Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1998-08-25
2003-02-25
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C106S600000
Reexamination Certificate
active
06524543
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the production of soluble silicates from biogenic silica in substantially amorphous state.
BACKGROUND OF THE INVENTION
Soluble silicates are compositions in which sodium oxide and silica are combined in varying proportions, usually with some water. The different proportions allow a wide range of properties and applications. The proportion of silica to sodium oxide is expressed on a mole ratio basis with ratios ranging from 3.85 to 0.5. They are produced as either solids or water solutions—liquids—with the liquids usually made as concentrated as can be handled in the commercial applications. In 1977, the production of sodium silicate in the United States was about 760,000 tons for the most common grade—“water glass”—with a ratio of silica to sodium oxide of 3.2. Other grades, mostly more alkaline (lower ratio), made up another 210,000 tons in that same year. Some of the principal uses of sodium silicates are: adhesives and cements; coatings; gels and catalysts; silica sols and water treatment; detergents and soaps; foundry molds and cores; drilling muds; soil stabilization; chemical fixation/solidification of wastes.
Sodium silicate is conventionally made by fusing high purity soda-ash and silica sand in furnaces at temperatures of 1300° to 1500° C. and higher to produce a solid glass. The liquid is made by dissolving the glass with steam and hot water. This is known as the open hearth process which is the foundation of all commercial processes for making sodium silicate today. Both processes are very energy intensive. Therefore, any method which requires the use of less energy is advantageous and potentially competitive.
U.S. Pat. No. 1,293,008 (Blardone) discloses a boiling procedure for various lengths of time for producing a form of water glass from sodium silicate and sodium hydroxide; however, the boiling process cannot produce ratios higher than 1.5:1 of silica to sodium oxide. In subsequent paragraphs a fusion process is disclosed wherein sodium carbonate or sodium sulfate is fused with rice hull ash. While any ratio of sodium silicate desired can be produced by this fusion process, the energies adequate to couple sodium and silica at various ratios are at temperatures well over 2,000 to 3,000° F. These fusion products are then boiled in water to produce solutions, the open hearth process.
In U.S. Pat. No. 4,488,908 (Goodwin, et al.) ash and sodium hydroxide solutions are heated in an open container to give dry, but hydrated solids. Even when the hydrated solids are added to water, about 25% of the original mass would dissolve generating a slurry of unreacted ash in the sodium metasilicate (1:1 ratio).
As described in more detail subsequently herein, the present invention is directed to producing sodium silicate solutions with biogenic silica which is clear and homogenous, essentially free of unreacted silica and comprising controlled ratios of silica to sodium oxide, both with respect to the feed stock and the recovered product. The yields obtained by the hydration process of this invention are close to theoretical.
In obtaining soluble silicates from, biogenic silica, such as rice hull ash, in which the hull fibers have been burned off, the resulting soluble silicates have an amber color which is very difficult to remove. For example, attempts to remove the amber color proved inadequate by the following material and methods: activated carbon (perculation and filtration); activated, amorphous silica; zeolites (perculation and filtration); ion exchange resins; EDTA (ethylenediaminetetraacetic acid disodium salt); black rice hull ash (original and residual); PHPAA (partially hydrolyzed poly acrylic acid); sodium peroxide; chlorine; silica foam; silicate foam; and sodium gluconate.
Since commercial grades of soluble silicates, such as sodium silicate, are water white, the amber color is unacceptable for most commercial applications.
Commercially available rice hull ash is prepared by burning rice hulls as an energy source in a furnace. In the process, raw rice hulls are continuously added to the top of the furnace and the ash is continuously removed from the bottom. Temperatures in the furnace range from 800° to about 1400° C., and the time factor for the ash in the furnace is about three minutes. Upon leaving the furnace, the ash is rapidly cooled to provide ease in handling. When treated by this method, silica remains in a relatively pure amorphous state rather than in the crystalline forms known as quartz, tridymite or crystobalite. Transition from the amorphous to the crystalline state generally takes place when the silica is held at very high temperatures, for example 2000° C., or longer periods of time. The significance of having the silica in an amorphous state is that the silica ash maintains a porous skeletal structure rather than migrating to form crystals, and the amorphous form of silica does not cause silicosis thus reducing cautionary handling procedures. The burning of the rice hulls is time-temperature related, and burning of these hulls under other conditions can be done so long as most of the ash is in an amorphous state with a porous skeletal structure.
On a commercial burning of rice hulls as an energy source, the resultant ash had the following chemical analysis (by weight):
TABLE 1
SiO
2
93
percent
Carbon
5.5
percent
Moisture
<1
percent
The remaining ½ percent by weight which converts to 5,000 parts per million (5000 ppm) by weight consists of minor amounts of magnesium, barium, potassium, iron, aluminum, calcium, copper, nickel mangonese, and sodium. Apparently, it is these metal salts, as well as organic material, which impart the amber color to the sodium silicates and which are very difficult to remove once the soluble silicate is formed.
The carbon content was in a dispersed state throughout the material. Depending upon the time and temperature of burning of the biogenic source of silica, and the particular furnace used, the carbon content can vary considerably, for example, up to and above 12%.
SUMMARY OF THE INVENTION
The present invention comprises a hydration method of making soluble silicates such as sodium silicates by dissolving biogenic silica in aqueous alkali solution such as sodium and potassium hydroxide in a closed container. By controlled burning of the rice hull ash, a “black ash” can be obtained with a residual carbonaceous content. This provides a method and material which, surprisingly, generates a clear, homogenous water white solution of alkali silicate when digested in aqueous sodium or potassium hydroxide in a closed container at temperatures and pressures which do not cause discoloration by the inherent organic material and trace minerals of the ash. Temperatures from ambient to the order of 275° F. are suitable for most black ash. Higher temperatures and pressures may cause discoloration, for example, by the breakdown of the carbonaceous residue. While the mechanism of the prevention of the color formation is not known, it is possible that the carbonaceous residue in the ash is similar to “activated carbon” which may absorb or react with color forming agents before they are released to the alkali solution during the digestion of the ash. Surprisingly, perculation or filtration of amber colored sodium silicate through a bed or column of “black ash” did not remove the color. The isolated black residue recovered from the digestion of black ash in alkaline solution was also ineffective in removing color from an amber solution. Such amber solutions result from biogenic silicas which contain less than 1% carbonaceous matter.
Accordingly, it is an object of the present invention to provide a method of producing a soluble silicate solution in which biogenic silica, in a closed container is dissolved in a strong alkali solution effective to produce the soluble silicate, in the presence of an active carbonaceous material in an amount sufficient to absorb or react with the inherent organic material or minerals thereby preventing discoloration of the soluble silicate
Conner Jesse R.
Mallow William A.
Rieber Roy S.
Hendrickson Stuart L.
Weiler James F.
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