METHOD FOR CONTROLLING THE PROPERTIES OF BIOGENIC SILICA

Abstract

Porous amorphous silica can be obtained from siliceous plant matter containing non-siliceous inorganic substances. The siliceous plant matter is soaked in an aqueous solution which includes a chelating agent. The chelating agent is present in an amount which helps to extract at least some of the non-siliceous inorganic matter. The aqueous solution is then separated from the siliceous plant matter. Beneficial properties are imparted to the siliceous plant matter by controlling the amount of at least one preselected non-siliceous inorganic substance in the siliceous plant matter. At the end of the process, the siliceous plant matter is heat treated in the presence of oxygen at a temperature to produce the resulting amorphous silica having the beneficial properties.

Claims

1-25. (canceled)

26. A process for production of porous amorphous silica from siliceous plant matter containing non-siliceous inorganic matter, the process comprising the steps of: a) manipulating the amount of at least one preselected non-siliceous inorganic substance in said siliceous plant matter by controlling the amount of the chelating agent, or by introducing mineral acids into said siliceous plant matter, or by mixing the desired amount of preselected non-siliceous inorganic matter into the siliceous plant matter, such that the amount of the preselected non-siliceous inorganic substance remaining in the plant matter is established at a preselected amount of from 20 ppm to 25,000 ppm, said preselected amount of the at least one preselected non-siliceous inorganic substance selected to control the surface area of the porous amorphous silica obtained after heat treatment within the range of 10 m.sup.2/g and 450 m.sup.2/g, the preselected non-siliceous inorganic substance including elements or compounds of alkali metals, alkali earth metals, aluminum, boron, iron, manganese, titanium, and/or phosphorus; and b) heat treating said siliceous plant matter in the presence of oxygen at a temperature in the range of 200° C. to 1,000° C. wherein the resulting silica is comprised of silica of porous amorphous form.

27. The process of claim 26, wherein the manipulating of the amount of the at least one preselected non-siliceous inorganic substance in said siliceous plant matter is by controlling the amount of the chelating agent.

28. The process of claim 27, wherein the chelating agent is selected from the group consisting of citric acid, acetic acid, ethylenediamine, ethylenediaminetetracetic acid, di mercapto succinic acid, trimethylaminetricarboxylic acid, alphalipoic acid, and diethylenetriaminepentaacetic acid.

29. The process of claim 28, wherein the amount of chelating agent is from 0.001 kg per kg of plant matter to 1 kg per kg of plant matter.

30. The process of claim 29, wherein the chelating agent is citric acid and the amount of citric acid is from 0.01 kg per kg of plant matter to 0.1 kg per kg of plant matter.

31. The process of claim 26, wherein the manipulating of the amount of the at least one preselected non-siliceous inorganic substances in said siliceous plant matter is by introducing mineral acids into said siliceous plant matter.

32. The process of claim 31, wherein said at least one mineral acid is selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, and perchloric acid.

33. The process of claim 26, wherein the manipulating of the amount of the at least one preselected non-siliceous inorganic substances in the siliceous plant matter is by mixing the desired amount of preselected non-siliceous inorganic matter into the siliceous plant matter.

34. The process of claim 26, wherein the surface area of said amorphous silica is controlled in a specified narrow band within the range of 10 m.sup.2/g and 450 m.sup.2/g by controlling the amount of said preselected non-siliceous inorganic matter in the siliceous plant matter and/or the heat treatment temperature.

35. The process of claim 26, wherein the pore volume of said amorphous silica is controlled in a specified narrow band within the range of 0.50 cc/g and 0.05 cc/g by controlling the amount of said preselected non-siliceous inorganic matter in the siliceous plant matter and/or the heat treatment temperature.

36. The process of claim 26, wherein the pore diameter of said amorphous silica is controlled in a specified narrow band within the range of 10 Angstroms and 200 Angstroms by controlling the amount of said preselected non-siliceous inorganic matter in the siliceous plant matter and/or the heat treatment temperature.

