The effect of diameter and height of the inserted rod in a dense medium cyclone to suppress air core

Studies were carried out by suppressing the air core in a 100 mm DMC using a metal rod inserted through the spigot. The effect of the diameter of the inserted rod and its height of insertion on the Ep of the cyclone was studied. A spigot blocking ratio of 0.4 with 60% of free VF clearance gave the best separation efficiency. Multiphase CFD studies showed that the improved performance of the cyclone was because of (i) higher tangential velocity and hence higher centrifugal force acting on the particles (ii) lower radial and axial velocities at the VF reducing misplacement.     

Fundamentals on the spigot capacity of dense medium cyclones

The capacity of dense medium cyclones is often restricted by the solids-carrying capacity of the underflow, referred to as spigot capacity. Cyclone manufacturers normally recommend the spigot capacity for a cyclone of a particular size; however, it is not clear how these capacities are determined and whether they can be increased. In the literature, spigot capacity has previously been associated with roping flow at the underflow, although this notion has neither been clearly proven nor disproved. Furthermore, the effect of overloading the spigot on the operation of a dense medium cyclone has not been adequately studied.In this study, test-work on a 165 mm diameter dense medium cyclone was performed to investigate its spigot overloading behaviour. This work established clearly that spigot capacity was reached at the onset of roping flow, and that there was a critical underflow ore concentration at which roping/spigot overloading occurred. This ore concentration was shown to be a useful tool in anticipating and avoiding spigot overloading. Spigot capacities obtained in this study were higher than those specified in the DSM handbook, indicating that dense medium cyclone could possibly be operated at higher spigot capacities. However, separation efficiencies were not monitored during this investigation.     

Using a high pressure hydrocyclone for solids classification in the submicron range

A 10 mm hydrocyclone was operated using a barite suspension with a maximum particle size of d^max = 7 μm. The test rig was equipped with a piston diaphragm pump for pressures up to 60 bar. At 40 bar and 20 °C, cut sizes d₅₀ were obtained down to 0.7 μm; increasing the temperature to 50 °C resulted in d₅₀ values down to 0.5 μm for a throughput of 0.6 m³/h. Another experiment was conducted at 40 bar using a batch hydrocyclone technology. Only the overflow was recirculated to the feed box, whereas the underflow was discharged via a collection box. Increasing the number of recirculations increased the separation of fines in the submicron range. The results showed that after 20 min particles with d^max = 1 μm were obtained in the hydrocyclone overflow. After 120 min, the particles size distribution had a d^max = 0.5 μm and a mean size of d⁵⁰ = 0.2 μm. This procedure requires high energy consumption and is thus suitable only for fractionating small quantities of particles in the submicron range.     

CFD simulation and experimental validation studies on hydrocyclone

Hydrocyclone is a key unit operation in mineral process industry and simulation of which using CFD techniques is gaining popularity in process design and optimization. The success of the simulation methodology depends primarily on how best the results are matching with the experimental values and the computational time it requires for obtaining such results. In the present investigation, attempts are made to develop a methodology for simulating the performance of hydrocyclone. Initial work included comparison of experimental and simulated results generated using different turbulence models i.e., standard k–ε, k–ε RNG and RSM in terms of water throughput and split with the help of suitably designed experiments. Among the three modeling methods, predictions using RSM model were found better in agreement with experimental results with a marginal error between 4% and 8%. Parametric studies have indicated that a decrease in the spigot opening increased the upward vertical velocity of water more compared to a decrease in the downward vertical velocity. An increase in the inlet pressure has increased the axial velocities of water in both the upward and downward directions and increased the mass flow rates through the cyclone. An increase in the inlet pressure has also increased the static pressure differential along the radius within the cyclone body and hence more water split into overflow. Further, an increase in the inlet pressure has also increased the tangential velocities and reduced the cyclone cut size. The simulated particle distribution values generated using the particle injection technique are found matching with the experimental results while achieving cut sizes between 4.9 and 14.0 μm.     

