1 EDITORIAL Rice husk is an agricultural residue abundantly available in rice producing countries around the world. India alone has produced around 31 million tons of rice husk and thus generated 4.65 – 5.58 million tons (15-18% of rice husk) of Rice Husk Ash (RHA) in 2014. The major States under rice cultivation in India are West Bengal, U.P., Andhra Pradesh, Punjab, Odisha etc. During processing of paddy, rice husk is being produced during de-husking operation. This rice husk finds its application as fuel mainly in industrial boilers being used in dyeing industry, biomass based power plants, paper mills, rubber industry, par-boiled rice mills etc. for steam generation required for various process applications. In Punjab, Ludhiana - the hub of the Indian Hosiery industry - is consuming around 4.50 lakh MT of rice husk as fuel in industrial boilers, thus generating around 0.7 – 0.8 lakh MT of RHA annually. Presently, this RHA is being disposed off in low lying areas and along road sides which leads to deterioration of ambient air quality due to low bulk density thus posing problems to the nearby residents. The quantum of RHA generation is substantial but did not find any productive use in any of the manufacturing process. The current issue of newsletter documents a study “Techno-Economic Feasibility Study for Silica Recovery from Rice Husk Ash” carried out by Punjab State Council for Science & Technology (PSCST) with support from Punjab Pollution Control Board in 2015. This article highlights the outcome of study, different technologies available for silica recovery from RHA along with its cost economics for the scientific management of RHA. Published by Editorial Team Punjab ENVIS Centre Punjab State Council for Dr. Jatinder Kaur Arora Article Prepared by Science & Technology (PSCST), Chandigarh, INDIA Dr. S.S. Ladhar Consultancy Cell, PSCST Mr. Gurharminder Singh Er. Pardeep Kumar Garg Ms. Ravleen Singh Sponsored by Ms. Inderdeep Gill Er. Mandeep Singh Ministry of Environment, Forests & I.T. Assistance Climate Change, Government of India Mr. Dinesh Kumar ENVIS Centre, PSCST is a partner in Regional Centre of Expertise (RCE) Chandigarh on Education for Sustainable Development (ESD) of United Nations University - Institute of Advanced Studies, Japan. This article recommends to promote the technologies to recover silica for scientifc disposal of RHA within the region for sutainable living. 2 Introduction Rice Husk (RH) is one of the most widely available Table 1. Typical Analysis of Rice Husk agricultural wastes in many rice producing countries Property Range around the world. It is majorly prevalent in South-East Asia because of the high rice production in this area due Bulk density (kg/m3) 96 - 160 to its fertile land and tropical climate. Hardness (Mohr’s scale) 5-6 During milling of paddy, about 80 % of weight is received Ash % 22 - 29 as rice, broken rice and bran. Rest 20 % of the weight of Carbon % ~ 35 paddy is received as husk. RH contains 75-90% organic Hydrogen % 4 - 5 matter (cellulose, lignin, etc) and rest mineral Oxygen % 31 - 37 components (silica, alkali) & trace elements. A typical Nitrogen % 0.23 - 0.32 analysis of RH is shown in Table 1. The content of each constituent depends on rice variety, soil chemistry, Sulphur % 0.04 - 0.08 climatic conditions and even the geographical location. Moisture 8-9 The rice milling industry generates a lot of RH that creates Source: Kumar et al., 2012 disposal problem due to its less commercial interest. The handling and transportation is also a problem due to its its ash is the alternative solution to disposal problem low density. Thus, the RH is mostly used as a fuel in the (Kumar et al, 2012). It is estimated that for every 1000 boilers (to generate steam for the parboiling process) for Kgs of paddy milled, about 200 Kgs of husk is produced, processing of paddy. and when this husk is burnt in the boilers, about 50 Kgs When RH is burnt, Rice Husk Ash (RHA) is generated. On of RHA is generated (www.ricehuskash.com). burning, cellulose and lignin are removed leaving behind The use of RHA mainly depends on chemical composition silica ash. Under controlled conditions (temperature and of ash predominantly silica content in it. Due to presence environment) the burning yields better quality of RHA of large silica content in ash, extraction of silica is possible. determined by particle size & surface area. Completely There is a growing demand for fine amorphous silica in burnt RHA is grey to white in color, while partially burnt the production of special cement and concrete mixes, RHA is blackish. high performance concrete, high strength, low RHA is an environmental threat causing damage to land permeability concrete, for use in bridges, marine and surrounding area where it is dumped. The improper environments, nuclear power