37. The process of claim 26, wherein the amount of wherein the amount of non-siliceous inorganic substances in the in the siliceous plant matter, is controlled in a range within the range of 300 ppm and 15,000 ppm.

38. The process of claim 26, wherein the content of non-siliceous inorganic substances in the heat treated silica is controlled in a range within the range of 10 ppm and 1,000 ppm by post washing the silica with at least one of water, mineral acids, chelants, and pH adjustment chemicals.

39. The process of claim 38, wherein the content of non-siliceous inorganic substances in the heat treated silica is controlled in a range within the range of 100 ppm and 500 ppm by post washing the silica with at least one of water, mineral acids, chelants, and pH adjustment chemicals

40. The process of claim 26, wherein said heat treatment temperature is in the range of 200° C. to 1,000° C.

41. The process of claim 26, wherein said preselected non-siliceous inorganic matter in the siliceous plant matter is comprised of at least one of lithium, sodium, potassium, magnesium, calcium, aluminum, boron, iron, manganese, titanium or phosphorus.

42. The process of claim 26, wherein the content of said preselected non-siliceous inorganic matter in said siliceous plant matter is chemically controlled.

43. The process of claim 26, wherein the content of preselected non-siliceous inorganic matter and heat treating temperature are controlled to prevent formation of crystalline structures in said amorphous silica.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a graph showing the results of an experiment to determine the ability to control the surface area of biogenic silica by controlling the total content of non-siliceous inorganic substances (flux agents) retained in the rice hull composition.

[0013] FIG. 2 is a graph showing the results of an experiment to determine the ability to control the pore characteristics of biogenic silica by controlling the non-siliceous inorganic substances (flux agents) retained in the rice hull composition.

PREFERRED EMBODIMENT OF THE INVENTION

[0014] While the present invention will be described with reference to preferred embodiments, it will be understood by those who are skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. It is therefore intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments and legal equivalents thereof which are within the scope of the appended claims.

[0015] Porous amorphous silica can be obtained from siliceous plant matter containing non-siliceous inorganic matter. In prior practice, the non-siliceous inorganic matter would be removed during a soaking and chelation step. It has been found that if certain of the non-siliceous inorganic matter is preferentially retained within the plant matter prior to heat treatment, preferred properties can be imparted to the resulting amorphous silica.

[0016] In a presently preferred method, the siliceous plant matter is soaked in an aqueous solution comprising a chelating agent. The chelating agent is present in an amount which extracts at least some of the non-siliceous inorganic matter. The aqueous solution is then separated from the siliceous plant matter. Beneficial properties are imparted to the siliceous plant matter by controlling the amount of at least one preselected non-siliceous inorganic substance in the siliceous plant matter. At the end of the process, the siliceous plant matter is heat treated in the presence of oxygen at a temperature wherein the resulting silica is comprised of silica of porous amorphous form.

[0017] The preselected non-siliceous it substances can include any or all of the following materials: elements or compounds of alkali metals (most preferably from lithium, sodium, and potassium), alkali earth metals (most preferably magnesium and calcium), aluminum, boron, iron, manganese, titanium, or phosphorus.

[0018] One method of controlling the amount of the preselected non-siliceous inorganic substance is to control the amount of chelating agents in the siliceous matter. Suitable chelating agents include citric acid, acetic acid, ethylenediamine, ethylenediaminetetracetic acid, dimercaptosuccinic acid, trimethylaminetricarboxylic acid, alphalipoic acid, and diethylenetriaminepentaacetic acid. Preferably, the amount of chelating agent is maintained between 0.001 kg per kg of plant matter to 1 kg per kg of plant matter. In the case of a citric acid chelating agent, the amount of citric acid preferably ranges between 0.01 kg per kg of plant matter to 0.1 kg per kg of plant matter. By controlling the amount of chelating agent present the plant material, the amount of non-siliceous inorganic substances remaining in the plant matter can be established at a preselected amount sufficient to impart desired properties to the silica resulting from the heat treatment of the plant matter.

