DESC:
TECHNICAL FIELD

The present disclosure relates to synthesis of silica from rice husk ash, and particularly relates to synthesis of silica from rice husk ash using a zero solid waste technology.

BACKGROUND OF THE DISCLOSURE

Rice is one of the major crops grown through the world. Once the paddy is separated from the rice grain, the kernel is removed from rest of the grain. This constitutes about, one third of the total mass of grains, commonly termed as “Rice Husk” or “Rice Hull”. Rice husk is an agricultural residue abundantly available in rice producing countries. Rice husk is generally not recommended as cattle feed since its cellulose and other sugar contents are low. Furfural and rice bran oil are extracted from rice husk. Industries use rice husk as fuel in boilers and for power generation. Among the different types of biomasses used for gasification, rice husk has a high ash varying from 18-20%. Rice Husk Ash (RHA) is the product of incineration of rice husk. Most of the evaporable components of rice husk are slowly lost during burning and the primary residues are the silicates.

The characteristics of the RHA are dependent on (1) composition of the rice husks, (2) burning temperature, (3) burning time and (4) type of boiler. Rice husk ash is a by-product from power / energy plants that use rice husk as a source of fuel. Other plants involved in use of sustainable technologies or turning waste-to-wealth by substituting fossil fuels with rice husk, are also generators of rice husk ash, which is a waste and requires disposal. Rice husk ash is a rich source of silica, which is an essential raw material for several industries, notably the tyre industry. The existing processes for synthesizing silica from rice husk produce undesirable wastes. Hence, there is a need for a process that synthesizes silica from rice husk ash producing zero waste.

SUMMARY OF THE DISCLOSURE

A method for producing precipitated silica from different types of boilers is disclosed. The method includes processing of rice husk ash (RHA) that has cristobalite in a range of 1 to 50 wt%, quartz in a range of 1 to 10 wt% and amorphous silica in a range of 50 to 98 wt% generated from three different type of boilers. The rice husk ash is heated to a temperature of 900-1025oC to remove any residual carbon, moisture, or any other volatile substance. Caustic soda flakes of 97% purity are added to hot water in a reactor along with the desired quantity of RHA with continuous agitation. The contents are leached at a temperature of 75 to 95oC. The resulting slurry containing sodium silicate in solution and unreacted solid is vacuum filtered to separate a solid fraction called as leach cake and sodium silicate solution. The leach cake comprises of unreacted RHA. The cake is washed with water and dried in an oven at a temperature of about 110 to 120oC. The sodium sulphate solution is first diluted with hot water (70-90oC) and then sulphuric acid having a molarity in a range of 6 to 9M is slowly added with continuous agitation under hot conditions to precipitate silica of required purity and morphology. The pH is maintained at 5.7-6.0 during the reaction. The precipitated silica is filtered to separate silica from filtrate which contains sodium sulphate. Silica is washed and dried to obtain precipitated silica having purity in a range of 98 to 99.6%. Sodium sulphate solution is evaporated and crystallized to obtain high grade sodium sulphate crystals.

BRIEF DESCRIPTION OF DRAWINGS

The disclosed process will be described and explained with additional specificity and detail with the accompanying figures in which:

Fig. 1 is a schematic representation of a flowsheet of a process for synthesizing silica from rice husk ash, according to an embodiment of the disclosure; and

Fig. 2 illustrates particle size distribution of a typical silica product.

Further, persons skilled in the art to which this disclosure belongs will appreciate that elements in the figures are illustrated for simplicity and may not have necessarily been drawn to scale. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the figures by conventional symbols, and the figures may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the figures with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiment illustrated in the figure and specific language will be used to describe them. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Such alterations and further modifications to the disclosure, and such further applications of the principles of the disclosure as described herein being contemplated as would normally occur to one skilled in the art to which the disclosure relates are deemed to be a part of this disclosure.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such a process or a method. Similarly, one or more devices or sub-systems or elements or structures or components preceded by "comprises... a" does not, without more constraints, preclude the existence of other devices, other sub-systems, other elements, other structures, other components, additional devices, additional sub-systems, additional elements, additional structures, or additional components. Appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.

Embodiments of the present disclosure will be described below in detail with reference to the accompanying figure.

