Reduction of phosphogypsum to calcium sulfide (CaS) using metallic iron in a hydrochloric acid medium

peer reviewedOur study aims to decompose phosphogypsum (PG), mainly composed of CaSO4.2H2O, by reduction in an acidic medium. We evaluated the decomposition of PG by various reaction mechanisms. Sulfate ions from the acid digestion of PG are reduced to sulfide by the hydrogen gas produced in the solution by hydrochloric attack of the metal iron. The solid residues obtained have been determined and monitored by X-Ray Diffraction, Fourier-Transform Infrared spectroscopy and Ultraviolet-visible spectroscopy. The microstructure of residues was observed by scanning electron microscope (SEM). The results show that hydrogen gas formed by hydrochloric acid attack of iron reduces the sulfur from S(VI) to S(-II). CaSO4.2H2O, insoluble in water, gives a residue containing CaS, which is only sparingly soluble in water. The residue also contains anhydrite, bassanite and ferrous chloride. The monitoring of the quantities of residue obtained under varying experimental conditions (temperature, attack time, mass of iron and PG and volume of acid on PG) and volume of HCl showed that the amounts of residue obtained are less than 32% of mass. When the volume of the HCl added increases, the obtained mass of the solid residue decreases sharply. The residue stabilizes at 10% of mass when the volume of HCl added is higher than that required to attack metal iron

Removal of impurities released from phosphogypsum waste in aquatic environment using bentonite

Phosphogypsum (PG), CaSO4,2H2O, is an acidic residue derived from the phosphate industry produced in very large quantities. Phosphogypsum is mainly calcium sulfate dihydrate, it contains elevated amount of impurities that originated primarily from the phosphate rock used for the production of phosphorus fertilizers. Large quantities are produced world-wide and are being thrown out on the seas, thus causing adverse effects on marine environment. It is known that PG causes disturbances, mainly chemical pollution of the aquatic environment (e.g. phytoplankton efflorescence and continuous regressions of Posidonia herbarium). In order to fully understand the environmental impact of phosphogypsum, it is necessary to understand the geochemical processes that control the composition of phosphogypsum leachates and the attenuation of environmentally sensitive chemical species when these leachates enter aquatic environments. The aim is also to use natural adsorbent as raw bentonite to reduce the impact of the acidity and heavy elements on the environment. For that purpose, XRD, XRF, LOI and ATD/TG analyses were performed on raw bentonite (B). Afterwards, in distillated water, we mixed 10g of different proportion of PG and bentonite (0% ≤PG/B≤ 100%) in order to follow the characteristics of the liquid phase (pH, conductivity, chemical composition ...). The results indicate that the pH of the solutions containing only PG remain constant, however, for the PG and bentonite mixtures, the environment became more alkaline. A proliferation of algae starts to be observed in the mixture 5PG/5B, and after a long stay in natural conditions, a proliferation of algae is observed in all the mixtures. We note the absence of algae when only the phosphogypsum or bentonite is put in contact with water. Overall, the preliminary results show that the mixture "PG + bentonite" has two important effects: (1) fertilization of the aquatic environment, (2) depollution by trapping of the chemical impurities released by the PG into the solution

Development of a new ecological material based on Moroccan industrial waste for road construction

Currently, the recovery of industrial waste to reduce their quantities and their impact on the environment is one of the problems to be tackled in research. The Phosphogypsum (PG) is an industrial waste produced by the attack of sulfuric acid on natural phosphate. PG contains harmful elements to the ecosystems and human health, in particular heavy metals and radionuclides [1]. The electric steel is a growth industry worldwide, in particular in the United States, it raised from less than 50% in 2000 to more than 68% in 2018 (Létard, 2019). This growth is accompanied by a significant increase in slag waste. In Morocco, OCP (Jorf Lasfar) generates more than 15 million tonnes per year of and the steel company (SONASID) produces slag waste (SC) 150,000 tonnes per year. Adding this waste to concrete may increase its mechanical strength. The Aim of this study is to incorporate these wastes with Bentonite of deposit Trebia (Nador, Morocco) stabilized by Lime to assess stabilization effect of bentonite in the presence of water (W). For this purpose the mixtures (BLW), (BL-PG-W) and (BL-PG-Ss-W), cured for 3, 7, 15 and 28 days, were analyzed by X-ray diffraction (XRD), infrared (IR), thermal analysis (DTA/TGA or DSC/TGA), scanning electron microscopy (SEM) and unconfined compressive strength (UCS). The kinetics setting and hardening of B-L and B-L-PG mixtures are slow. The addition of PG to the mixture (B-L) allows ettringite formation after 7 days of hydration and stratlingite after 28 days. The Slag accelerates the setting of the mixture and activates the pozzolanic reactions of bentonite. Mechanical strength is accelerated for the mixture (B-L-PG-Ss) comparable to other mixtures; it increases only after 3 days of setting. This improvement was confirmed for other curing times. The presence of Ye’elimite and other hydraulic binder phases present in slag could explain the increased hardening showed for B-L-PG-Ss mixtur

Hydrochemical study of some bottled water produced in the city of N'Djamena (Republic of Chad).