Characterizing hydrocyclone performance for grit removal from wastewater treatment activated sludge plants

Typically, 15–45% of the mixed liquor (sludge) in biological wastewater treatment plants (WWTPs) consists of inorganic (fixed) suspended solids. A portion of these inorganic compounds is grit (sand) originating from the influent. Grit accumulation impacts WWTP design and operating costs as these unbiodegradable solids reduce the effective treatment capacity of the bioreactor and other unit operations that must be sized to carry this material.The goal of this study was to characterize the performance of a hydrocyclone to selectively separate grit from activated sludge. Laboratory experiments were conducted with a 13 mm diameter Krebs hydrocyclone treating sludge from eight WWTPs. Reduced efficiencies of 17 ± 7% on fixed suspended solids and 9 ± 6% on volatile suspended solids were obtained. Grade efficiency curves enabled the development of a modified definition for cut size useful for this application. The characterization of hydrocyclone performance for grit removal from activated sludge will enable modelling of the process for integration into wastewater treatment simulators used for process performance prediction and design.     

Flow conditions in the air core of the hydrocyclone

The geometry and movement of the air core are sensitive indicators of the operational state of hydrocyclones. Therefore, an increased knowledge of the air core behaviour is desirable.Measurements of the air-core diameter in transparent hydrocyclones operated with a suspension of glass spheres are presented. The experiments indicate the dependencies between feed-solids content, air-core diameter, and spray angle of the underflow.Additionally, the air flow inside the air core in a transparent 50 mm water cyclone was experimentally investigated. Visualization techniques show the whirling motion of the air-core flow that is driven by the rotating liquid boundary of the air core. The radial distribution of the tangential air velocity was determined. The mean axial air velocity was shown to be dependent on the pressure drop and the hydrocyclone geometry. At the overflow discharge the air core is dispersed, and forms an aero suspension.Thus, the paper presents new aspects of the complex multi-phase flow in the hydrocyclone considering the gaseous phase.     

Density based separations in the Reflux Classifier with an air–sand dense–medium and vibration

The Reflux Classifier is a device consisting of parallel inclined channels above a fluidized bed. Water-based versions of the system have been successfully employed in industry for gravity separation of ?2 + 0.25 mm coal and mineral matter. In this study an air-fluidized system was investigated using a single 2 m long inclined channel with 100 mm wide channels and 20 mm perpendicular spacing. Sand (?355 + 125 micron) was used as a dense–medium and vibration at two distinct levels was used to improve fluidization stability. Tracer particles of ?6.35 + 1.00 mm nominal diameter and 1300 to 2400 kg/m³ density were used to study the effects of the vibration energy and vibration direction on the separation efficiency. The device was able to separate coal particles from a maximum of 8 mm down to 1 mm on the basis of density; hence this method has potential for industrial application. Results were analysed using a simple 2-parameter dispersion–convection model. Dense particles had negative slip velocities, low-density particles had positive slip velocities and the slip velocities were proportional to particle diameter.     

Formulating and optimising the compressive strength of controlled low-strength materials containing mine tailings by mixture design and response surface methods

Controlled low-strength materials (CLSM), like other cement-based backfill materials, are typically formulated by trial-and-error methods to yield the desired product characteristics. This paper presents the use of mixture design and response surface methods as tools to optimise formulations of CLSM to achieve desirable mechanical integrity with a minimum amount of statistically-sound experiments; while minimising the amount of cement and maximising the amount of by-products used. Statistical combinations of three-component mixtures were formulated to investigate the unconfined compressive strength (UCS) of CLSM comprising: Portland cement, fly ash and mine flotation tailings from a Ni–Cu ore. The data is analysed using the response surface method (using a mixture design of a constrained triangular surface) and ANOVA. Optimum formulations are simulated using a desirability function set at lower (1.0 MPa), target (2.0 MPa) and upper (3.0 MPa) UCS values after 28 days curing. All mix combinations had a constant spread diameter of 229 ± 10 mm, the standard workability for conventional CLSM. Results are compared to conventional CLSM incorporating silica sand in the place of the tailings. A significant quantity of tailings (up to 80 wt% solids) and low quantity of cement (up to 5 wt% solids) produced CLSM with UCS within the 2 MPa target value of excavatability. UCS of CLSM is a function of the individual component proportions, and the mixture design approach can be an important tool to help develop and optimise formulations of cement-based materials consisting of several components.     