The present technology for extracting silica from rice husk ash produces no waste, is environmentally sustainable and cost effective that makes it possible to implement these end-of-pipe technologies in manufacturing / energy sector plants. The process developed is economically viable and can be used for variety of RHA generated from different kind of boilers. Grade of silica produced is consistent irrespective of type of boiler and is of superior quality. The silica produced has a narrow range particle size distribution (as shown in Fig.2), indicating uniformity in particle size. The process developed has zero waste discharge as the sodium sulphate solution after recovery of silica is subjected to evaporation and water is recycled back to process. Further the solid generated after leaching can be used as an adsorbent.

Three types of rice husk ash samples were used for making precipitated silica. The total silica content in the samples varied between 83 to 94%. The XRD patterns showed cristobalite in a range of 1 to 50 wt%, quartz in a range of 1 to 10 wt% and amorphous silica in a range of 50 to 98 wt%. Bench tests were conducted at 100g scale by heating the RHA samples at 900-1025o C to remove carbon, free moisture and any other volatile material. These samples were leached in a glass reactor in alkaline medium using caustic flakes as lixiviant. Silica present in RHA was converted to sodium silicate during leaching. Sodium silicate solution was first diluted with hot DI water and in a 2nd reactor and then sulphuric acid was slowly added to get precipitated silica. The precipitated silica was given thorough wash using deionized water till desired conductivity of wash solution was achieved. Washed silica was then dried to get fine powdery precipitated silica product.

The chemical composition of rice husk ash varies from sample to sample, which is due to different geographical conditions, type of paddy, climatic variation, soil chemistry and fertilizers used in the paddy growth. The carbon value and the silica form (amorphous or crystalline) in RHA depend on the process conditions used in the power plant.

The main feature of the developed process is the production of high purity silica having = 99.6% SiO2 for all three RHA samples. The silica recovery varied between 71.0%- 86.2%. The leach cake generated during the process may be used as an adsorbent in chemical industries or may find some other alternate usage. The developed process provides good opportunity to utilize RHA to produce amorphous silica which have several applications in different industries including high end applications in rubber industry.

A method 100 for producing precipitated silica is disclosed. At step 102, rice husk ash is incinerated to remove moisture, carbon, and any other volatile material. Step 102 is followed by step 104, wherein RHA is leached with sodium hydroxide solution at a temperature of 75 to 95oC to with continuous agitation for a period of 4 to 6 hours. At step 106, the slurry is vacuum filtered to separate a leach cake and filtrate containing sodium silicate solution. The leach cake comprises of unreacted RHA. Step 106 is followed by step 108 where the leach cake is washed with water and dried in an oven at about a temperature of 110 to 120oC. At step 110, the sodium silicate solution is diluted with hot deionized water at a temperature of 70 to 90oC. At step 112 silica is precipitated with slow simultaneous addition of sulphuric acid (6 - 9M) and dilute sodium silicate solution while maintaining a pH of 6. At step 114, the slurry is filtered to separate silica from sodium sulphate solution. Step 114 is followed by step 116, where silica is washed and dried to obtain precipitated silica of purity in a range of 98.0 - 99.6%. Further, at step 118, sodium sulphate solution is evaporated and crystallized to obtain sodium sulphate crystals.
The BET surface area of the final silica product may vary between 129-160 cm2/g. It is a function of molarity of sulphuric acid and time of addition during precipitation. The surface area can also be varied using combination of the two parameters. Also, at constant addition time of 6h BET surface area was found to be 159.32 cm2/g when 6M sulphuric acid was used. The specific gravity of precipitated silica can vary in a range of 2.10 to 2.20.
Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. The scope of embodiments is by no means limited by the specific examples described below. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible.
The following examples illustrate the process:

Examples 1 to 3
Three examples are being given to illustrate the process in 100 g scale for three RHA samples.
The chemical composition of the samples used are given in Error! Reference source not found..