[0019] An alternative method of controlling the amount of the preselected non-siliceous inorganic substance is to introduce mineral acids to interface with the siliceous plant matter. Suitable mineral acids include sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, and perchloric acid. The mineral acids will aid to extract some of the non-siliceous inorganic substances, with a portion remaining in the plant matter. The remaining non-siliceous inorganic substances are selected to impart desired properties to the silica resulting from the heat treatment of the plant matter.

[0020] An additional method of controlling the amount of the preselected non-siliceous inorganic substance is to control the amount of chelating agent added to the aqueous solution when soaking the plant matter. It has been found that the amount of the preselected inorganic non-siliceous substances can be manipulated within a range of 20 ppm to 25,000 ppm, and more preferably within a range of 300 ppm and 15,000 ppm, by controlling the amount of chelating agent added to the aqueous solution.

[0021] A further method of controlling the amount of the preselected non-siliceous inorganic substance is to add back to the plant matter a desired amount of a preselected non-siliceous inorganic substance in an amount sufficient to impart desired properties to the silica once the plant matter has been heat treated.

[0022] A further method of controlling the amount of the preselected non-siliceous inorganic substance is to add back to the plant matter a desired amount of a preselected non-siliceous inorganic substance in an amount sufficient to impart desired properties to the silica once the plant matter has been heat-treated. It has been found that the alkali metals (more specifically lithium, sodium, and potassium) have a strong influence on the properties of the silica; an increased amount of these alkali metals results in a decrease of the surface area and pore volume, when compared to a control. It has also been found that the alkaline earth metals (more specifically magnesium and calcium) will have a moderate influence on the properties of the silica; an increased amount of those elements will also result in a decrease of the surface area and pore volume. Finally, it has been found that other typical inorganic impurities will not influence the properties of the silica (more specifically boron, zinc, aluminum, manganese, phosphorous, and iron).

[0023] One of the properties of the amorphous silica that can be controlled using the method of the present invention is the surface area of the silica. It has been found that the surface area of the amorphous silica can be controlled within a narrow range within the broader range of 10 m.sup.2/g and 450 m.sup.2/g, and more preferably, within a narrow range within the broader range of 30 m.sup.2/g and 400 m.sup.2/g.

[0024] Another property of the amorphous silica that can be controlled using the method of the present invention is the pore volume of the silica. It has been found that the pore volume of the amorphous silica can be controlled in a narrow range within the broader range of 0.50 cc/g and 0.05 cc/g, and more preferably, within a narrow range within the broader range of 0.40 cc/g and 0.10 cc/g.

[0025] A further property of the amorphous silica that can be controlled using the method of the present invention is the pore diameter of the silica. It has been found that the pore diameter of the amorphous silica can be controlled in a narrow range within the broader range of 10 Angstroms and 200 Angstroms, and more preferably, within a narrow range within the broader range of 30 Angstroms and 100 Angstroms.

[0026] It has been found that the content of non-siliceous inorganic substances in the heat treated silica can be controlled in a range within the range of 10 ppm and 1,000 ppm, and more preferably, in a range within the range of 100 ppm and 500 ppm, by post washing the silica with at least one of water, mineral acids, chelants, and pH adjustment chemicals.

[0027] The preferred heat treatment temperature is in the range of 200° C. to 1,000° C.

[0028] Certain aspects of the invention are demonstrated in the following experiments.

