Hazard Assessment of Rice Husk Ash Generated from Rice Mills in Bangladesh and Way Forward to Overcome or Adopt the Hazards

Rice husk (RH), an agricultural waste, is abundantly available in Bangladesh, where the major agricultural crop is paddy consisting of 73.94 percent of all crops. Bangladesh produced 37.61 million metric tonnes of paddy in 2020-21, among which the volume of rice husk was 7.52 million metric tonnes. These are mainly used for low-value applications such as fuel in the parboiling of the rice mills, where the burning of rice husk produces more than 5 million metric tonnes of rice husk ash (RHA). The disposal in landfills or open fields causes serious environmental and human health hazards due to the low bulk density of RHA. As silicon dioxide (SiO2) has the highest amount of presence in the rice husk ash, long-term exposure can lead to lung cancer, silicosis, chronic obstructive pulmonary disease, or tuberculosis. This research primarily focuses on the feasible and cost-effective technology to convert the rice husk ash into environment-friendly valuable products namely amorphous silica, crystalline silica, and porous silica, apart from ash’s usage as construction material, pesticide, or soil improvement material.


Introduction
Rice is one of the most consumed starchy cereal grains, where almost half of the world's population depends on rice.Based on the statistics of the year 2020 of the Food and Agriculture Organization (FAO) of the United Nations, Bangladesh was the third highest rice-producing nation in the world after China and India.Occupying 96 percent of the cereal agriculture land, rice is the staple food for Bangladeshi people and it provides 48 percent of the rural employment [1].While producing rice, around 16 to 25 percent of its weight is generated as a by-product, named husk or hull, which is the outermost shell of the rice seed and acts as a protective layer [2].Based on the information provided by the Bangladesh Bureau of Statistics (BBS), Bangladesh produced 37.61 million metric tonnes of paddy in 2020-21.Among these, 20 percent is considered rice husk, which equals 7.52 million metric tonnes.The production of rice as well as rice husk is steadily increasing in Bangladesh.Despite the massive amount of annual production, rice husk has been used or recycled only for lowvalue applications in Bangladesh.At present, almost 90 percent of the total rice is processed by a parboiling system, and the rest is produced by a non-parboiled system.Again, 70 percent of the total 1305 (2024) 012016 IOP Publishing doi:10.1088/1757-899X/1305/1/012016 2 parboiled rice is processed in conventional small and medium-sized rice mills (locally known as "Chatals") which use inefficient traditional boilers (with a thermal efficiency of 20 to 30 percent).Thus, these low-efficient rice husk-fired boilers are posing safety hazards and creating significant air pollution.In contrast, Improved Rice Parboiling System (IRPS) and rice husk-based auto rice mill boilers can attain a thermal efficiency of 45 percent and 65 percent respectively, and thus reduce the waste and environmentally demonstrate less pollution.There are around 50,000 Chatals and 500 auto rice mills in Bangladesh [3].Burning of rice husk produces rice husk ash (RHA) during rice parboiling by rice mills.These ashes are usually dumped in open places near the rice mills.Due to extremely small particle sizes (59 percent of particles having sizes less than 0.05 mm) and the high silica content of the ashes (more than 90 percent of the total content) [4], these can easily fly around and pose a serious threat to the public health scenario.Long-term exposure to silica may cause lung cancer, silicosis, chronic obstructive pulmonary disease, and tuberculosis [5].Identifying the main clusters of rice as well as rice husk or ash generation, this research will primarily focus on developing cost-effective technology to convert the rice husk and its ash into environmentfriendly valuable products namely amorphous silica, crystalline silica, and porous silica.

Rice Husk Ash Generation Mapping in Bangladesh: Hazard Location Identification
Based on Bangladesh Bureau of Statistics data on rice generation in the year 2020-21, three mappings are created to show district-wise rice husk ash generation in three different seasons, known as Aush (April to July), Aman (July to November) and Boro (November to April).From the mapping, Dinajpur, Mymensingh, Bogura, Naogaon, Rangpur, and Jashore, are found as the highest ash generation areas.Hence, these areas can be considered suitable zones for the set-up of rice husk ash recycling or processing industry.