International audienc

Posphogypsum waste valorization by acid attack with the presence of metallic iron

During the classical production of phosphoric acid using phosphate rock, a high quantity of phosphogypsum “PG” waste is produced. Storage and management of this quantity present a serious problem, because the elevated levels of impurities which originate primarily from the source phosphate rock. The consumption or the re-use of this waste generates several environmental risks due to its composition. Several valorization processes of the PG, economic and environmental, are in phase of development and study. Among the valorization processes, we can find desulphurization, bacteriological or thermal methods, allowing to produce sulphur dioxide SO2 used in the synthesis of the sulphuric acid. The main objective of the present study is to develop an alternative technique to valorize the PG and produce sulphur dioxide “SO2”, using an alternative procedure based on a strong acid attack with the presence of a metal catalyst. The results indicate that the leaching of the PG with acid alone does not allow reduction of the sulfates ions dissolved in SO2. However, when the leaching occurs with the presence of the metal elements, the release of SO2 is observed. The values of the calculated ΔGr and ΔHr permit to discuss the possibility of the reactions involved and to argument the experimental results found

Phosphogypsum waste as additives to lime stabilization of bentonite

Waste recycling with increasing the lifecycle of resource is a novel approach for enhancing circular economy. Within this context, this study aims to give a second life cycle to phosphogypsum (PG) waste, which is a by-product from phosphoric acid manufacture, as useful resources. This study evaluates the sustainability of this harmful waste, which is accumulated in large stockpiles and becomes hazardous during storage, and to stabilize bentonite-based concrete intended for road construction. The effect of raw bentonite (Ca-Na montmorillonite), PG, and lime on the properties of cementitious materials has been investigated through various tests. The properties examined include chemistry (X-ray Fluorescence Spectrometry), mineralogy (X-ray Powder Diffraction and Fourier-Transform Infrared Spectroscopy), calorimetry (Differential Thermal Analysis/Thermogravimetry) and microstructure (Scanning Electron Microscopy) of the mortar based on different mixtures of bentonite, PG and lime. The result show that lime and PG addition involves in the formation of new nanocrystalline phases and the disappearance of certain minerals as portlandite. Calcium silicate hydrate gel appeared in the mixture with 8% of lime, whereas strätlingite was neoformed in the mixture with 8% of lime and 8% of PG. These nanocrystalline phases are responsible for enhancing mechanical strength through the pozzoloanic reaction

Geotechnical and chemical properties of stabilized bentonite with lime-phosphogypsum

Phosphogypsum (PG), produced from phosphoric acid production, is accumulated in large stockpiles and occupies vast areas of land. The phosphate industry in Morocco produces 15 million tons of phosphogypsum waste by year (Hakkou, 2016). Those wastes contain toxic elements which may cause environmental damage during storage, especially by contact with aquatic environments (Gaudry, 2007). The aim of this study was to propose a sustainable management of those harmful wastes. We test different mixings to trap the toxic elements. We mix some phosphogypsum waste with raw bentonite (B). Bentonite is abundant clay material in Morocco and is successfully used for decades as an adsorbent for removing toxic heavy metals from aqueous solutions (Chiban, 2012). The raw bentonite was sampled from the Tribia deposit located in Northeast of Morocco. We also test several mixings with both bentonite and lime (L). Chemical (XRF), mineralogical (XRD), thermal (ATG), geotechnical (Atterberg limits) and mechanical (Proctor and compressive strength) analyzes analysis were performed on the different mixtures. The results display that lime and phosphogypsum significantly enhance the behavior of bentonite by increasing the compressive strength through different reactions. The optimal results are reached when 8% of PG is added to the bentonite, highlighted by an increase of 20% compared to the mixture made only by bentonite and lime at 28 days (Fig. 1). Over time, long-term modifications occur. In particular, lime as a strong base raises the pH of the clay and causes silica and alumina mobilization. Phosphogypsum decreases the time of setting. It forms then aluminates and hydrated calcium silicates which, by crystallizing, act as a binder between the grains thus causing an increase in the compressive strength. In addition, immediate changes in geotechnical properties of bentonite were observed. Those changes are expressed by a decrease in the density of the Proctor optimum values and increase in the optimum water content with the addition of 5%, 8%, 10% and 15% of lime (Fig. 3). We suggest that the additional water is retained in the agglomerates resulting during the reaction of the lime with bentonite. The mixture evolves almost instantaneously from a plastic state to a solid, friable, non-tacky state and partially loses its sensitivity to water. The increase in unconfined compressive strength was the highest with 8% lime and 8% phosphogypsum (Fig. 2). Furthermore, the addition of phosphogypsum increases the liquid limit compared to the bentonite and lime mixture

