M. Teresa Carvalho

Control of a flotation column using fuzzy logic inference

This paper describes a fuzzy controller that was developed for the control, in real time, of three main variables of the column flotation process. Column flotation is a solid-solid separation process based on the physical and chemical properties of mineral particle surfaces. It is applied to the concentration of low grade ores and to ores that require very fine grinding to achieve mineral liberation. The economic exploitation of the process, guaranteeing the desired final product characteristics from a feed of varying composition, requires a stable operation, which is accomplished through the automatic control of some key process variables. The difficulties encountered in the development of process controllers for this process are related to the lack of reliable measuring devices for the process variables involved, with the non-existence of mathematical dynamical models that can be used in a controller synthesis and with its non-linear and time-varying behaviour. The process and the problems related with its control are briefly explained and the main reasons for fuzzy logic control application are discussed. The real time control system and the fuzzy controller structure and parameters are presented. A general overview of the pilot plant where the experimental work was performed is presented. Finally, results of the experimental work, carried out with the two-phase system (air and water), are summarised and the conclusions are drawn.

Separation of packaging plastics by froth flotation in a continuous pilot plant

The objective of the research was to apply froth flotation to separate post-consumer PET (Polyethylene Terephthalate) from other packaging plastics with similar density, in a continuously operated pilot plant. A representative sample composed of 85% PET, 2.5% PVC (Polyvinyl Chloride) and 11.9% PS (Polystyrene) was subjected to a combination of alkaline treatment and surfactant adsorption followed by froth flotation. A mineral processing pilot plant, owned by a Portuguese mining company, was adapted for this purpose. The experimentation showed that it is possible to produce an almost pure concentrate of PET, containing 83% of the PET in feed, in a single bank of mechanical flotation cells. The concentrate grade attained was 97.2% PET, 1.1% PVC and 1.1% PS. By simulation it was shown that the Portuguese recycling industry specifications can be attained if one cleaning and one scavenger stages are added to the circuit.

Optimization of wet shaking table process using response surface methodology applied to the separation of copper and aluminum from the fine fraction of shredder ELVs

With the purpose of reducing the waste generated by end-of-life vehicles (ELVs) by enhancing the recovery and recycling of nonferrous metals, an experimental study was conducted with the finest size fraction of nonferrous stream produced at an ELV shredder plant. The aim of this work was to characterize the nonferrous stream and to evaluate the efficiency of a gravity concentration process in separating light and heavy nonferrous metal particles that could be easily integrated in a ELV shredder plant (in this case study the separation explicitly addressed copper and aluminum separation). The characterization of a sample of the 0-10mm particle size fraction showed a mixture of nonferrous metals with a certain degree of impurity due to the present of contaminants such as plastics. The majority of the particles exhibited a wire shape, preventing an efficient separation of materials without prior fragmentation. The gravity concentration process selected for this study was the wet shaking table and three operating parameters of the equipment were manipulated. A full factorial design in combination with a central composite design was employed to model metals recovery. Two second order polynomial equations were successfully fitted to describe the process and predict the recovery of copper and aluminum in Cu concentrate under the conditions of the present study. The optimum conditions were determined to be 11.1° of inclination, 2.8L/min of feed water flow and 4.9L/min of wash water flow. All three final products of the wet shaking table had a content higher than 90% in relation to one of the metals, wherein a Cu concentrate product was obtained with a Cu content of 96%, and 78% of Cu recovery and 2% of Al recovery.

Separation of plastics: The importance of kinetics knowledge in the evaluation of froth flotation.

Froth flotation is a promising technique to separate polymers of similar density. The present paper shows the need for performing kinetic tests to evaluate and optimize the process. In the experimental study, batch flotation tests were performed on samples of ABS and PS. The floated product was collected at increasing flotation time. Two variables were selected for modification: the concentration of the depressor (tannic acid) and airflow rate. The former is associated with the chemistry of the process and the latter with the transport of particles. It was shown that, like mineral flotation, plastics flotation can be adequately assumed as a first order rate process. The results of the kinetic tests showed that the kinetic parameters change with the operating conditions. When the depressing action is weak and the airflow rate is low, the kinetic is fast. Otherwise, the kinetic is slow and a variable percentage of the plastics never floats. Concomitantly, the time at which the maximum difference in the recovery of the plastics in the floated product is attained changes with the operating conditions. The prediction of flotation results, process evaluation and comparisons should be done considering the process kinetics.