plants etc. Further, it can disposal may cause pollution in addition to being a health be used in a variety of applications as enumerated in and traffic hazard. Therefore, commercial use of RH and Plate 1. Loading of Rice Husk Ash in trolley Disposal of Rice Husk Ash along roadside 3 Main by-products of Rice Parts of Paddy Potential Use of Rice Husk Common - products from Rice Husk Rice Husk (loose form) Rice Husk briquettes Rice Husk pellets Rice Husk ash Carbonized Rice Husk Source: www.knowledgebank.irri.org 4 surface area silica or active silica (AS) from RHA consists of a thermal treatment at various temperatures. The aim of this step is to increase the relative amount of silicon oxide by reduction of carbonaceous materials present in the samples, as well as to burn out other undesirable components detected by chemical analysis. Heating cycles were carried out in air in an electric oven with a As per literature, there is a potential to recover silica heating rate of 10 0C/min. Each sample was held at a from RHA, which can be used in rubber & tyre industry, maximum temperature (400, 500, 600 or 700 0C) for 1, 3 construction industry etc. RHA contains the highest or 6 hours. The samples were cooled down inside the amount of biogenic silica in its amorphous form (in oven. The RHA sample after burning out at 7000C for 6 excess of 95 wt% silica, SiO ) (Kaupp, 1984; Kapur, 1985; hours represented the highest amount of silica (94.95%) 2 James and Rao, 1986) compared to other biomass as compared to the other samples. materials, such as ash from sugarcane bagasse (57 – 73% Subbukrishna et al., 2007 reported in their article SiO ) (Jenkins et al., 1996; Natarajan et al., 1998; Stephens 2 “Precipitated Silica from Rice Husk Ash by IPSIT (Indian et al., 2003). In addition, the percentage of ash in RH is Institute of Science Precipitated Silica Technology) many times higher (at 13 – 25 wt%, dry basis) (Jenkins et process” that experiments have been carried out al., 1998; Natarajan et al., 1998; Armesto et al., 2002) successfully under lab scale and pilot scale to extract the compared to that of sugarcane bagasse (at only 1.9 – 6.8 silica from Rice Husk ash. This not only provides value wt%, dry basis) (Jenkins et al., 1998; Natarajan et al., addition but also solves the problem of large amount of 1998; Das et al., 2004). A brief of the various R&D ash disposal. The patented Indian Institute of Science activities on silica recovery from RHA is outlined as under: Precipitated Silica Technology (IPSIT) developed at CGPL, Mittal D., 1997 in his article “Silica from Ash” presented IISc, Bengaluru is a novel method of extracting that RHA is one of the most silica rich raw materials precipitated silica in commercially viable way from RHA. containing about 90-98% silica (after complete combustion) Thuadaij and Nuntiya, 2008 reported in their article that among the family of agro wastes. The chemical process the RHA sample after being burnt at 700 0C for 6 hours discussed not only provides a solution for waste disposal presented higher silica content compared to the other but also recovers a valuable silica product, together with sample at 700 0 C for 3 h. The RHA sample after being certain useful associate recoveries. extracted by 2.5 N sodium hydroxide generated the yield Kalapathy et al., 2000 reported that RHA, a waste of pure silica up to 90.3%. The concentration of sodium product of the rice industry is rich in silica. A simple hydroxide had strong effect on the dissolution of silica method based on alkaline extraction followed by acid from RH and it also removed some impurities which were precipitation was developed to produce pure silica not dissolved from the main product xerogels from RHA, with minimal mineral contaminants. Mamdouh et al., 2015 in their article presented the The silica gels produced were heated to 800°C for 12 preparation of RHA by Open-field burning and by hours to obtain xerogels. Silica and mineral contents of combustion at 1000 0C for 4 hours. X-ray diffraction xerogels were determined by Energy Dispersive X-ray studies of produced RHA revealed that increase in (EDX) and Inductively Coupled Plasma (ICP) emission temperature of burning will increase the crystal growth spectrometers, respectively. Xerogels produced from rate with the occurrence of different varying degrees of RHA had 93% silica and 2.6% moisture. The major quartz (Q), Cristobalite (C) and Tridymite (T). The purity impurities of silica produced from RHA at an extraction concentration of silica in RHA samples were measured by yield of 91% were Na, K, and Ca. Acid washing prior to X-Ray Fluorescence and found to be in the range of 82.7- extraction resulted in silica with a lower concentration of 91.6 % with major impurities of K O, P O and Cr O . RHA Ca (<200 ppm). However, final water washing of the 2 2 5 2 3 produced via open-field burning was treated with an xerogel was more effective in producing silica with lower activating reagent sodium hydroxide (3.5 mol/L), the overall mineral content. activated RH so obtained was heated at a temperature of Della et al., 2002 in their article describes the processing 900°C to get activated carbon. Silica was precipitated and characterization of high specific surface area silica from sodium silicate by acidification using sulphuric acid from RHA. The first step for producing a high specific with a % yield of 97% of extracted silica. 