Pierre Moszkowicz

THE EFFECTS OF CARBONATION AND DRYING DURING INTERMITTENT LEACHING ON THE RELEASE OF INORGANIC CONSTITUENTS FROM A CEMENT-BASED MATRIX

A Portland cement mortar was submitted to cycles of intermittent wetting (IW) in which tank leaching was interspersed with periods of storage in either an inert or a reactive atmosphere. Relative humidity (RH) (23%, 48% and 98%) was maintained during storage to control the drying process. The effects of IW were qualified by comparing flux and cumulative release of matrix constituents (Ca, OH, Na, K and Cl) to that of continuous water saturation. The carbonation process was associated with the degree of drying occurring due to storage. Cumulative release of most major constituents was suppressed in samples storage under 100% CO2 in comparison to the inert atmosphere (100% N2). Results suggest that accurate long-term performance assessment must account for the potential impact of phenomena associated with IW.

Leaching behavior of pollutants in stabilized/solidified wastes

The objective of the study is to characterize pollutant retention in wastes solidified with hydraulic binders in order to predict the long term leaching behavior. The stabilized/solidified wastes are APC MSW (air pollution control residues from municipal solid waste incineration), lead secondary smelting slag and different salts. The binders used are ordinary Portland cement (OPC 55) or blast furnace slag cement (CLK 45) and additives, such as metakaolin. Leaching tests are used in the characterization procedure. Modelling of the observed leaching behavior is conducted to improve the description of the physical and chemical phenomena involved in the release of soluble species. The main results of the study allow a distinction to be made between the species whose solubility is not sensitive to the chemical context of the porewater, and the other species, such as amphoteric metals, whose solubility is sensitive to the context (especially the pH). In the first case, a diffusion or shrinking front model is appropriate. In the second case, a coupled solubilization/diffusion model must be developed in order to describe the leaching behavior of heavy metals (lead, etc.) contained in the stabilized/solidified wastes.

Batch extractive distillation: The process and the operational policies

The batch realisation of the extractive distillation and the role played by the different steps in the process are analysed and on the basis of this analysis several operational policies are presented. The different BED policies are studied by simulation and compared (with the example of the mixture acetone–methanol+water). A method is suggested for the comparison of the different BED operational policies. In a BED process the mole fraction of the more volatile component (A) in the distillate (xd,A) can be influenced not only by the variation of reflux ratio (R) but also by changing the flow rate of the solvent (F). The simulation results are experimentally investigated and verified for each operational policy. The original R=const.policy is modified by shortening the second preparatory step of the BED (R=∞,F>0). The possibilities of performing a constant distillate composition (xd,A=const.) policy are discussed. In order to increase the efficiency of the modified R=const.policy further, it is combined with the xd,A=const. policy, maintaining a high value of xd,A for a longer period during the production of A by gradually increasing R or F or R and F simultaneously. The feasibility of the cycling policy is also studied for the BED.

Behaviour of ordinary Portland cement during the stabilization/solidification of synthetic heavy metal sludge: macroscopic and microscopic aspects

Abstract Chromium and/or zinc hydroxide sludges were mixed at variable proportions with OPC, in order to evaluate the macroscopic and microscopic effects caused by the waste on the cement hydration process. Initial setting time, heat production during hydration, leaching characteristics and microstructure of the samples were investigated using varied techniques. For zinc hydroxide sludge, hydration was stopped after a few minutes, because of the precipitation of a solid hydrated phase. Since the sludge:cement ratio remains under 2–3:1, chromium hydroxide sludge accelerated the cement hydration. For higher ratios, hydration was hindered. Some microstructural modifications have been detected, such as the possible formation of a U-phase analog, related to the presence of chromate ions in the sludge. The influence of the oxidation degree of chromium on its fixation in the matrix is discussed.

Dynamic simulation of a batch extractive distillation process

Abstract The process of batch extractive distillation may provide the advantages of both the batch and the extractive distillation. So far this process has not been applied at all probably due to its complexity. An algorithm and a computer program were developed for simulating the experiments of a batch extractive distillation process (separation of acetone and methanol on a pilot-plant column containing 32 bubble cap trays applying water as solvent) on a PC/AT/486. For the integration of the set of nonlinear differential equations (component material balances) the Runge-Kutta method was used. For saving computation time a two-dimensional linear interpolation method was applied for the ternary mixture when calculating VLE. The experimental and calculated results are compared.

Pollutants leaching behaviour from solidified wastes : A selection of adapted various models.

