). The experimental results showed that the favorable fluidization performance could be achieved with the weight proportion of paigeite powder in the mixture 25% ≤ α ≤ 35%, while the relative deviation in the bed density should be Δρ ≤ 0.025 g/cm3. The corresponding simulation results were obtained by selecting suitable mathematical models and parameters. It also indicated that the fluidized bed with mixed solids could provide a good fluidization performance and stable density distribution. The laboratory-scale DGSFBS was used to conduct the separation experiments on raw coal with size range of 13–6 mm and ash content of 33.43%. With the parameter α = 30%, the experiment resulted in the coal ash content being reduced from 33.43% to 11.63% with a probable error, E P of 0.07 and a recovery efficiency of 98.59%. Utilization of either magnetite or paigeite powder alone was not effective; however, fluidization performance and separation efficiency of DGSFBS improved with the utilization of mixed solids.

Separation of metal laden waste using pulsating air dry material separator

Separation of metal laden solid wastes for their recycling utilization using passive pulsed air and active pulsing air classifiers was studied. Laboratory investigation showed that the active pulsing air separator performs more efficiently than the passive pulsed air separator due to the ability to accurately control operating parameters. By studying the difference of drag coefficients of the particles moving through the airflow of varying Reynolds numbers, models of the dynamic particle motion were developed and a computer simulation was prepared. Results of the simulation were reported to predict the observed results with artificial tracing spheres being separated by the laboratory equipment. Two different, real world feed materials were separated with the laboratory scale active pulsing air classifier. The discarded catalyst consisting of precious metal components and sintered magnetic beads was separated with the separation efficiency, of 97.6 %. The second real-world feed, electronic scrap crushed to a size of 0.5 to 2 mm, showed a separation efficiency of 92.41 %. At the same time, the grade of the recovered concentrate of metals was above 98 %.

Triboelectric separation technology for removing inorganics from non-metallic fraction of waste printed circuit boards: Influence of size fraction and process optimization.

Removing inorganics from non-metallic fraction (NMF) of waste printed circuit boards (WPCBs) is an effective mean to improve its usability. The effect of size fraction on the triboelectric separation of NMF of WPCBs was investigated in a lab triboelectric separation system and the separation process was optimized in this paper. The elements distribution in raw NMF collected from typical WPCBs recycling plant and each size fraction obtained by sieving were analyzed by X-ray fluorescence (XRF). The results show that the main inorganic elements in NMF are P, Ba, Mn, Sb, Ti, Pb, Zn, Sn, Mg, Fe, Ca, Cu, Al and Si. The inorganic content of each size fraction increased with the size decreasing. The metal elements are mainly distributed in -0.2mm size fraction, and concentrated in middle product of triboelectric separation. The loss on ignition (LOI) of positive product and negative product is higher than that of the middle product for the -0.355mm size fraction, while the LOI presents gradually increasing trend from negative to positive plate for the +0.355mm size fraction. Based on the separation results and mineralogical characterizations of each size fraction of NMF, the pretreatment process including several mineral processing operations was added before triboelectric separation and better separation result was obtained.

Experimental and Numerical Simulation of Spent Catalyst Separation in an Active Pulsing Air Classifier

Research on recycling spent catalyst is at the forefront of preventing environmental pollution and recycling resources. An active pulsing air classifier is applied to recover the valuable concentrate from spent catalyst. A multi-factor separation experiment was designed using Box-Behnken module of Design-Expert software, the results show that the separation efficiency ranged from 96.26 to 99.29%. When the airflow velocity is 8.06 m/s, pulsing frequency is 3.55 Hz, feed frequency is 35 Hz, and thus 99.29% separation efficiency can be achieved. A sharp separation is achieved with a wider performance range of operation conditions. A DEM-CFD model is developed to study the gas-solid flow in a lab-scale active pulsing air classifier. In the model, the motion of particles is obtained using the Discrete Element Method facilitated with the concept of “particle-particle,” while the gas flow by Computational Fluid Dynamics based on the local averaged Navier-Stokes equations. The simulated results agree reasonably well with the experimental observation. It will provide a cost-effective way to study the gas-solid flow in the larger-scale active pulsing air classifier with different geometrical, operational and material conditions.

Hydrodynamic characteristics of the dense medium gas–solid fluidized bed for coal beneficiation and cleaning

ABSTRACT The hydrodynamic characteristics of the dense medium gas–solid (DMGS) fluidized bed for coal beneficiation and cleaning were analyzed and discussed by the experimental measurement and numerical simulation. The magnetite powder with a wide particle size of 0.3–0.15 mm was selected as the major fluidized medium in this study. The simulation and experiment results indicate that the bed pressure drop can reach stable condition in a relatively short period with the increase of superficial gas velocity. Afterwards, the bed pressure drop shows minor fluctuations within the gas velocity of 1.8Umf ≤ U ≤ 2.2Umf. Meanwhile, the bed density shows good homogeneity at various bed heights and axial directions with slight density fluctuation. Therefore, the results clearly demonstrate that the hydrodynamic characteristics of the fluidized bed, including the bed pressure drop and separating density distribution, overall maintain stable and uniform conditions without intense fluctuations. It provides favorable fluidization and separation conditions for coal beneficiation and cleaning in the three-dimensional spaces of DMGS fluidized bed.

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.