Source: UNIVERSITY OF ARKANSAS submitted to NRP
SILICA-BASED INDUSTRIAL PRODUCTS FROM RICE HULL ASH SILICA
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
COMPLETE
Funding Source
Reporting Frequency
Annual
Accession No.
0185982
Grant No.
2001-35504-10044
Cumulative Award Amt.
(N/A)
Proposal No.
2000-01921
Multistate No.
(N/A)
Project Start Date
Dec 1, 2000
Project End Date
Nov 30, 2005
Grant Year
2001
Program Code
[(N/A)]- (N/A)
Recipient Organization
UNIVERSITY OF ARKANSAS
(N/A)
FAYETTEVILLE,AR 72703
Performing Department
FOOD SCIENCE
Non Technical Summary
Rice hulls are a major waste product of rice milling and pose a significant disposal problem. This project examines the possibilty of producing ceramic materials from soluble silica extracted from rice hulls.
Animal Health Component
100%
Research Effort Categories
Basic
(N/A)
Applied
100%
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
5111530200040%
5111530201030%
5111530202030%
Goals / Objectives
1. To produce light-weight silica blocks from rice hull ash silica 2. To investigate the physical properties of the silica blocks
Project Methods
Rice hull silica will be solubilized by alkaline extraction to produce a silicate solution. Silica gel will then be precipitated by decreasing solution pH. Gel strength will be increased by aging and subsequent drying. Structural analysis of the blocks will be done by X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy. Atomic force microscopy and scanning electron microscopy will be used to examine xerogel microstructure. Average surface area and pore size will be determined by BET gas adsorption. Mechanical properties and the effect of temperature on mechanical strength will be determined by DMA. Changes in structure with temperature will be measured by temperature controlled X-ray diffraction. Infrared imaging will provide information on radiant emissions and a semi-quantitative determination of heat flow properties.

Progress 12/01/00 to 11/30/05

Outputs
Commercial rice hull ash is a rich source of amorphous silica obtained by rice hull combustion. This form of natural silica can more easily be used to produce silica-based adsorbents and ceramics than sand, which is traditionally used. Ceramic silica xerogels were produced by alkali extraction of silica from commercial rice hull ash with subsequent precipitation and drying. Xerogels were produced from alkali solutions whose volumes were reduced to produce a range of silica concentrations of 4-12%. Samples from each concentration were adjusted to a pH of 9, 10, 10.5 and 11 to precipitate silica gels, which were then dried for 24 hours at 80 degree C. X-ray diffraction studies showed that the samples were all amorphous silica and infrared spectroscopy analysis showed that silica polymerization, or silica chain lengthening, was greater at a higher precipitation pH. However, cross-linking between polymers was the major interaction at lower pH and increased with silica concentration. Glassy solids were formed by precipitation at high pH, whereas porous silica gels were obtained with higher silica concentrations. Xerogel formed from the concentrated silica solutions precipitated at pH 10, produced a ceramic block material that had the greatest mechanical strength due to the optimum combination of silica polymerization and cross-linking. In addition, high purity, ceramic silica xerogel was obtained by removing residual metal ion impurities by extensive water washing of the precipitated silica, before drying. Water washing was found to be more effective than acid washing.

Impacts
Rice Twenty percent of harvested rice is composed of rice hulls. Environmental regulations limit landfill disposal of hulls, and despite their use as chicken litter and in animal feed there is no large scale use that will significantly reduce the waste problem. Many rice mills are using hulls as a source of fuel and produce an amorphous silicate ash, some of which is used by the steel industry as refractory material. However, there is still the need to find large markets for hull and ash material to alleviate the waste disposal problem, despite reduction in the waste volume by combustion. Rice hull ash is a low value material that can readily be converted to useful silicate materials. The rice and silicate industry has expressed interest the novel silica-based products we have made and are seeking commercialization.

Publications

  • Kalapathy, U., Proctor, A and Shultz, J. 2000. Silica xerogels from rice hull ash: Structure, density and mechanical strength as affected by gelation, pH and silica concentration. J. Chem. Tech. and Biotech. 75:464-468.
  • Kalapathy, U., Proctor, A and Shultz, J. 2000. A simple method for production of pure silica from rice hull ash. Bioresource Technology 73:257-262.
  • Kalapathy, U., Proctor, A. and Shultz, J. 2002. An improved method for production of silica from rice hull ash. Biores. Tech. 85:285-289.
  • Kalapathy, U., Proctor, A. and Shultz, J. 2003. Silicate thermal insulation material from rice hull ash silica. Ind. Eng. Chem. Res. 42:46-49.
  • Kalapathy, U. and Proctor, A. 2000. A new method for free fatty acid reduction in frying oil using silicate films produced from rice hull ash. J. Am. Oil Chem. Soc. 77:593-598.


Progress 01/01/04 to 12/30/04

Outputs
Rice hulls are a major waste material of processing and are industrially combusted as a bioenergy source. However, the residual ash has limited use. Nevertheless, rice hull ash is an unusual material as it contains over 60 percent amorphous silica that is much more soluble that crystalline geological silica sources. Hull combustion processes are controlled as to minimize silica crystallization and maximize the amount of silica that can be alkali extracted. We have previously produced low density ceramic insulation material from alkali extracted silica and more recently investigated the effect on combustion to reduce the residual carbon in the ash. Heating ash at lower temperatures reduced the carbon content with minimal loss of amorphous structure, relative to higher temperature combustion. Reducing the ash carbon content before alkali extraction would increase silicate yields and reduce the amounts of insoluble residues.