An experimental study of size segregation in a batch jig

This paper presents the findings of an experimental investigation into size segregation in a 200 mm diameter batch jig that was conducted to provide background information for the development of a stratification model that accounts for the effects of both particle size and particle density on separation performance. The investigation focused on a simple system in which the only variable was particle size; i.e. binary systems involving 50% mixtures of two differently sized spherical glass beads from 14 mm to 4 mm diameter in 2 mm increments. The density of all beads was 2520 kg/m³. The study revealed four different types of size segregation patterns that may occur in a jig bed, and gave some indication of the factors that determine the transition from one type to another under the specific experimental conditions of the test work carried out. It also developed a conceptual picture of the dynamics affecting size segregation in batch jigs operated under equilibrium conditions and highlighted three mechanisms: the interplay between stratification and dispersive processes; interstitial tricking of smaller particles; and convective remixing of smaller particles in the bed. Interpretation of the findings suggests a compositional regime where one segregation mechanism dominates, i.e. the stratification/dispersion interplay, and that our modelling efforts should concentrate on this mechanism and this regime.     

Volume split control of a hydrocyclone group

Experience obtained with a new hydrocyclone control system is reported. As command variable of the control serves an optical sensor detecting the underflow shape (rope or spray). These signals are transmitted to the computer and the control loop is completed with the pump power input, pressure in the feed and in the overflow. A volume split control of the hydrocyclone is performed by regulation of the overflow using a control valve. Groups of hydrocyclones can be controlled by installing only one control valve in a collecting pipe of all overflows. This control system stabilizes the separation at the transition point between spray and rope discharge at the cyclone underflow which in most cases is the operational optimum. One of the first applications was the regeneration of bentonite suspensions in tunnel driving. Since excavation of the material is discontinuous the solids content of the hydrocyclone feed varies over a wide range. By controlling the 150 mm hydrocyclone group it was possible to compensate for feed fluctuations and to stabilize the separation.     

Boundary conditions for gas rate and bubble size at the pulp–froth interface in flotation equipment

This paper describes the effective boundary conditions for the gas dispersion parameters of bubble size, superficial gas velocity and bubble surface area flux, in mechanical and column flotation cells. Using a number of previously derived correlations, with appropriate simplifying assumptions, and experimental data reported from plant practices, the boundary conditions were identified. Thus, it was shown that these constraints typically allow for a mean bubble diameter range of d_b = 1–1.5 mm and superficial gas rate of J_g = 1–2 cm/s, in order to maximize the bubble surface area flux, S_b = 50–100 s⁻¹. Under these conditions there is no carrying capacity limitation, while keeping a distinctive pulp–froth interface.     