Experiment 1

[0029] Several tests were conducted using identical mixing systems comprised of a top entering mechanical mixer, hot plate (preferably with temperature feedback) and a 2-liter capacity beaker. One test was a control and the other tests used different citric acid chelating agent concentrations. The test procedure was as follows: [0030] 1. Each beaker was filled with 1,800 ml. of 160° F. distilled water. [0031] 2. 100 grams of 60 mesh ground rice hulls (from the same sample batch) were added to each beaker and the mixers were operated at identical speeds. Hot plates were set to the same (low) level of heating or if they have a feedback mechanism were set to 160° F. [0032] 3. The control batch was filtered to remove water, post dried at 50° C., and heat treated at 600° C. Silica was analyzed for inorganic elements content. [0033] 4. The other batches were treated with increased amounts of a 10% solution of citric acid while allowing time for proper chelation, followed by a sequence of purified water soak and drain cycles. A 30% hydrogen peroxide solution was added into the previous cycle to the final wash. Material from the tests were filtered out to remove water, post dried at 50° C. and heat treated at 600° C. Silica was analyzed for inorganic elements content

[0034] Once the treated hull samples were analyzed for residual inorganic elements via ICP, the relative removal rates of each treatment were compared. The results of the comparison are presented in Table 1 below:

TABLE-US-00001 TABLE I 5 g 10% 10 g 10% 20 g 10% 30 g 10% 40 g 10% 50 g 10% 60 g 10% ppm +60 Mesh citric/100 g citric/100 g citric/100 g citric/100 g citric/100 g citric/100 g citric/100 g ICP OES Control rice hulls rice hulls rice hulls rice hulls rice hulls rice hulls rice hulls Li 3 3 3 2 2 3 4 4 Al 19 10 24 17 11 14 9 17 B 29 19 22 16 15 10 19 15 Fe 87 65 63 69 58 60 59 56 P 726 193 205 230 247 331 305 269 Ca 3,305 1,860 1,444 812 452 335 223 203 K 6,367 51 44 27 17 23 47 37 Mg 985 312 199 119 69 69 53 49 Na 193 29 18 77 39 27 7 20 Flux (K, 10,850 2,252 1,705 1,035 577 454 330 309 Na, Ca, Mg)

[0035] As shown in Table 1, increased amounts of citric acid will improve inorganic substances removal. The removal can be modeled assuming 1 mol citric acid will chelate 1 mol of the element. Therefore specific inorganic elements concentration, in particular flux elements (such as K, Na, Ca, Mg) that will affect final silica product properties such as surface area, pore volume, pore diameter and dispersibility can be achieved with a specific amount of citric acid concentration, for a given feedstock.

Experiment 2

[0036] An experiment was conducted to determine other ways to further reduce inorganic substances in the chelation step. Four tests were conducted using identical mixing systems comprised of a top entering mechanical mixer, hot plate (preferably with temperature feedback) and a 2-liter capacity beaker. One test was a control and the other tests used citric acid chelating agents on the same concentrations. One test had the introduction of a mineral acid during the chelation and the other test had the introduction of a mineral acid on a soak step prior to the chelation step to further reduce pH of the solution. The test procedure was as follows: [0037] 1. Each beaker was filled with 1,800 ml of 160° F. distilled water. [0038] 2. 100 grams of +60 mesh ground rice hulls (from the same sample batch) were added to each beaker and the mixers were operated at identical speeds. Hot plates were set to the same (low) level of heating or if they have a feedback mechanism were set to 160° F. [0039] 3. Control batch was filtered out to remove water, post dried at 50° C. and heat treated at 600° C. Silica was analyzed for inorganic elements content. [0040] 4. The second batch was treated with a 10% solution of citric allowing time for proper chelation, followed by a sequential of purified water soak and drain cycles. A 30% hydrogen peroxide solution was added into the previous cycle to the final wash. Material from the tests was filtered out to remove water, post dried at 50° C. and heat treated at 600° C. Silica was analyzed for inorganic elements content. [0041] 5. The third batch was treated with a 10% solution of nitric acid, 50% of the total 30% hydrogen peroxide and a 10% solution of citric allowing time for proper chelation, followed by a sequential of purified water soak and drain cycles. The remaining 50% of the 30% hydrogen peroxide solution was added into the previous cycle to the final wash. Material from the tests was filtered out to remove water, post dried at 50° C. and heat treated at 600° C. Silica was analyzed for inorganic elements content. [0042] 6. The fourth batch was treated with a 10% solution of nitric and 50% of the total hydrogen peroxide on an initial soak step. This soak step was conducted prior to the chelation. The soak was followed by a chelation step with 10% citric acid solution and a sequential of purified water soak and drain cycles. The remaining 50% of the 30% hydrogen peroxide solution was added into the previous cycle to the final wash. Material from the tests was filtered out to remove water, post dried at 50° C. and heat treated at 600° C. Silica was analyzed for inorganic elements content.