Impacts
A commercial facility producing sodium silicate and rice hull carbon by alkali-extraction of rice hull ash has been in production in Stuttgart, Arkansas since 2004 using the technology developed, which brings much needed rural development to southern Arkansas. Opportunities to industrially produce ceramic materials will develop as sodium silicate production increases.

Publications

  • No publications reported this period


Progress 01/01/03 to 12/31/03

Outputs
Rice hull ash contains over 60% silica and can be economically viable raw material for the production of silica products. The silica is mostly in an amorphous state that enables it to be readily extracted with aqueous solvents. In contrasts commercial sources of silica are highly crystalline and require high temperature furnace smelting processing to produce a usable product. Silicate blocks were made from sodium silicate obtained from rice hull ash. Porous light-weight material with a various of densities was obtained by boiling the silicate solutions at different heating rates. Density, strength and thermal conductivity increased with increasing heating rate. The density of the materials ranged from 0.33-0.42 grams per cubic centimeter with a mechanical strength that enables the silicate not to fracture below forces of 145-196 Newtons per square centimeter. The thermal conductivity range was 0.103-0.128 Watts per meter per kilogram. The insulation properties would allow the material to be used to substitute asbestos and organic polymers. It is anticipated that work to examine the effect of rice hull ash surface properties on its chemical reactivity will continue.

Impacts
Recent work shows that materials with a diverse range of properties can be produced from rice hull ash to meet a variety of market needs. A facility to produce commercial rice hull sodium silicate and carbon from rice hull ash is now built in Arkansas. The possibility of the commercial development of value-added rice hull products will increase as production exceeds the current demand.

Publications

  • Kalapathy, U., Proctor, A. and Shultz, J. 2003. Silicate thermal insulation material from rice hull ash silica. Ind. Eng. Chem. Res. 42:46-49.


Progress 01/01/02 to 12/31/02

Outputs
Rice hull ash contains over 60% silica and can be economically viable raw material for the production of silica products. The silica is mostly in an amorphous state that enables it to be readily extracted with aqueous solvents. In contrasts commercial sources of silica are highly crystalline and require high temperature furnace smelting processing to produce a usable product. We have previously synthesized gels and xerogel by simple alkaline extraction of silica as sodium silicate with subsequent precipitation by acid addition at pH 7. However, we found that the products contained significant mineral contamination. We found that we could significantly reduce the mineral contamination of the precipitated silica by adding the extracted sodium silicate to the acid rather than adding the acid to the silicate. Thus, we were precipitating the silica at pH 7 by increasing the pH rather than lowering it. The higher purity material would have greater suitability in food and non-food adsorption applications were mineral contact with the food would be undesirable. Additional work of development of sodium silicate blocks from rice hull ash showed that increasing the rate of drying of sodium silicate solution produced stronger, denser material whereas a slower heating rate produced a low density material which had excellent insulation properties.

Impacts
Recent work shows that materials with a diverse range of properties can be produced from rice hull ash to meet a variety of market needs. Silica gel was the first product we developed from rice hull ash and a facility to produce commercial rice hull sodium silicate will be built soon in Arkansas by a rice company.

Publications

  • Kalapathy, U., Proctor, A. and Shultz, J. 2002. An improved method for production of silica from rice hull ash. Biores. Tech. 85:285-289.


Progress 01/01/01 to 12/31/01

Outputs
Rice hull ash contains over 60% silica and can be an economically viable raw material for the production of silica based products. The method previously developed in our laboratory for producing silica xerogel involved dissolving rice hull ash silica with alkali solution to form sodium silicate solution and subsequently forming silica aquagel by adding hydrochloric acid to lower the pH to 7, followed by washing and drying aquagel to form xerogel. The silica xerogel produced using this method had over 4% sodium as a contaminant. Recent work in our laboratory has led to an improved technique to produce silica xerogel with < 0.5% sodium. The improved method involved production of silica aquagel by adding silicate solution to pH 1.5 hydrochloric, citric or oxalic acid solutions until pH 4.0 was reached. Aquagel is then washed and dried to form silica xerogel. Silica produced at acidic pH of 4.0 using oxalic or citric acid had a lower sodium content than the silica produced at an alkaline pH 7.0 using any of the three acids, and silica produced at pH 4.0 using hydrochloric acid. This new procedure has eliminated the extensive washing of the dried gel required by the previous method for pure silica gel production. In addition, sodium silicate solution produced from rice hull ash by reacting the silica with 1M NaOH was used to produce highly porous low-density thermal insulation blocks using a simple evaporation method. The heating rate to produce silicate blocks had significant effect on block density, mechanical strength, and thermal conductivity. Density, peak force at break, and thermal conductivity of silicate blocks produced at a higher heating rate were 0.33 gcm-3, 150 Ncm-2, and 0.103 Wm-1K-1, respectively.

Impacts
Rice hull ash is a low-value co-product of the rice industry and there is a commercial need to utilize this material to produce value-added products to avoid disposal costs. The amorphous silica content can be readily extracted and used to produce a number of silica based products. The development of the materials described in this work offers the opportunity to produce high value products inexpensively. Current commercial production of silica involves use of significant amounts of energy. Methods utilizing rice hull ash is are simple, requiring very low energy relative to conventional methods using traditional silica sources. In particular, highly porous silicate blocks produced from rice hull ash can be used as an environmentally safe thermal insulation material.

Publications

  • Kalapathy, U., and Proctor, A. and Shultz, J. 2001. A lightweight thermal insulation material from rice hull silica: Production and properties. In review J. Chem. Tech and Biotech. In review.
  • Kalapathy, U., and Proctor, A. and Shultz, J. 2001. An improved method for production of silica from rice hull ash. Biores. Technol. In review.