Recovery of glass from the inert fraction refused by MBT plants in a pilot plant

Selective collection is a common practice in many countries. However, even in some of those countries there are recyclable materials, like packaging glass, erroneously deposited in the Mixed Municipal Solid Waste (MMSW). In the present paper, a solution is proposed to recover glass from the inert reject of Mechanical and Biological Treatment (MBT) plants treating MMSW aiming at its recycling. The inert reject of MBT (MBTr) plants is characterized by its small particle size and high heterogeneity. The study was made with three real samples of diverse characteristics superimposed mainly by the different upstream MBT. One of the samples (VN) had a high content in organics (approximately 50%) and a particle size smaller than 16 mm. The other two were coarser and exhibited similar particle size distribution but one (RE) was rich in glass (almost 70%) while the other (SD) contained about 40% in glass. A flowsheet was developed integrating drying, to eliminate moisture related with organic matter contamination; magnetic separation, to separate remaining small ferrous particles; vacuum suction, to eliminate light materials; screening, to eliminate the finer fraction that has a insignificant content in glass, and to classify the >6mm fraction in 6-16 mm and >16 mm fractions to be processed separately; separation by particle shape, in the RecGlass equipment specifically designed to eliminate stones; and optical sorting, to eliminate opaque materials. A pilot plant was built and the tests were conducted with the three samples separately. With all samples, it was possible to attain approximately 99% content in glass in the glass products, but the recovery of glass was related with the feed particle size. The finer the feed was, the lower the percentage of glass recovered in the glass product. The results show that each one of the separation processes was needed for product enrichment. The organic matter recovered in the glass product was high, ranging from 0.76% to 1.13%, showing that drying was not sufficient in the tests but that it is a key process for the success of the operation.

Supervisor fuzzy controller of a flotation column

Abstract This paper describes a supervisor fuzzy controller of a pilot flotation column. The flotation column processes a clay composed mainly of kaolinite and quartz, reducing the iron grade of the final product to be used in the preparation of ceramic pastes. The main control objective (top level control) is to keep the iron content in the underflow stream close to target specification. At the intermediate level, three process variables are controlled by fuzzy logic inference: collection zone height, air holdup in the collection zone and bias water flow rate. The lower level of control manipulates the process stream flow rates by means of PID controllers.

Using factorial experimental design to evaluate the separation of plastics by froth flotation

This paper proposes the use of factorial experimental design as a standard experimental method in the application of froth flotation to plastic separation instead of the commonly used OVAT method (manipulation of one variable at a time). Furthermore, as is common practice in minerals flotation, the parameters of the kinetic model were used as process responses rather than the recovery of plastics in the separation products. To explain and illustrate the proposed methodology, a set of 32 experimental tests was performed using mixtures of two polymers with approximately the same density, PVC and PS (with mineral charges), with particle size ranging from 2 to 4 mm. The manipulated variables were frother concentration, air flow rate and pH. A three-level full factorial design was conducted. The models establishing the relationships between the manipulated variables and their interactions with the responses (first order kinetic model parameters) were built. The Corrected Akaike Information Criterion was used to select the best fit model and an analysis of variance (ANOVA) was conducted to identify the statistically significant terms of the model. It was shown that froth flotation can be used to efficiently separate PVC from PS with mineral charges by reducing the floatability of PVC, which largely depends on the action of pH. Within the tested interval, this is the factor that most affects the flotation rate constants. The results obtained show that the pure error may be of the same magnitude as the sum of squares of the errors, suggesting that there is significant variability within the same experimental conditions. Thus, special care is needed when evaluating and generalizing the process.

SEMEC PILOT PROJECT: MECHANIZED RECOVERY OF GRANULATED PLASTICS FROM PACKAGING

Abstract This paper describes a project under development in Portugal aiming the design of a pilot plant that will be used in the mechanical separation of granulated plastics from packaging waste. The objective of the project is to minimize or, if possible, to eliminate the current practice of manual sorting. The flow-sheet will use mineral processing based techniques, namely gravity concentration and froth flotation. The project also seeks the development of a procedure for the quick and inexpensive analysis of plastics mixtures allowing the future automation of equipment and process control.

MSWSampling - Software Tool for Solid Wastes Sampling

Abstract The paper describes a software tool, called MSWSampling , to support the evaluation of the solid wastes sampling quality. The software is a random sample generator program that simulates the sampling process of solid wastes, calculating the size of the sample that assures a desired representativeness level with respect to the content of a critical component of the lot to be sampled or the variance of the fundamental error associated to a sample of given size. The solid wastes consists of very different active classes of solid components, which are characterized by their size distribution, specific gravity, weight proportions and shape factors. The results of a case study, the municipal solid wastes that feed a composting plant nearby Lisbon, are presented.