[0043] Once the treated hull samples were analyzed for residual inorganic element impurities via ICP, the relative removal rates of each treatment were compared. The results of the comparison are presented in Table 2 below:

TABLE-US-00002 TABLE 2 10 g 10% 10 g 10% nitic 10 g 10% citric 10 g soak followed citric 10% citric by 10 g citric ppm  60 chelation/ chelation/ chelation/ ICP Mesh 100 g 100 g rice 100 g OES Control rice hulls hulls rice hulls Li 3 3 2 2 Al 19 24 10 15 B 29 22 15 5 Fe 87 63 47 53 P 726 205 220 274 Ca 3,305 1,444 86 144 K 6,367 44 10 6 Mg 985 199 32 42 Na 193 18 8 4 Flux (K, Na, 10,850 1,705 136 196 Ca, Mg) indicates data missing or illegible when filed

[0044] As shown in Table 2, the addition of a mineral acid to promote a pH change in the chelation process or in a presoak process preceding chelation increased amounts of inorganic substances removed from the rice hulls. It is noted that citric acid buffer the solution pH to 3 and the mineral acid will promote further reduction to a pH of 2 in this example. With the test results it is possible to see that further purification of a silica can be achieved by various methods of washing, with or without chemical agents and at temperature ranges.

Experiment 3

[0045] An experiment was conducted to determine the ability to control the surface area, pore characteristics and dispersibility by controlling the level of flux agents on the rice hull composition. In current practice, the goal is to remove inorganic substances that can act as flux agents as fully as possible before combustion in order to yield a high surface area and good dispersibility. Controlling the level of inorganic substances remaining in the rice hulls will provide lower surface areas and greater degrees of abrasiveness. Because silica qualities may also be affected by combustion temperatures, a 600° C. combustion temperature was used for all evaluations.

[0046] Several samples of various batches with different inorganic element impurities concentrations, that can act as flux agents (expressed by the sum of Na, K, Ca and Mg) were heat treated at 600° C. The silica was analyzed for flux agents (Na, K, Ca and Mg), surface area and pore volume. The results where plotted on the graphs shown in FIGS. 1 and 2:

[0047] With the test results it is possible to see that silica properties such as surface area, pore volume and pore diameter, leading to other important properties such as dispersibility and abrasivity can be obtained with a specific flux agent element concentration at a given heat treatment temperature.

Experiment 4

[0048] An experiment was conducted to evaluate the changes in surface area, pore volume, and pore diameter of a given batch of treated rice hulls, with a specific flux agent (expressed by the sum of Na, K, Ca and Mg) level content. The treated rice hull batch was heat treated at four different temperatures and results are shown in Table 3 below:

TABLE-US-00003 TABLE 3 Temper- Flux ature Agents Pore Samples (C) (ppm) SA (m2/g) PV (m3/g) Width (Å) 41118 #3 600 238 326 0.34 42.19 41118 #3 700 238 301 0.32 43.28 41118 #3 800 238 264 0.29 44.72 41118 #3 900 238 196 0.24 48.20

[0049] With the tests results it is possible to see that a given treated rice hull will produce different results of pore volume, surface area and pore diameter on different heat treatment temperatures, being able then to be manipulated for a specific final product desired property

Experiment 5

[0050] An experiment was conducted to evaluate the effect of post washing the silica to further eliminate metals and achieve higher degrees of purification.