Literature Review
No study has been conducted yet regarding the adverse impact of rice husk ash in Bangladesh.However, a study conducted in Vietnam showed that 66 percent of the residents near the rice mills had respiratory problems, 26 percent of the rice mill workers had skin problems and crop production declined by 20 percent due to the ash covering the plants [6].Another study in Nigeria showed the presence of heavy metals in the rice husk ash, such as Chromium, Nickel, Copper, Zinc, and Arsenic, which adversely impact the environment [7].Rice husk is a renewable agricultural waste and a major source of amorphous silica, which is used in the production of semiconductor circuits to isolate different conducting regions [8].Nanostructured silica powder can be prepared from rice husk by acid or alkaline leaching, followed by calcination at higher temperatures [9], electrodeposition and reduction [10], and acid precipitation [11].Rice husk ash is predominantly silica together with some minor oxides.The nature of this silica depends on the burning temperature.Controlled combustion of rice husk produces reactive silica which is suitable for making pozzolana cement [4].It can also be used as a filler material to enhance the strength of the concrete blocks [12].Rice husk ash can be used for the production of precipitated silica, which can be used in different industries like the tire industry, food industry, paint industry, cosmetics, etc.It can also be used as a reinforcing agent in rubber, a thickening agent in paints, anti-caking agent in the food industry.Based on different silica grades, characterized by surface area, tapped density, oil adsorption capacity, silica content, etc., all these different applications can be achieved [13] [14].Rice husk ash can also be utilized for the production of various advanced materials, such as Silicon Carbides, which is a third-generation semiconductor material, used to fabricate high-temperature, highpower, and high-frequency feature-enabled electronic and optoelectronic devices.It can also be used for the production of Silicon Nitride, which can be used in a variety of structural, electronic, and orthopedic applications due to its high strength, fracture toughness, hardness, wear resistance, and good chemical and thermal stability [15].Besides all these high-value conversion processes of rice husk ash, it can also be utilized directly in various other applications such as soil stabilization for different types of soil.Combined with different chemical binders and waste materials, it can ensure soil improvement [16].Its use is also found in the simultaneous removal of pesticides from contaminated water as a low-cost and widely available adsorbent [17].

Methodology -Rice Husk Ash to Amorphous Silica: A Feasible Solution from Bangladesh Perspective
An integrated rice husk ash processing plant can be used for amorphous silica production.A schematic representation of the Rice Husk Ash to Amorphous Silica Production process is shown in Fig 4 .A brief description of the process is elaborated below: • Rice husk ash will be fed into a digester with sodium hydroxide (NaOH) solution.Agitation will be performed at a temperature of 110ºC in the digester.It will form a sodium silicate (Na2SiO3) solution with approximately 15 percent carbon black.• After the preliminary filtration with a centrifuge separator (small-scale plant) or filter press (medium or large-scale plant), a viscous, transparent, colorless Na2SiO3 solution will be separated.• In the next step, silica (SiO2) will be precipitated from this Na2SiO3 using sulfuric acid (H2SO4).
The temperature will be kept at 10ºC above the ambient temperature in this precipitation step with a more vigorous agitation than the digestion step.• After that preliminary filtration with a centrifuge separator (small-scale plant) or filter press (medium or large-scale plant), silica will be separated from the sodium sulfate (Na2SO4) solution.The resulting silica will be washed with 5 percent hydrochloric acid (HCl) to remove impurities like calcium oxide (CaO), magnesium oxide (MgO), etc.It is passed again through another filter system and then washed with water.
• The washed amorphous silica will be dried in a dryer at a temperature of 120ºC using a steam dryer (drum flaker).The resulting amorphous silica can be bagged for commercial selling.• Carbon black can be activated at 800ºC temperature and then can be sold as a commercial product known as Activated Carbon.• Separated Na2SO4 solution will be evaporated to derive Na2SO4 salt, which is commercially known and sold as Glauber's salt

Cost Benefit Analysis
An industrial setup for the conversion of rice husk ash to amorphous silica with a daily capacity of handling 1 tonne of rice husk ash will take approximately BDT 50 million.For the regular operation, it is considered that the expenditure will be BDT 500 thousand every month with regular raw material purchases at BDT 5 per kg.After the production, silica is assumed to be sold at BDT 80 per kg as a conservative approach, which is BDT 20 lower than the usual market price.Besides it is assumed that the price of the raw material and operation expenditure increases by 2 percent every year while the selling price increases by 1 percent every year.With the assumptions, it is found that the business model reaches break even in the third year, and from the fifth year, IRR goes above 20 percent, which indicates it is a profitable venture.At a 25-year lifetime, IRR reaches 34 percent.As illustrated, rice husk ash is a very low-value waste product and it is almost freely available in Bangladesh.From these ashes, it is highly feasible to produce metallurgical-grade silicon (MG-Si).Silica (SiO2) has a present market value of Tk 100 per kg.Then, through the plasma arc reduction process, silica can be transformed into Metallurgical Grade Silicon.In this process, Argon (Ar) gas is introduced into the furnace and the gas produces the plasma arc.As a result, the temperature of the interior of the furnace reaches 2000℃ to 5000℃.Henceforth, the mixture of Argon (Ar) gas, Silica (SiO2), and Hydrogen (H2) gas (Ar + SiO2 + H2) in definite proportion are charged into the furnace.