TABLE 1 Chemical composition of sample 1, Sample 2 and Samlple3 ________________________________________________________________________
% metal/compound
Sample1 Sample 2 Sample 3
SiO2 (Total) 93.27 94.93 93.74
SiO2 (Reactive) 73.90 71.53 59.36
Al 0.093 0.071 0.547
Fe 0.088 0.102 0.085
Mn 0.078 0.085 0.090
Ca 0.303 0.271 0.291
Pb 0.001 0.015 0.014
Mg 0.433 4.903 0.333
Zn 0.013 0.015 0.008
Cu 0.004 0.007 0.003
LOI @ 1025 oC 3.750 6.090 5.460
Particle Size, (-) 100 Mesh 37.72 60.48 36.29

The results showed highest percentage of reactive silica for samples 1 and 2 were almost same while the 3rd sample had least reactive silica. The XRD patterns had shown that there are three major phases present in the RHA samples, namely cristobalite, quartz and amorphous silica. The percentage of these three phases in samples 1,2 and 3 are given in Error! Reference source not found..

TABLE 2 Percentage of various phases as determined from XRD patterns
Parameter
Sample 1 Sample 2 Sample 3
% Cristobalite 2.00 10.00 47.00
% Quartz 3.00 6.00 -
% Amorphous 95.00 84.00 53.00

As is apparent, sample 3 had low reactive silica because amorphous silica is only 53%. All three samples were roasted at 1025oC for 2 hr. Step 102 is followed by step 104, a leaching step, where caustic soda flakes (as lixiviant) of 97% purity were used to make the leachant for RHA samples. Leaching was conducted in a glass reactor where recycled wash solution was taken and then slowly targeted quantity of caustic soda flakes having 97% purity were added. Heater was switched on to raise the temperature to 90oC and when target temperature was achieved the rice husk ash (RHA) was slowly added under continuous agitation. Time zero was started after complete addition of RHA. During leaching sodium silicate solution is produced as per the following reaction.

2xNaOH + ySiO2 ? xNa2O·ySiO2 + xH2O

TABLE 3 Leaching Parameters Maintained for all Three Samples

Parameter Value
Lixiviant NaOH
NaOH Molarity 1.43 M
Solids, wt/wt 10.0 %
Reaction Time 4.0 hour
Reaction pH 11.0-12.0
Temperature 90oC
Degree Of Agitation Medium

After completion of reaction time, heater was switched off and the agitation was stopped. All required process parameters pH, Eh (Redox Potential), temperature were closely monitored during the test and recorded.
Step 104 was followed by step 106 where the leach cake and filtrate containing sodium silicate solution were separated through vacuum filtration using Buchner filter. At step 108, the leach cake was given a thorough wash using DI water. Wet cake was later dried in an oven at a temperature in a range of 110oC. Sodium silicate solution was separately preserved for silica precipitation. Comparative analysis of sodium silicate solution is shown in Error! Reference source not found..

TABLE 4 Chemical analysis of sodium silicate solution for the three samples
Sample 1 Sample 2 Sample 3
Si g/L 43.54 44.43 37.62
Al ppm ND 0.870 0.189
Ca ppm 1.635 1.940 1.250
Mn ppm 0.364 0.990 0.364
Fe ppm 1.182 1.770 1.182
__________________________________________________________
Sodium silicate produced during leaching was treated with sulphuric acid to precipitate silica. The conditions maintained during precipitation are given in Table 5.

Table 5 Conditions for silica precipitation
Parameters Value
Lixiviant H2SO4
Feed H2SO4 Concentration 9.0 M
Reaction Temperature 80 – 82 0C
Reaction Time 2.0 hour
Reaction pH 5.7 - 5.8
Degree Of Agitation intense

In order to control the particle morphology initially a fixed quantity of deionised (DI) water is first taken in a glass reactor and heated to 80oC at step 110. Step 110 was followed by step 112, where sodium silicate solution and H2SO4 were simultaneously added while maintaining the slurry pH at 5.7-5.8. The molarity of sulphuric acid is 9M. After completion of reaction, slurry was filtered, and precipitated silica was separated at step 114. The precipitated silica was then washed with DI water till desired conductivity of wash solution was achieved. The washed cake was dried in an oven at 108oC till constant weight was achieved at step 116. The following chemical reaction takes place during precipitation:

xNa2O·ySiO2 + xH2SO4 ? ySiO2 + xNa2SO4 + xH2O

Percentage recovery during leaching and precipitation of silica are given in table 6.

Table 6 % Recovery of Silica during Leaching and Overall Recovery for Three Samples
Sample 1 Sample 2 Sample 3
Silica Recovery during leaching 80.60 86.40 72.10
Overall Silica Recovery 80.48 86.17 71.05

The precipitated silica produced for all three types of raw materials were dried in an oven at 108oC for 12 hours till constant weight was achieved. The chemical analysis of the final product is given in Table 7.

