Gianni Schena

A method for a financially efficient design of cell-based flotation circuits

A procedure is derived that enables to select the configuration and size of complex cell-based flotation circuits without re-grinding. The method is driven by a user-defined financial objective function that incorporates metallurgical efficiency through concentrate sales revenues and elements of both capital and operating costs. The suggested design method is likely to be superior to traditional empirical procedures that aim to maximise metallurgical performance without including in a formalised way the cost side of the circuit. Technically feasible solutions which are too costly compared to their metallurgical results are rejected and the best techno-economic solution is iteratively sought. The method allows the solution of green field problems and the optimisation of operating circuits. An example application is given to illustrate the innovative method.

Grade of fine composite mineral particles by dual-energy X-ray radiography

Abstract Standard X-ray radiographs of a composite particle made up by two mineralogical species with different linear attenuation coefficients produce 2D attenuation images that do not allow differentiating between a thin high stopping power and a thick low stopping power-mineralogical inclusion. Dual-energy (DE) imaging—i.e., a pair of radiographs taken at different incident X-ray energy—followed by digital image subtraction allows transforming the log processed energy images into two thickness images. This linear transformation permits the estimation of the particle volumetric content in the two phases and it is fast, as required for the development of an online ‘mineral liberation’ or a ‘washability’ sensor or a ‘valuable-phase-inclusion’ detector. In this paper, micro-CT was used to reconstruct fine particle slices so that the thickness of the two phases could be precisely calculated and compared with that one derived by DE. Preliminary experiments with monochromatic X-ray demonstrated the potential of the DE method to retrieve the particle grade. The tests were conducted with fine composite particles made up of two mineralogical species. Due to the nonhomogeneous composition and density variation within the two mineralogical phases segmented and also to partial volume effects in the tomographic reconstruction of particle slices, each of the two segmented phases have a Gaussian distribution of the linear attenuation coefficient μ (rather that one single value) and values of the σ ( μ ) such that the density distribution right tail of the species with the lower μ overlaps the left tail of the other species giving rise to a bimodal distribution of μ within the particle. This material feature adds further complexity to the problem.

Column flotation circuits in Chilean copper concentrators

Abstract This paper reviews the column flotation practice in the largest Chilean copper-moly concentrators where this process unit has been increasingly chosen to replace multi stage cleaning circuits in new and existing operations. Process flowsheets incorporating columns are presented for selected concentrators. Procedures for scaling up, and process control are also illustrated.

Modelling of screening operations

Abstract This paper reviews the Ferrara-Preti screening model and shows that recent laboratory and pilot plant data validate this approach. The model successfully predicts the partition curve (selectivity function) and therefore the product size distributions, given the feed size distribution. Some improvement in the application of the model, concerning numerical solution of the equations and parameter estimation, are indicated. Information is then given on the influence of some operating conditions on the screen performance studied through the modification of the model parameters, a way that could be useful for a more complete approach to modelling of screening operations. Some applications of the model to improvement of screening operations, sizing of screens, simulation of screening and of integrated plant operations are discussed.

A general expression for the global recovery of a chain of reactors

We present a generalization of the well-known formula for the total recovery of a two-stage reactor with recycling that allows the reduction of chain based systems to one single equivalent reactor. Complex flotation and separation networks can be reduced to simple systems and the total recovery directly calculated.

A stereologically posed mass balance for calculating the distributed efficiency of particle separation systems

Abstract Size and composition specific — i.e., distributed — efficiency of particle separation systems is calculated by exploiting measurements of particle composition in low dimension — i.e., areal grade. A stereologically based framework for the mass balance around the separator was set out and an original regularization criterium was used for its solution. It allows retrieving the distribution of volumetric grade of the streams entering and exiting the separator from the respective areal grade distributions, and makes possible the calculation of the distributed efficiency. Synthetic data are used to demonstrate the method proposed.

An assessment of the efficiency of a stereological correction for recovering the volumetric grade of particles from measures on polished sections

An application of the kernel-based inversion method for recovering the volumetric grade distribution of an assembly of particles from computer generated areal grade measures is presented. The application involves: 1. generation of a spatial mineral texture of a mineralized rock sample, Ž . 2. derivation of the volumetric liberation spectrum of the particulate resulting from the simulation of the sample fragmentation, 3. calculation of the inversion kernel based on measures of areal grade made on the intersection of random testing planes with the particles generated, 4. generation of areal measures on a virtual polished section, 5. solving the inversion problem for comparison of the recovered volumetric distribuŽ . tion with the original simulated liberation spectrum. The whole process allows to evaluate the efficiency of the inversion method achieved with an exact kernel and the suitability of the mathematical methods used for the Ž . inversion and their statistical significance; a plain by number of occurrences kernel and

Analysis of 3D computed tomographic imaging of ground-wall insulation for AC rotating machines

The X-ray Computed Tomography (X-CT) has been used to reconstruct the internal features of two types of ground-wall insulation for ac rotating machines. Four samples whose insulation was designed for a rated voltage of 11 kV, were extracted from different coils and scanned using X-CT. The 3D virtual volumes have been visually inspected stacking the tomographic slices to identify the internal morphological features that is, the different materials in the inner structure, the presence of minute voids, gas bubbles, anomalous densities, wrinkled or damaged tape layers. The tomographic volumes have been segmented into different gray-levels resorting to luminosity histograms with the purpose to differentiate the structural materials and extract only specific anomalies. In particular, the binary segmentation has been adopted to investigate the distributions of the void enclosures. The use of X-CT to investigate the internal structure of complex insulation systems, opens significant perspectives in improving the insulation manufacturing quality.

Multiple-energy X-ray radiography and digital subtraction for a particle-composition sensor

Composite mineral particles in a monolayer were X-ray radiographed with monochromatic beams at different incident energies. The digital radiographic images were subtracted according to the dual-energy (DE) principle, which is extended to multiple-energy (ME) for the purpose of this work. The subtraction allows the selective derivation of two images thicknesses, one for each mineralogical phase component. The integration of the thickness gives the volumetric content in the phase. Preliminary experimental results were compared to the grade obtained by micro-CT, showing good agreement. The possible developments related to the fast DE evaluation of particle grades are discussed.

Characterisation of water-oil drainage process through a reservoir rock sample by digital core analysis

Oil-water relative permeability and capillary pressure of a water-wet digital core are computed using the finite-volume/volume of fluid method and mimicking traditional laboratory steady-state experiments carried out on a water saturated sample. Commercial software is used for immiscible multiphase fluid flow simulations, carried out on a low-cost multicore workstation, and the porescale data are upscaled to derive relative permeability and capillary pressure curves as function of water saturation. The digital sample is obtained by high resolution X-ray computed tomography scanning.