Assessment of a Mining-Waste Dump of Galena Mine in the East of Morocco for Possible Use in Civil Engineering

Mining dumps, particularly inactive or abandoned mines located near makeshift mining towns, have significant environmental and social impacts. The Touissit-Boubker lead mine, operated for years by the Touissit Mining Company (CMT) and abandoned without rehabilitation, is an example of this socio-economic and environmental collapse. Large quantities of harmful solid waste containing clayey aggregates rich in lead sulphide have been dumped in dykes on the edge of the village of Touissit. These mining wastes were rewashed to extract galena causing a depletion of lead sulphide. The objective of this study is to evaluate the possibility of using washed mining waste as sandy aggregate for the manufacture of masonry mortar. Cylindrical mortar tests, made with various proportions of sand and mine waste were characterized by X-ray diffraction, scanning electron microscopy and mechanical analysis by uni-axial compressive strength after curing for 3, 14, 28 and 60 days. The results obtained revealed that the mining waste consists of dolomite, quartz and clay. The dehydration rate of the mortar specimens is strongly affected by the amount of the waste added and the grain size. The mechanical strength of the mortar specimens mostly depends on the grain-size of the aggregates than on the amount of mine waste added. The microstructure of the mortar did not change when sand was replaced by mine waste of the same

Physico-chemical, mineralogical, and technological characterization of stabilized clay bricks for restoration of Kasbah Ait Benhadou- Ouarzazate (south-east of Morocco)

Due to ageing, the unfired brick masonry may detach and fall off, or deteriorate to such an extent that it becomes necessary to restore them. Such is the case, for instance, of the Ksar of Ait Benhadou in Ouarzazate, Morocco. Our study aims to provide compatible and sustainable earthen bricks to restore this monument. Samples were collected from facades that were under conservation/restoration at the time when sampling was performed. Clayey soil samples were collected vicinity to the Ksar and analyzed by X-ray diffraction and X-ray fluorescence. A representative sample was stabilized with three aggregates (lime, cement and straw). The effect of ageing of the stabilized briquettes on shrinkage, water absorption, mechanical and thermal properties, compressive strength and thermal conductivity was studied on stabilized specimens. The samples consist mainly of clay minerals, calcite and quartz. They are rich in iron, aluminium and potassium. These samples are sandy with low plasticity (PI = 7%), which is slightly lower than the plasticity value required by the Moroccan standard for earth constructions. The results showed that the stabilized clayey soils have suitable properties such as density, porosity, water absorption and high thermal insulation. The best compressive strength performance is obtained for clay-stabilized samples with a high sand fraction. The thermal conductivity of clay-stabilized specimens increases as the specimens become denser and more compact, lime and straw have the opposite effect. Copyright 2022 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 3rd International Congress on Materials & Structural Stabilit

Élaboration de nouveau matériau pour la construction routière à base des déchets industriels locaux : scories noires d’aciérie électrique et phosphogypse

The Black Steel slag (Ss) and phosphogypsum (PG) are industrial wastes produced in Morocco. In order to reduce these two wastes and to evaluate their pozzolanic reactivity in the presence of water, they were incorporated into bentonite (B) mixed with lime (L). The studied mixtures (BLW, BL–PG–W and BL–PG–Ss–W) were analyzed by X-ray diffraction, Infrared spectroscopy, Raman spectroscopy and SEM/EDX analysis. Compressive strength tests were performed on hardened specimens. The results obtained show that the hydration kinetics of the B– L–W and B–L–PG–W mixtures are slow. The addition of PG to a bentonite––lime mixture induces the formation of new microstructures such as hydrated calcium silicate (C–S–H) and ettringite, which increases the compressive strength of the cementitious specimens. The addition of the Ss to a mixture composed of 8%PG and 8%L–B accelerates the kinetics of hydration and activates the pozzolanic reaction. The presence of C2S in the slag helps to increase the mechanical strength of the mixture B–L–PG–Ss. The compressive strength of the mixtures BL–W, BL–PG–W and BL–PG–Ss–W increases from 15 to 28 days of setting. After 28 days of setting, 8% of Sc added to the mixture 8% PG–8%L–B is responsible for an increase of the compressive strength to 0.6 MPa