[0051] Several tests were conducted using identical mixing systems comprised of a magnet, hot plate (preferably with temperature feedback) and a 500 ml capacity beaker. One test was a control and the other tests used different citric acid chelating concentrations. The test procedure was as follows: [0052] 1. Each beaker was filled with 300 ml. of 160° F. distilled water. [0053] 2. 30 grams of silica (from the same sample batch) were added to each beaker and the mixers were operated at identical speeds. Hot plates were set to the same (low) level of healing or if they have a feedback mechanism were set to 160° F. [0054] 3. Control silica was analyzed for metals content. [0055] 4. The other batches were treated w ith increased amounts of a 10% solution of citric, followed by one of purified water soak and drain cycles. Material from the tests was filtered to remove water and post dried at 50° C. Silica was analyzed for inorganic element impurities content.

[0056] The results of the comparison are presented in Table 4 below:

TABLE-US-00004 TABLE 4 ppm Control 1 g 10% citric 2 g 10% citric 4 g 10% citric 8 g 10% citric 10 g 10% citric ICP OPS Silica solution solution solution solution solution Li 10 5 5 5 5 5 Al 136 38 39 37 38 35 B 22 3 4 4 3 2 Ca 87 38 38 31 30 30 Fe 130 50 47 46 40 36 K 34 12 13 11 12 11 Mg 27 20 20 19 19 18 Na 5 4 4 3 4 4 P 47 40 47 45 42 41 Total 498 210 217 201 193 182

[0057] These test results show that it is possible to further reduce the residual inorganic substances in the silica, thus creating levels of impurities for each market application requirement.

Experiment 6

[0058] Several tests were conducted using a procedure to reintroduce the desired inorganic element into a control pretreated rice hull sample where most of the inorganic contaminants had been removed. The inorganic contaminant, usually from a salt solution with a given concentration, was sprayed on to a dry rice hull sample with vigorous agitation for full incorporation and wetting of the rice hulls. The concentrations of desired contaminant were calculated based on the elemental quantity on each solution and the silica quantity on each rice hull sample. The wetting was calculated in such a way that the rice hulls were capable of absorbing all the excess water so that the distribution of the elemental impurity would be uniform.

[0059] Wet samples were post dried in an oven and then calcined at different temperatures to evaluate surface area and pore volumes. Results of the experiments are presented below:

[0060] A lithium hydroxide solution was incorporated into a control rice hull sample with three different concentration targets measured on the calcined silica sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm. The results of this experiment are presented in Table 5 below:

TABLE-US-00005 TABLE 5 Metals ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control Li Li Li Li (lithium hydroxide) 4 752 1,558 2,491 % relative to target (75%) (78%) (80%) B 9 10 10 10 Mg 30 28 27 33 Zn 4 21 5 4 P 276 283 279 306 Al 7 3 4 4 Ca 74 68 71 75 Fe 49 42 48 48 K 9 8 9 12 Mn 49 46 47 50 Na 3 3 5 9 Total 514 1,264 2,063 3,042 SA (m.sup.2/g) 600° C. 391 280 247 221 PV (cc/g) 600° C. 0.40 0.31 0.29 0.27

[0061] The results in Table 5 show that the increased amounts of lithium in the rice hull had a significant effect in decreasing the surface area and pore volume of the calcined silica sample.