Rice Husk Silica to Various Grades of High-Value Silicon
Here, argon acts as a carrier gas, and hydrogen acts as the reducing element.Hydrogen reduces silica and produces the expected metallurgical-grade silicon, which is deposited at the bottom of the furnace as melt.Later on, it is poured out from the furnace.By this process, it can yield metallurgical grade silicon, whose purity can reach as high as 99.9%.It is to be noted that Metallurgical Grade Silicon, Solar Grade Silicon, and Electronic Grade Silicon are very important for solar cell production, renewable energy production (hydrogen fuel production), electronic devices production, different uses in the medical sector, and many other applications.

Comparison between Rice Husk Ash (RHA) Silica and Silica Sand
Rice husk ash is a waste but sand is a mineral.So, for the production of silicon, using rice husk ash will be more environmentally friendly than mineral silica.Mineral sand contains a lot of impurities that can lead to the formation of defects, and enhance the formation of dislocations, which act as recombination centers of photo-carriers and can compromise both mechanical and electrical properties as well as decrease the solar cell efficiency.Moreover, impurities in solar cells generally introduce allowed levels into the forbidden gap and thereby act as recombination centers, and an increased density of such centers decreases cell efficiency.More importantly, rice husk ash silica is free from boron and phosphorous contamination.Therefore, to produce Solar Grade Silicon less purification is required as compared to the Silica Sand.The boron content should be controlled to less than 1 mass ppm because it is one of the doping elements.However, it is also one of the most difficult elements to remove from silicon because its vapor pressure in molten silicon is too low to remove in the form of its elemental gaseous species.Furthermore, since the segregation coefficient is nearly unity, conventional directional solidification is useless.So, it is very clear that the boron content in MG-Si is not expected, which comes from the raw materials (sand/Quartz).If rich husk ash silica is used as the raw material for the production of MG-Si & SoG-Si, the presence of boron and phosphorous will be virtually zero allowing efficient and cost-effective production processes with generating manifold value-added products.

Conclusion
In this research, the production of different grades of silicon (Si) is proposed from just a waste or extremely low-value product.In each step of purification, a more valuable product is produced.From metallurgical grade silicon (98.5% to 99.9% purity), more valuable products such as solar grade silicon (99.9999% purity), electronic grade silicon (99.9999999% purity), pure silicon crystal, polished silicon

Figure 1 .
Figure 1.Rice husk ash generation in the Aush season (April to July) of the year 2020-21

Figure 2 .Figure 3 .
Figure 2. Rice husk ash generation in Aman season (July to November) of the year 2020-21

Figure 4 .
Figure 4. Rice Husk Ash to Amorphous Silica Production Schematic

Figure 5 .
Figure 5. Rice husk ash Silica to Metallurgical Grade Silicon (MG-Si) to Solar Grade Silicon (SoG-Si) to Electronic Grade Silicon to Silicon Wafer
The price of this Metallurgical Grade Silicon is Tk 225 per kg.To produce a more valuable product of 6N (99.9999% pure) Solar Grade Silicon (SoG-Si) by the purification of Metallurgical Grade Silicon, a rotating plasma arc furnace can be used.The market price of Solar Grade Silicon is Tk 1,250 per kg.By further purification of Solar Grade Silicon with the same rotating plasma arc furnace, Electronic Grade Silicon of purity 9N (99.9999999%) can be obtained, whose market price is Tk 7,000 per kg.It is possible to produce Pure single-crystal silicon from Electronic Grade Silicon by the crystal growth process, and its market price is Tk 45,000 per kg.By wafering and polishing, Pure Silicon Crystal can be transformed into a Polished Silicon Wafer.The present price of a Polished Silicon Wafer is Tk 275,000 per kg.From Polished Silicon Wafers, IC Chip Wafer can be produced and its market price is Tk 700,000 per kg.