Continuous flow synthesis of silicon compounds as feedstock for solar-grade silicon production

This thesis describes the key steps in the production of high purity (solar-grade) silicon from metallurgical-grade silicon for use in the production of photovoltaic cells as alternative renewable, environmentally benign and cheap energy source. The initial part of the project involves the development and optimization of a small chemical production platform system capable of producing alkoxysilanes from metallurgical-grade silicon as green precursors to solar-grade silicon production. Specifically, the main aim of the study was to synthesize trialkoxysilanes in continuous flow mode, although the synthesis on monosilane was also done in batch mode. The alkoxylation reaction was carried out in a traditional slurry phase batch reactor, packed bed flow tubular reactor and also attempted in a continuous flow falling film tubular reactor. The effect of key parameters which affect the silicon conversion and selectivity for the desired trialkoxysilane were investigated and optimized using ethanol as a reagent model. The synthesis was then extended to the other alcohols namely methanol, n-propanol and n-butanol. Copper catalysts which were tested in the alkoxylation reaction included: CuCl, Cu(OH)2, CuO and CuSO4. CuCl and Cu(OH)2 showed comparable activity in the batch mode but the former was more efficient in the packed bed flow tubular reactor. Cu(OH)2 could be used as a non-halide catalyst but its activity is limited to short reaction cycles (500 oC) resulted in a lower rate of reaction and selectivity than when slightly lower temperatures are used (95% in batch and >97% in flow) and conversion (about 88% in batch and about 64% in flow) as compared to all other alcohols studied showing that it could be the most efficient alkoxylation alcohol for this reaction. Overally, the packed bed flow tubular reactor resulted in higher selectivity to trialkoxysilanes than the batch system. Performing the reaction under pressure resulted in increased conversion but selectivity to the desire trialkoxysilane diminished. Synthesis in a continuous flow falling film tubular reactor was not successful as it resulted in very poor conversion and selectivity. Monosilane was successfully synthesized from the disproportionation of triethoxysilane using homogeneous and heterogeneous catalysts in batch mode. The results obtained from homogeneous catalysis showed that the reaction can be conducted at room temperature. The heterogeneous catalysis method resulted in slow conversion at room temperature but mild heating up to 55 oC greatly improved the reaction. Conducting the reaction under neat conditions produced comparable results to reactions which were carried out using solvents. The disproportionation reaction was best described by the first order kinetic model. The results obtained in this research indicate that the packed bed flow tubular reactor can be utilized with future modifications for continuous flow synthesis of alkoxysilanes as feedstock for the solar-grade silicon production

Recent Natural Corrosion Inhibitors for Mild Steel: An Overview

Traditionally, reduction of corrosion has been managed by various methods including cathodic protection, process control, reduction of the metal impurity content, and application of surface treatment techniques, as well as incorporation of suitable alloys. However, the use of corrosion inhibitors has proven to be the easiest and cheapest method for corrosion protection and prevention in acidic media. These inhibitors slow down the corrosion rate and thus prevent monetary losses due to metallic corrosion on industrial vessels, equipment, or surfaces. Inorganic and organic inhibitors are toxic and costly and thus recent focus has been turned to develop environmentally benign methods for corrosion retardation. Many researchers have recently focused on corrosion prevention methods using green inhibitors for mild steel in acidic solutions to mimic industrial processes. This paper provides an overview of types of corrosion, corrosion process, and mainly recent work done on the application of natural plant extracts as corrosion inhibitors for mild steel

Synthesis and selective derivatization of 1,2,4-triols as a model for fluorous tagging and separation of compounds in avocado oil

M. Tech. Chemistry.While there are existing methods for the analysis and authentication of avocado oil based on the composition of the saponifiable fraction, the projected growth in the market for avocado oil as heart healthy edible oil or for use in cosmetics and pharmaceuticals may lead some producers to cheat via adulteration with inferior vegetable oils of similar fatty acid composition. Thus new methods based on the analysis of the unsaponifiable fraction could be used to complement the existing methods of detecting undesirable additives. The aim is to develop a method of selective benzoylation of 1,2-diols and 1,2,4-triols based on the Martinelli protocol

Biosorption of zinc(II) ions from aqueous solution using msasa tree (brachystegia spiciformis) leaf powder

The purpose of this study was to explore the effectiveness of the Brachystegia spiciformis leaf powder for the removal of zinc (II) ions from aqueous solution. Batch experiments for the effect of pH, contact time, biomass dose and initial metal ion concentration were carried out in the laboratory. The adsorbent was characterized before and after biosorption of zinc ions by Fourier Transform-Infrared Spectroscopy (FT-IR). The investigation showed that the highest metal uptake was at solution pH 6, contact time of 120 mins and biomass dose of 2.0 g. The experimental data for the biosorption zinc (II) ions by Brachystegia spiciformis leaf powder was modelled to the Langmuir and Freundlich isotherms. The biosorption of zinc (II) ions fits better to the Langmuir than to the Freundlich model having correlation coefficients of 0.9902 and 0.9569 respectively. The maximum adsorption capacity Qmax was found to be 1.85 mg/g indicating the ability of the biosorbent to remove zinc from aqueous solution