5 Production of Rice Husk in Punjab In the year 2015, Punjab State Council for Science & Technology carried out a project titled “Techno-Economic Punjab holds place of pride among the Indian States for Feasibility Study for Silica Recovery from Rice Husk Ash” its outstanding achievements in agricultural development. supported by Punjab Pollution Control Board (PPCB). The Over the years, Punjab is amongst the highest ranking study focused on Ludhiana which is identified as one of states in terms of productivity of rice. Punjab Agriculture the critically polluted area by Ministry of Environment, Department has estimated that its paddy (basmati and Forests & Climate Change (MoEFCC), GoI vide office non-basmati rice varieties) production is around 186 lakh memorandum J-11013/5/2010-IA (I) dated 13/1/2010. tonnes for the 2016-17 kharif season which is nearly 10 During the study, a team of PSCST Engineers visited lakh tonnes more than last year’s production. During various industrial clusters and 2-3 dyeing units of each 2014-15 the paddy yield in state (3838 Kgs per hectare) cluster was studied. It was observed that dyeing industry was highest in the country and contributed 24.2 % of rice is the major sector, where Rice Husk is used as fuel in to Central Pool during the same year. The national and boilers for steam generation. the global rice production is given in Box 1. It was further observed that the industries have installed In Punjab, Ludhiana which is known as the hub of Indian fire tube/ water tube boilers manufactured by Hosiery industry is consuming around 4.50 lakh MT of RH M/s Thermax Limited, M/s Cheema Boilers Limited and as fuel in industrial boilers, thus generating around 0.7 – M/s Misra Boilers Pvt. Ltd. The boiler capacity in small 0.8 lakh MT of RHA annually. Presently, this RHA is being scale units varies from 2- 10 T/hr generating steam at a disposed off in low lying areas and along road sides which pressure of 10 -12 kg/cm2. The steam is mainly being leads to deterioration of ambient air quality due to low supplied for various process applications at a pressure of bulk density thus posing problems to the nearby residents. 5 - 8 kg/cm2. The quantum of RHA generation is huge but did not find The boiler system consists of feed water system, fuel any productive use in any of the manufacturing process. feeding system and steam distribution system. The feed Box 1. Production of Rice and Rice Husk: A Global & National Scenario As per Food and Agricultural Association of the United Nations, the worldwide production of rice in 2014 was around 740 million tons hence 148-185 million tons (20-25% of crop approx.) of RH was produced. As per Indian National Agricultural Research Systems of ICAR-IARI, the country has largest area under paddy in the world and ranks second in the rice production after China (www.krishikosh.egranth.ac.in). Further, in 2014, China and India have produced 208 and 157 million tons of rice, respectively. Thus, India and China together account for 50% of global rice production. Together with Indonesia, Bangladesh, Vietnam, Myanmar, Thailand, Philippines, Japan, Pakistan, Cambodia, Republic of Korea, Nepal and Sri Lanka, Asian countries account for 90% of the world’s total Map 1 : Top 5 Rice Producing Countries in the World rice production. Other major non-Asian rice producing countries include Brazil, United States, Egypt, Madagascar and Nigeria, which together account for 5% of the rice produced globally. The top 5 rice producing countries in the world in 2014 are as shown in Map 1. India alone has produced around 31 million tons of RH and thus generated 4.65 – 5.58 million tons (15-18% of RH) of RHA in 2014 (Department of Agriculture and Cooperation, GoI, 2015). The major States under rice cultivation in India are West Bengal, U.P., Andhra Pradesh, Punjab, Odisha etc. 6 Fig 1 . FBC furnace integrated with boiler water system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair. The steam distribution system collects and controls the steam produced in the boiler. Steam is distributed through a piping system to the point of usage. A schematic view of the Fluidized Bed Combustion (FBC) furnace along with boiler is shown in Fig 1. Rice Husk Ash Generation Graph 1. Industries