[0062] A potassium hydroxide solution was incorporated into a control rice hull sample with three different concentration targets measured on the calcined silica sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm. The results of this experiment are presented in Table 6 below:

TABLE-US-00006 TABLE 6 Metals ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control K K K K (potassium hydroxide) 12 679 1,398 1.871 % relative to target (68%) (70%) (62%) Al 14 9 47 18 Ca 81 85 87 85 Mn 52 52 53 52 P 231 248 253 248 Li 4 4 4 4 B 17 17 14 14 Fe 53 81 47 46 Mg 30 32 32 30 Na 4 7 5 4 Zn 6 6 6 6 Total 504 1,220 1,946 2,378 SA (m.sup.2/g) 600° C. 387 330 300 286 PV (cc/g) 600° C. 0.40 0.34 0.32 0.31

[0063] The results in Table 6 show that the increased amount of potassium in the rice hull had a significant effect in decreasing the surface area and pore volume of the calcined silica sample.

[0064] Sodium hydroxide and sodium sulfate solutions were incorporated into a control rice hull sample with three different concentrations targets measured on the calcined silica sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm. The results of this experiment are presented in Table 7 below:

TABLE-US-00007 TABLE 7 Metals ppm 1,000 ppm 2,000 ppm 3,000 ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control Na Na Na Na Na Na Na (sodium sulfate) 0 935 2,072 3,041 % relative to target (93%) (104%) (101%) Na (sodium hydroxide) 0 1,007 2,329 3,479 % relative to target (100%) (116%) (116%) Al 12 23 55 22 11 143 109 Ca 109 113 129 118 113 116 111 K 9 8 12 12 9 12 11 Ma 80 73 78 78 76 75 75 P 298 305 346 353 318 332 347 Li 4 4 4 4 4 4 4 B 17 14 12 13 7 13 11 Fe 80 188 128 84 83 84 84 Mg 40 37 43 42 39 38 38 Zn 4 4 5 4 4 6 5 Total 653 1,704 2,887 3,771 1,671 3,152 4,274 SA (m.sup.2/g) 600° C. 387 318 298 285 298 276 251 PV (cc/g) 600° C. 0.40 0.35 0.32 0.31 0.32 0.30 0.28

[0065] The results in Table 7 show that the increased amount of sodium in the rice hull had a significant effect in decreasing the surface area and pore volume of the calcined silica sample.

[0066] A calcium oxide solution was incorporated into a control rice hull sample with three different concentrations targets measured on the calcined silica sample: 1,000 ppm, 2,000 ppm and 3,000 ppm. The results of this experiment are presented in Table 8 below:

TABLE-US-00008 TABLE 8 Metals ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control Ca Ca Ca Ca (calcium oxide) 81 802 1,549 2,096 % relative to target (80%) (77%) (70%) Al 14 10 13 16 K 12 14 13 12 Mn 52 53 53 51 P 231 232 247 250 Li 4 4 4 4 B 17 11 12 9 Fe 53 46 49 47 Mg 30 37 38 42 Na 4 3 4 3 Zn 6 5 4 4 Total 504 1,217 1,986 2,534 SA (m.sup.2/g) 600° C. 387 385 375 370 PV (cc/g) 600° C. 0.40 0.40 0.39 0.39 SA (m.sup.2/g) 900° C. 230 232 215 202 PV (cc/g) 900° C. 0.26 0.26 0.24 0.22

[0067] The results in Table 8 show that the increased amount of calcium in the rice hull had a moderate effect in decreasing the surface area and pore, volume of the calcined silica sample.

[0068] A magnesium sulfate solution was incorporated into a control rice hull sample with three different concentrations targets measured on the calcined sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm. The results of this experiment are presented in Table 9 below:

TABLE-US-00009 TABLE 9 Melals ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control Mg Mg Mg Mg (magnesiam sulfate) 40 976 2,054 2,950 % relative to target (98%) (102%) (98%) Al 12 11 22 17 Ca 109 112 121 123 K 9 21 38 59 Mn 80 79 81 77 P 298 305 329 340 Li 4 4 4 4 B 97 10 10 8 Fe 80 82 84 85 Na 0 44 0 0 Zn 4 3 2 3 Total 653 1,647 2.745 3,666 SA (m.sup.2/g) 600° C. 387 369 364 352 PV (cc/g) 600° C. 0.40 0.39 0.38 0.38 SA (m.sup.2/g) 900° C. 214 222 195 197 PV (cc/g) 900° C. 0.27 0.26 0.23 0.24

[0069] The results in Table 9 show that the increased amount of magnesium in the rice hull had a moderate effect in decreasing the surface area and pore volume of the calcined silica sample.