Leaching tests are essential in the environmental assessment of stabilized wastes. Research programmes were conducted on their interpretation in order to develop tools for the evaluation of long term release of pollutants contained in solidified wastes. Models for the leaching of porous materials are discussed in this paper according to the specificity of the chemical species (i.e. transport model with total dissolution of species-diffusional model; transport model with progressive dissolution of species due to limitation of solubility-shrinking core model; and the model coupling transport and chemical phenomena). The leaching behaviour of pollutants (i.e. lead) solidified in a cement matrix was studied under different chemical conditions. Results have shown that the release of species whose solubilities depend on the physico-chemical conditions, and especially the pH (e.g. amphoteric metals), is governed by the solubility of the species in the pore water at local conditions and by the pH evolution within the matrix. A coupled dissolution/diffusion model was developed to describe the release of chemically complex species contained in a porous medium in contact with water. Leaching tests of cement matrices and artificial porous matrices containing calcium hydroxide and pollutants were conducted in order to validate the coupled dissolution/diffusion model. A good assessment of the retention of some pollutants contained in cement matrices could then be obtained by the association of two tests: solubilization of the pollutants related to the chemical context (pH) under steady state conditions and monolithic long term dynamic leaching tests in order to characterize the evolution of the chemical context (pH) and consequently the release of pollutants. The objective is to integrate this approach in the standardization process (CEN TC 292- WG 6, in progress).

Retention mechanisms in mortars of the trace metals contained in Portland cement clinkers

Abstract This work aimed to assess the retention in the mortars of the heavy metals fed in a cement kiln with natural raw materials, fossil fuels or waste derived fuels. Industrial cement has been studied, along with laboratory samples enriched during clinkering with lead, zinc and chromium. The relevant mortars were tested for metal release through leaching studies in various chemical contexts. Zinc appeared to be insoluble in the pH range 7–13. Lead is released only in alkaline medium above pH 12.5. Hexavalent chromium, whose salts are usually very soluble, is retained though in an hydrated phase, that is stable in the chemical environment ensured by the cement matrix. Lead and chromium releases are linked to the metal levels in the mortars. The solubility studies of a chromium+sulfate ettringite revealed that chromium release is linked to its proportion in the crystal. A selective dissolution of the chromium part of ettringite has been pointed out. This leads to a lowering of the chromium level in such a solid contacted with water. Theses results improved the understanding of the release mechanisms of chromium by the mortar blocks.

Diffusion and dissolution in a reactive porous medium: mathematical modelling and numerical simulations

In this work a simple mathematical model for diffusion and dissolution in reactive porous medium is presented. The case of lime and lead in solid phase enclosed in cement matrices is considered more specifically. A numerical method based on finite difference and on a marching technique is proposed and some numerical results are provided. In a simple case, the results obtained are compared with numerical results available in literature.

Homoazeotropic distillation of maximum azeotropes in a batch rectifier with continuous entrainer feeding II. Rigorous simulation results

Abstract The separation of maximum azeotropes in a batch rectifier by extractive distillation was investigated by rigorous simulation. The influence of the most important parameters was studied. The results obtained under continuous feeding of the entrainer were compared with those obtained by the batch distillation, where the whole quantity of entrainer is added to the charge before the start of the distillation. The calculation results obtained for the mixture acetone-chloroform using benzene and toluene as entrainer showed the benefits of the continuous feeding of the separating agent.

Arsenic stability in arsenopyrite-rich cemented paste backfills: A leaching test-based assessment

Arsenic (As) is a toxicant in tailings from sulphur deposits. It represents an environmental risk because of its high solubility. Tailings can be mixed with water (typically 25%) and a low proportion of hydraulic binder (3-7%) to produce a cemented paste backfill (CPB), stored in underground mine openings. CPB is a tailings storage technique, but it could also provide environmental advantages by stabilization of polluting elements such as As. Tailings from Casa Berardi mine (QC, Canada) contain As (3800 ppm), mainly in arsenopyrite form. For this study, three different CPBs were synthesized in laboratory using Casa Berardi tailings and three different binders. These pastes were submitted to various leaching tests after 28 days of curing. The results indicate that As is released at higher concentration for a fly ash-based CPB than for slag- and Portland cement-based CPB. However, at lower pH, As is better stabilized in fly ash-based samples. These differences can be explained by a variation of solubility of As-compounds in each CPB. Several mechanisms of As release occur, as diffusion and/or dissolution/precipitation. The accelerated weathering test results show that sulphide reactivity is buffered by the neutralizing minerals contained in CPB, and influence the As release behaviour by decreasing the oxidation of As-bearing sulphides.