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EXTRACTION OF SILICA FROM RICE HUSK ASH

CHAPTER ONE

INTRODUCTION

1.1   BACKGROUND TO THE STUDY

Critical economic and environmental situations of the current days encourage companies and researchers to develop and improve technologies intended to reduce or minimize industrial wastes. As a consequence, much effort has been expended in different areas, including the agricultural production. Rice is the second largest produced cereal in the world. Its production is geographically concentrated in Asia with more than 90 percent of world output. The United States and Brazil are the most important non-Asian producers and Italy ranks first in Europe. The rice world production was approximately 400 million tons of milled rice in 2003. In most varieties rice is composed by approximately 20 % of rice hull, which contains a fibrous materials and silica; however the amount of each component depends on the climate and geographic location of rice crop. Therefore, due to its high percentage in the grain composition, the hull is considered a by-product in the mills and creates disposal and pollution problems.

Burning rice hull as a fuel substitute in order to generate energy is a useful solution which is used by many industries; however it results in a new waste, named rice husk ash (RHA). This residual ash obtained from the combustion can contain over 60 % of silica and some amount of metallic impurities. Depending on the burning process, RHA can contain silica in the amorphous form; therefore, this residue can be considered as a new economically viable raw material to produce silica or to be used as silica resource. Silica has been used in many applications, including production of nanomaterials. Tailored materials composed of nanoparticles have potential for application in numerous technological fields. The expression nanodispersed silica covers the entire variety of silica forms including sols, gels, suspensions, and pastes. Silica sol or colloidal silica refers to a stable dispersion of colloidal silica particles in water medium. It is used in many applications, such as in refractory materials, binder for inorganic paint, and stiffener for hard coating reagents, abrasive particles, adsorbents, and catalyst. Various raw materials can be used in the manufacturing of monodispersed sols but the two main ones are tetralkyl orthosilicates and sodium silicate solution.

This latter has the advantage over the first one because it is less expensive and uses water as the solvent. Sodium silicate solutions (commercially called sodium water-glass) are complex mixtures of silicate anions and polymer silicate particles especially when silica module (SiO2: Na2O molar ratio) is >2. The manufacture process of sodium silicates is generally considered expensive due to the energy required to reach high temperatures during the calcination stages, in addition to producing considerable air pollution by emission of dust, nitrogen and sulphur oxides. Although this calcination process is widely used in industrial scale, there is another process based on the reaction of silica with aqueous sodium hydroxide (NaOH) in autoclave. This latter one has an advantage when compared with the conventional calcinations process as it requires less energy.

1.2  SCOPE OF THE STUDY 

The study will cover the qualitative and quantitative analysis that is involved in the extraction of silica from rice paddy (husk ash). This analysis will determine the:

· Micrographs of silica

· Chemical composition of RHA analysis

· Effect of NaOH concentration and temperature on the silica conversion

· Particle size and the PH of Silica Sol

1.3  OBJECTIVES OF THE STUDY

The present study has been designed to evaluate the extraction of silica from rice paddy (husk ash). The study has also explain the production of a sodium silicate solution with silica module of approximately 3 (M ≈ 3; where M = SiO2/Na2Omolar ratio) using RHA as the silica resource and then, use it to obtained silica sol via Ion-Exchange Method. In order to develop the study, the following sequence was performed: (i) characterize the RHA; (ii) investigate the influences of NaOH molar concentration and temperature on the silica conversion to sodium silicate production and (iii) utilize it to produce a silica sol.

1.4  SIGNIFICANCE OF THE STUDY

The study is significance because it shows that:

• Rice husk is a widely available agricultural waste India produces around 25 million tons of Rice Husk

• It is largely used as a fuel—in small scale, and in large scale for electrical power generation and thermal needs

• Rice husk contains 20 % ash and leaves large amount of residue (about 25 %) after it is burnt causing a disposal problem

• Silica is the main constituent of the Rice husk ash (~ 90 %)

• Precipitated silica is a high value product (Rs. 40 per kg) having applications in rubber, cosmetics, tooth paste and many other industries

• Production of precipitated silica from rice husk thus solves the disposal problem ash and provides additional revenue stream

• This process is cheaper – production cost about Rs. 22 – 24/kg of silica

Finally, students of chemical engineering will also find the work useful as it touches on their area of specialization.

1.5  LIMITATION OF THE STUDY

The major handicap of this study is that of time factor. The time under which this study was carried out was too short for the researcher to do a thorough and more comprehensive research work. This study was done coupled with academic stress and this may have resulted in some minor faults in the study. Financial problem, Ability to raise money for the project work was a big challenge.

1.6  CONCEPTUAL EXPLANATIONS OF TERMS

Silica (SiO2) is one of the valuable inorganic multipurpose chemical compounds. It can exist in gel, crystalline and amorphous forms. It is the most abundant material on the earth's crust. However, manufacture of pure silica is energy intensive. A variety of industrial processes, involving conventional raw materials require high furnace temperatures (more than 700°C). In this article, a simple chemical process is described which uses a non-conventional raw material rice husk ash for extraction of silica. Rice paddy of (husk ash) is one of the most silica rich raw materials containing about 90-98% silica (after complete combustion) among the family of other agro wastes. Rice husk is a popular boiler fuel and the ash generated usually creates disposal problems. The chemical process discussed not only provides a solution for waste disposal but also recovers a valuable silica product, together with certain useful associate recoveries.