[0070] Boric acid and zinc sulfate solutions were incorporated if to a control rice hull sample with three different concentration targets measured on the calcined sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm The results of this experiment are presented in Table 10 below:

TABLE-US-00010 TABLE 10 Metals ppm 100818 1,000 ppm 2,000 ppm 3,000 ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control B B B Zn Zn Zn B (boric acid) 9 941 1945 3228 8 0 0 % relative to target (94%) (97%) (108%) Zn (zinc sulfate) 4 7 5 5 910 1969 2570 % relative to target (91%) (98%) (86%) Li 4 4 4 4 4 4 4 Mg 30 31 28 36 91 35 30 P 276 268 270 307 277 273 275 Al 7 8 4 6 7 17 7 Ca 74 95 76 82 70 71 68 Fe 49 60 52 53 47 47 46 K 9 12 8 17 8 9 8 Mn 49 49 49 51 47 47 46 Na 3 17 3 3 7 8 3 Flux 116 155 115 138 171 123 109 Total 514 1492 2444 3792 1471 2480 3057 SA (m.sup.2/g) 600° C. 391 369 367 366 366 359 360 PV (cc/g) 600° C. 0.40 0.38 0.38 0.39 0.38 0.37 0.37 SA (m.sup.2/g) 900° C. 209 204 202 185 201 210 205 PV (cc/g) 900° C. 0.24 0.24 0.23 0.21 0.24 0.25 0.24

[0071] The results in Table 10 show that the increased amounts of boron and zinc in the rice hull had some effect on the surface area and pore volume, although the results did not indicate any trend.

[0072] Aluminum sulfate and manganese sulfate solutions were incorporated into a control rice hull sample with three different concentrations targets measured on the calcined sample: 1,000 ppm, 2,000 ppm, and 3,000 ppm. The results of this experiment are presented in Table 11 below:

TABLE-US-00011 TABLE 11 Metals ppm 110718 1,000 ppm 2,000 ppm 3,000 ppm 1,000 ppm 2,000 ppm 3,000 ppm ICP OES Control Mn Mn Mn Al Al Al Al (aluminum sulfate) 14 7 14 14 709 1399 1977 % relative to target (71%) (70%) (66%) Mn (manganese sulfate) 52 881 1790 2488 63 51 73 % relative to target (88%) (89%) (83%) K 12 13 15 12 12 19 22 Ca 81 81 83 84 84 79 87 P 231 239 238 245 272 263 331 Li 4 4 4 4 4 4 4 S 17 11 13 12 13 12 10 Fe 53 44 47 44 49 48 48 Mg 30 25 25 19 32 35 39 Na 4 3 3 3 4 3 5 Zn 6 3 1 0 4 4 5 Flux 127 122 126 118 132 136 153 Total 504 1311 2233 2925 1246 1917 2601 SA (m.sup.2/g) 600° C. 387 381 375 379 379 379 380 PV (cc/g) 600° C. 0.40 0.39 0.39 0.39 0.39 0.39 0.40 SA (m.sup.2/g) 900° C. 230 222 236 221 255 249 252 PV (cc/g) 900° C. 0.26 0.25 0.27 0.25 0.28 0.27 0.28

[0073] The results in Table 11 show that aluminum and manganese do not have an impact of the properties of calcined silica.

[0074] While the above description contains certain specifics, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment thereof. Accordingly, the scope of the present invention should be determined not by the embodiment(s) illustrated, but by the appended claims and their legal equivalents.