Valorization of steel slag by a combined carbonation and granulation treatment

This work reports the results of a combined accelerated carbonation and wet granulation treatment applied to Basic Oxygen Furnace (BOF) steel slag with the aim of producing secondary aggregates for civil engineering applications and of storing CO₂ in a solid and thermodynamically stable form. The tests were carried out in a laboratory scale granulation device equipped with a lid and CO₂ feeding system. In each test, humidified slag (liquid/solid ratio of 0.12 l/kg) was treated for reaction times varying between 30 and 120 min under either atmospheric air or 100% CO₂. Under both conditions, the particle size of the treatment product was observed to increase progressively with reaction time; specifically, the d₅₀ values obtained for the products of the combined granulation and carbonation treatment increased from 0.4 mm to 4 mm after 30 min and to 10 mm after 120 min. Significant CO₂ uptake values (between 120 and 144 g CO₂/kg) were measured even after short reaction times for granules with diameters below 10 mm and for the coarser particle size fractions after reaction times of 90 min. The density, mineralogical composition and leaching behavior of the obtained granules were also investigated, showing that the combined granulation–carbonation process may be a promising option for BOF slag valorization, particularly in terms of decreasing the Ca hydroxide content of the slag. Another interesting finding was that the leaching behavior of the product of the combined treatment appeared to be significantly modified with respect to that of the untreated slag only for coarse uncrushed granules, an indication that the carbonation reaction occurs mainly on the outer layer of the formed granules.     

Tangential velocity measurements in a conical hydrocyclone operated with a fibre suspension

The tangential velocity flow field in a conical hydrocyclone was measured using a self-cleaning pitometer. The influence of pulp fibre concentration on the tangential velocity profile was of particular interest.The measurements showed that the pulp fibres had a strong influence on the tangential velocity profile. When operating the hydrocyclone with pure water, the velocity profile showed the typical combination of free-vortex-like rotation close to the hydrocyclone wall and solid-body-like rotation in the proximity of the hydrocyclone axis. This profile changed significantly when pulp fibres were added. At a low fibre concentration (1.2 g/l), the fibres smoothened the transition between the free-vortex-like and the solid-body-like region of the velocity profile. The location of the maximum tangential velocity moved to a larger radius. At higher fibre concentration (7.5 and 11 g/l), the free-vortex-like behaviour in the outer area was virtually suppressed. Due to networking, the fibre suspension in the entire hydrocyclone behaved as a solid body.The radial acceleration profile and tangential rate-of-strain profile were determined. Based on these profiles, a hypothesis was proposed explaining the well-known observation that fractionation efficiency decreases significantly at higher fibre concentrations.     

Using an axial electromagnetic field to improve the separation density of a dense medium cyclone

To produce an on-line control method to improve the separation density of a given suspension density of a dense medium cyclone, a thin solenoid coil was placed in the cylindrical part of a cyclone. The dense medium distribution test and −3 + 0.125-mm coarse slime separation tests for different electric currents were performed. Float-and-sink analysis was performed for the separation products. The magnetic force of the particles under a magnetic field was also simulated. The results indicated that the presence of a magnetic field can improve the separation density by increasing the “separation cone density” caused by the inward radial motion and the upward axial motion of the magnetic particles. This approach provided a new separation density manipulation method for dense medium cyclones via application of a magnetic field.     

Ultrafine coal classification using 150 mm gMax cyclone circuits

A two-stage classification circuit using 150 mm diameter gMax cyclones was installed and evaluated in a coal preparation plant in an effort to achieve a clean coal product without the use of froth flotation. Particle size separations of around 37 μm were achieved while limiting ultrafine bypass to less than 10% in the circuit underflow stream. As a result, approximately 81% of the ash-bearing material in the circuit feed was rejected to the circuit overflow stream. The feed ash content was reduced from around 50% to values in the range of 22–30% in the circuit underflow stream with a mass recovery of about 30%. Further reductions in the coarse product ash content were limited due to the particle density effect and the remaining presence of a significant quantity of high-ash slime material in the coarse product. The typical D₅₀ for the coal particles was 40 μm while the estimated value for mineral matter was 17 μm. Based on the findings of the study, the use of classification to recover a low-ash, coarse fraction in the feed of a fine coal circuit is limited by the density effect regardless of the ability to eliminate ultrafine bypass.     

An empirical power model derived from 3D particle tracking experiments

The kinematic data resulting from 3D particle tracking experiments of a typical bulk charge particle is used to derive an empirical power model that forms the basis for comparison between the DEM and experiment. The model is derived from the center of circulation coordinates and given as a function of mill speed. The wide range of milling configurations coupled with experimental trajectory data accurate to within 0.15 mm in spatial resolution ensure that the comparisons are robust. The power prediction from both the DEM and experiment is done for a 142 mm diameter mill of the same length and charged with 4596 particles with a mean diameter of 6.1 mm.     