Table 7 Chemical Analysis of the Precipitated Silica
Sample 1 Sample 2 Sample 3
SiO2 % 99.73 99.69 99.74
Fe ppm 8.00 8.00 7.00
Cu ppm 2.00 1.00 1.00
Mn ppm 2.00 2.00 1.00

It is clearly observed that irrespective of initial raw material (RHA) used or crystallinity of the material, high pure amorphous silica is obtained which is the novelty of the present process.
Further, the filtrate containing sodium sulphate during this process was sent to sodium sulphate evaporation plant for production of sodium sulphate crystals and water is recycled back to process. The solution was taken for evaporation and crystallization at step 118 where sodium sulphate crystals were formed. The sodium sulphate solution formed during silica precipitation for all raw materials were separately subjected to evaporation and crystallization to get respective weights of sodium sulphate crystals. The wet sodium sulphate crystals were dried at a temperature of 108oC. The process condensate generated during evaporation is recycled back to process. Sodium sulphate evaporation was done in the lab using a heating mantle to boil the solution and saturated solution was crystallized using ice. In actual practice multiple effect evaporators operating under vacuum may be used for high steam economy. Sodium sulphate crystals have a purity of 98.35 and 98.38% for samples 1 and 2 respectively as given in Table 4. Sodium sulphate crystals were analyzed, and results are illustrated in Error! Reference source not found.8. No major impurities were observed in crystals.
Table 8 Purity of sodium sulphate crystals
purity Sample 1 Sample 2
element 98.35% 98.38%
% Mg 0.0202 0.0388
% Fe 0.0033 0.0045
% Co 0.0011 ND
%Cu 0.0030 0.0032
% Ca 0.1026 0.1731

For all three samples precipitated silica purity was found to be much higher than the purity required by the rubber industries. The foregoing examples clearly illustrate that the present process yields high purity amorphous silica product and high purity sodium sulphate by-product. The leach residue can be used as an adsorbent thereby generating no waste materials.
The figures and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.
,CLAIMS:
1. A method (100) for producing precipitated silica from Rice Husk Ash containing different % ages of reactive silica, the method comprising:

incinerating (102) rice husk ash (RHA) at 900 to 1025oC comprising of Cristobalite in a range of 1 to 50 wt%, quartz in a range of 1 to 10 wt% and amorphous silica in a range of 50 to 98 wt%;

leaching of RHA (104) with sodium hydroxide solution while maintaining continuous agitation and heating temperature of 75 to 95oC to form a leach slurry;

vacuum filtering (106) the slurry to separate a leach cake and sodium silicate solution, wherein the leach cake comprises of unreacted RHA;

washing (108) the leach cake with water and drying in an oven at about a temperature of 110 to 120oC;

adding (110) sodium silicate solution and sulphuric acid simultaneously to the deionized hot water for a period of 3 to 6 hours to form a slurry while maintaining (112) a pH of 6, wherein the sulphuric acid has a molarity in a range of 6 to 9M, and wherein the slurry comprises silica and sodium sulphate solution;

filtering (114) the slurry to separate silica from sodium sulphate solution;

washing and drying (116) silica to obtain precipitated silica of purity in a range of 98 to 99.6%; and

evaporating and crystallizing (118) sodium sulphate solution to obtain sodium sulphate crystals.

2. The method (100) as claimed in claim 1, wherein the silica is recovered from RHA in a range of 70 to 90%.

3. The method (100) as claimed in claim 1, wherein drying silica is carried out in an oven or by spray drying.

4. The method (100) as claimed in claim 1, wherein the silica has a surface area in a range of 120 to 160 m2/gm and a specific gravity of 2.10 to 2.20.

5. The method (100) as claimed in claim 1, wherein the silica has an iron content of 50 to 100 ppm.

6. The method (100) as claimed in claim 1, wherein sodium sulphate crystals are dried at a temperature in a range of 100 -110oC.

7. The method (100) as claimed in claim 1, wherein sodium sulphate crystals have a purity in a range of 98 to 99%.

8. The method (100) as claimed in claim 1, wherein the leach cake acts as an adsorbent.

9. The method (100) as claimed in claim 1, wherein the water used for washing the leach cake is recycled to ensure a zero-waste method.