Extraction of Water Treatment Coagulant from Locally Abundant Kaolin Clays

Rapid industrialisation is contributing to water pollution. There is a need to identify cheaper and efficient methods of removing contaminants as the demand for clean water rises. A study is carried out to investigate the extraction of alum from locally abundant kaolin clays using sulphuric acid. Alum is a coagulant that is used for raw water treatment. The kaolin clay and alum were characterized by Fourier transformation infrared spectroscopy (FTIR). The effects of particle size, calcination temperature, calcination time, acid-kaolin clay ratio, acid concentration, leaching temperature, and leaching time on extraction efficiency were investigated. The optimum leaching conditions for the calcined kaolin clay were found to be particle size 100 µm, acid-kaolin clay weight ratio 6 : 1, acid concentration 4 M, leaching temperature 100°C, and leaching time 90 min. Under optimised conditions, 66.95% (w/w) aluminum sulphate was extracted. The results showed that sulphuric acid could be used on a large scale to extract alum from kaolin clay. The extracted alum showed similar structural and physical characteristics compared with commercial alum. A dosage of 40 mg/L of the extracted alum showed effective coagulant properties with a great potential of treating raw water

Synthesis of fatliquor from waste bovine fat for use in small scale leather industry

In this study, fatliquor has been synthesized from waste bovine fat for its use in small scale leather industry. The physico-chemical properties of bovine fat are determined before sulphation with sulphuric acid followed by the subsequently neutralization with ammonia to produce the fatliquor. The fatliquor is subjected to chemical and physical analyses. The degree of sulphation is found to be 90% and the surface active groups of the fatliquor are observed in anionic form. The fatliquor has been applied onto light leather and physical tests are carried out on the fixed leathers. The results of the physical tests on fixed leather conform to the standard leather specifications. It is observed that the synthesized fatliquor could be used as an alternative in leather fixation

Application of central composite design in the Adsorption of Ca(II) on Metakaolin Zeolite

Metakaolin zeolite-A was synthesized from thermally activated kaolin clay and characterized by Fourier Transform Infrared Spectroscopy and X-Ray Diffraction Spectroscopy. The effects of pH (2–10), contact time (10–180 min), initial concentration (5–120 mgL−1), and dosage (0.1–2 g) and their interactions were investigated using response surface methodology following a central composite design. Optimum removal (87.70%) was obtained at pH 6, contact time 180 min, initial concentration 40.0 mgL−1, and adsorbent dosage 1.0 g by Excel Solver using the GRG solving method. The adsorption data fitted best to the Langmuir model with correlation coefficient 2 = 0.993 and Chi-square value 2 = 4.76. The Freundlich isotherm gave a correlation coefficient 2 = 0.933 and 2 = 37.91. The adsorption process followed the pseudo-second-order model. The calculated thermodynamic parameters showed that the adsorption process was endothermic and not thermodynamically spontaneous. The studied zeolite-A can therefore be used as a promising adsorbent for the removal of Ca(II) ions from aqueous solutions

Catalytic oxidation of cinnamyl alcohol to cinnamaldehyde using hydrogen peroxide

Platinum-bismuth bimetallic catalysts supported on activated carbon powder were prepared using a colloidal method. The catalysts and their support were characterized by physisorption, Brunauer Emmet Teller (BET), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The selectivity and activity of the catalysts were tested for the oxidation of cinnamyl alcohol using H2O2 as an oxidant. The effects of concentration, solution medium and modifier on catalysts selectivity and activity were investigated. The results showed that hydrogen peroxide could be used as an effective oxidant for the catalytic oxidation of cinnamyl alcohol. Selectivity of cinnamaldehyde (84%) was attained at 34% conversion of the cinnamyl alcohol. The catalysts were more active in water than in toluene

Synthesis and characterisation of Zeolites from Coal Fly Ash (CFA)

In this study, disposed coal fly ash samples from Zimbabwe were used in synthesis of zeolites. The method of direct hydrothermal treatment with sodium hydroxide at different concentrations (2, 2.5, 3.5 and 4.5 moldm-3) at a constant temperature of 100°C and activation time of 24 h was applied. Characterisation of coal fly ash and confirmation of occurrence of the zeolite material was done using XRF, XRD, FTIR, and BET techniques. The zeolite materials formed were a mixture of zeolite Na-X (NaAlSi1.1O4.2225H2O), zeolite Na-A (Na2Al2Si3.3O8.867H2O), zeolite NaP1 Na6Al6Si10O32H2O) and zeolite hydroxysodalite (Na1.08Al2Si1.68O7.441.8H2O). The variety and yield of zeolites formed depend on the concentration of sodium hydroxide used

Synthesis of zeolites and their applications in heavy metals removal: a review

Over 800 million tonnes of coal fly ash are produced worldwide every year posing an environmental threat due to disposal problems. Nowadays the increasing interest in the synthesis of zeolites from low cost materials has prompted extensive studies on their conversion into zeolites as one of promising coal fly ash utilization methods. Zeolite synthesized from coal fly ash is a minor but interesting product, with a number of environmental applications. This paper provides an overview on the methodologies for zeolite synthesis from coal fly ash, and a description of conventional alkaline conversion processes, of synthesis of different zeolitic products and their methods of characterisation.The examination of the studies presented by different authors reveal that one of the main potential applications of zeolite materials is the uptake of heavy metals and organic pollutants such as dyes from polluted waste waters and nearly all zeolite synthesis have been driven by environmental concerns