using different Fuel There are around 277 industries in Ludhiana which are using boilers (mainly FBC) for their process requirement. Out of these industries 11% are large scale units, 5% medium scale and remaining 84% are small scale units. Rice Husk, Pet Coke, Mustard Straw, Dung Cake, Wood etc are used as fuel. The industry-wise fuel utilization pattern reveals that 37.54% industries are using RH, 17.68% industries are using pet coke and remaining 44.76% industries are using agro waste / wood / dung cake etc. (Graph 1). Industries utilizing RH as fuel in FBC generates RHA which Graph 2. Category wise Rice Husk consumption is mainly collected from side doors of furnace & air pollution control device (SMEs have installed single cyclone/ set of cyclones to control air pollution). The water is sprayed on RHA so that it does not get air borne. The small and medium scale units are disposing off their RHA in low lying areas and along road sides. These units are paying Rs. 400-500/trolley to the contractor for disposing off moist RHA. The total consumption of RH in Ludhiana has been estimated as 1498 TPD with RHA generation of 225 – 270 TPD (15-18% of Rice Husk consumption) as shown in Graph 2 & Graph 3. Graph 3. Sector wise Rice Husk Consumption 7 Rice Husk Ash Collection and Disposal Mechanism Rice Husk Burning of Rice Husk in Furnace Use of Cyclone for collection of RHA Steam generation in Dyeing Industry RHA collection in Dyeing Industry Transportation of RHA by tractor-trolleys Disposal of RHA along road sides 8 Table 2. Characteristics of Rice Husk Ash in Study Area Sr. Sample 1 Sample 2 Sample 3 Sample 4 Parameters Unit No. (Ludhiana) (Dera Bassi) (Nawanshaher) (Ludhiana) 1. Silica % 82.82 84.31 80.20 81.24 2. Fixed Carbon % 3.40 0.90 3.32 7.69 3. Volatile Matter % 2.61 2.20 3.03 3.00 4. Moisture % 0.82 0.85 1.50 1.01 Characteristics of Rice Husk Ash During the meet, it was revealed that the estimated demand of amorphous precipitated silica across the globe is 2.2 MMT per annum with annual growth rate of The quality of RHA mainly depends upon its chemical 4-6%, whereas highly dispersible silica grade of composition, pre-dominantly silica content. The silica precipitated silica has market share of 20% of total content & its mineralogical structure depend upon the amorphous precipitated silica. Demand for this grade of combustion time, temperature and turbulence during silica is expected to increase at growth rate of 12% during combustion. During the study, 4 samples of RHA were 2013-2020 (Graph 4). collected for analysis from the industries, consuming RH in large quantities such as dyeing, dyeing with co- generation facility and tyre & tube manufacturers. The s n silica content in RHA was found to be in the range of 80- To n 1.00 84% indicating scope for silica recovery with details in o Table 2. Milli 0.80 n d i 0.60 n Identification of Technology Providers ma 0.40 e D al 0.20 b A stake holder meeting was organized on August 6, 2015 o Gl 0.00 by PSCST, wherein identified technology providers made 2013 2014 2015 2016 2017 2018 2019 2020 Year a presentation on their technology for silica recovery from RHA. Graph 4. Global demand of Highly Dispersible Silica (HDS) Stakeholders Meet at Chandigarh 9 Technological Options to Box 2: Major Components for Silica Production Recover Silica from RHA The major components required for the production of silica from RHA are enlisted below: In the conventional process, silica is recovered from silica • Reactor and Storage Vessels sand. The sodium silicate is manufactured by fusing pure • Filter Press silica sand with soda ash in rotary furnace at 1300°C. • Spray Drier When the mould cools down, a clear glass of sodium • Hot Air Generator silicate is obtained. The sodium silicate produced in the • Nano-filtration first stage undergoes acid precipitation to produce • Multi Effect Evaporator precipitated silica. This technique is highly energy • Crystallizer intensive as it requires the reactants to be heated to • Boiler elevated temperatures. However, the study reveals that silica can be recovered Carbonation Route from waste material (RHA) either by carbonation route The Silica recovery from RHA consists of three stages or acid route. The silica recovery from RHA is in the range named as digestion, precipitation and re-generation. The of 60-65%. The brief process details of both the first stage involves the digestion of RHA with caustic soda technologies are detailed as under and the major at specific conditions. In this stage, silica present in the components required for the production of silica from RHA are given in Box 2. ash gets extracted with caustic soda to form sodium silicate solution. After the completion of the digestion, the solution is filtered for the residual undigested ash Fig 2. Process Flow Diagram of Carbonation route 10