Sub-populations and patterns in blast induced fine fragmentation

The understanding of fundamental mechanisms of fragmentation is still being developed. As such the fine fragmentation produced in a blast has been the subject to study. By examining the fines in the context of surface area instead of mass, several patterns and sub-populations have been observed. This paper is a study of fine fragmentation generated in small scale blasts. Each size distribution was transformed from mass into surface area. Each explosive charge was simulated in context of the energy it applied to the sample during breakage. This paper represents a small part of a larger study.The samples blasted in this study produced several aspects of self-similarity in breakage. Samples of the same rock type produced a similar surface area size distribution that was observed at a wide range of applied explosive energies. Each rock type had a characteristic surface area size distribution. Shape characteristics of measured fragments in the √2 sieve series shared similarities across a wide range of size fractions.Also paradoxically, the products of fine fragmentation were found to be multi-component in nature. A change in surface area size distribution was observed at 1 mm, which was found to be statistically significant. The fractal dimension of particles larger than 1 mm were found to be statistically different to particles smaller than 1 mm. Another change in particle character was found at 10 mm fragment size.The generation of fine fragmentation characterisation paradoxically has two fragment size thresholds of note, 10 mm and 1 mm, and is self-similar in nature. It is postulated that fragmentation is not a true fractal but the individual fundamental mechanisms that generate it may well be.     

Gravity separation of coarse particles using the Reflux Classifier

A comprehensive study examining the potential of the Reflux Classifier to be applied to the beneficiation of coarser coal up to 8 mm in size was undertaken. It was demonstrated that efficient combustible recovery and control of the separation density to target low ash products could be achieved. The major finding from the study was the critical importance of providing sufficient fluidization water, though beyond the critical level the process was largely insensitive to the fluidization rate. It was concluded the required fluidization velocity is nominally 10 m/h per mm of top-size, hence for a nominal 4 mm top size the required velocity is 40 m/h. In an extended campaign the control of the process was investigated by varying the set point density from high to low levels and then returning the process to the original settings, and demonstrating a return to the original separation. Further analysis was conducted to determine the partition curves and the shift in the separation density with particle size. The variation in the D₅₀ with particle size approaches a level that is independent of the particle size. Previous data (Galvin et al., 2002, Galvin et al., 2004) covering particles up to 2 mm in size are consistent with the results from this study, involving feeds with top sizes of 4 mm and 8 mm. Beyond a particle size of 2 mm the Ep is typically less than 0.05 and approaches about 0.03 as the particle size increases to 8 mm.     

The effects of chamber diameter and stirrer design on dry horizontal stirred mill performance

This paper focussed on investigating the effects of chamber diameter and stirrer design on cement grinding performance of a horizontal type dry stirred mill. Within the scope, pilot scale test works were undertaken with two different chamber diameters (20.4 cm and 26.4 cm) having the same length and three different stirrer designs (wing, cross and disc) having the same diameter (16 cm). The chamber diameter tests were performed at the same stirrer design, media size and media filling. The studies concluded that, the use of larger chamber improved the grinding efficiency since 31.8% and 35.8% less energy was consumed than the smaller mill at the RR_d50 of 1.41 and 1.66 respectively. This behaviour of the larger mill can be attributed to the increased gap distance between the chamber wall and stirrer edge. With regards to stirrer design, the statistical evaluations, grinding results and temperature measurements all indicated that the disc design of stirrer ground the particles more effectively at high energy levels (>40 kW h/t). The use of the disc design reduced the energy consumption by 21% (at RR_d50 of 3.5). This was attributed to dissipation of energy as heat since the temperature measured for the wing and cross types were higher than the disc type.