IL104584A - Process for the manufacture of pure silica from rock containing it - Google Patents

Description

PROCESS FOR THE MANUFACTURE OF PURE SILICA FROM ROCK CONTAINING IT nniN 7»DDn y7on Π'ΪΙΙ ΠΪ7· 7 ·Ο II 24» · "7 η· 7ηη THE APPLICANTS: : nw7:ipn Rotem Amfert Negev Ltd. D"ya 233 OHDDN QUI Ί P.O.Box 435 435 .Τ.Π Dimona 86103 86103 Π3 ID'T THE INVENTORS: : D 1 N ' 3ΡΡΠ 1 . Alexander IOSEF ηθΙ ' -IT30D7N.1 5/2 Hakanayim St. 5/2 D»NJ|7n aim Arad 80700 80700 my 2. David BASHIRI •TBQ i n .2 23/1 Hatzvi St. 23/1 Ίϋη II m Beer-Sheva ynei-ii The present invention relates to a process for the manufacture of pure silica from rocks. More particularly the invention relates to a process for the manufacture of pure amorphous silica from rocks containing non-crystalline silica such as: porcel lani te , diatomite, and amorphic quartz.

BACKGROUND OF THE INVENTION.

Porcellanite is a typical example of a sedimentary rock comprising as a main compound non-crystalline active silica among other constituents considered as impurities. A typical analysis of the main constituents of porcellanite mineral, which exists in Israel is as follows: - Si02: 69% - CO2 : 7% and - Na20: 0.45%.

Rocks containing non-crystalline silica are found in many places of the world. In Israel, large amounts of porcellanite are covering the deposits of phosphate rocks near Nahal Zin without being utilized. Moreover, in order to mine the phosphate rock large amounts of porcellanite have to be removed fact which consists a serious ecological problem. Active silica is known as a valuable material useful for many purposes, such as: fillers, extenders, adsorbents, supports, dentrifices, etc.

The literature is quite abundant with many patents describing various approaches for obtaining active silica from alkali metal silicates and a mineral acid such as sulfuric acid or hydrochloric acid. According to the U.S. Patent Number 3,993,497, precipitated silica is obtained from a sodium silicate solution treated with sulfuric acid and aluminum containing ions.

In the East German Patent Number 293,097, powdered amorphous silica is obtained by reacting alkali metal silicate solutions with an acid or acidic substances, under stirring and adding a mixture of cationic and non-ionic surfactant before reaching an electrolyte concentration of 0.3N.

In the Israeli Patent Application Number 100,503, a process is described for the manufacture of amorphous silica having a purity of at least 90%. According to this process, granules of porcellanite rock are leached by a solution of sodium hydroxide at a temperature of up to 100°C. To the silica which is separated, gaseous carbon dioxide is added and it is claimed that a product which is defined as Si02nH20 is separated. Two Examples are given, but none of them mentions the amounts or ratio between the alkaline solution to the porcellanite nor the analysis of the final product. Actually, Example 2 as presented is nothing more than a pure description of the flowsheet, mentioned as Figure 1.

Although the known processes are yielding various high grades of pure silica, their main disadvantage is the fact that they require a relatively expensive starting reagent such as alkali metal silicate.

It is an object of the present invention to provide an process for obtaining amorphous silica from a natural rock containing it. Another object of the present invention is to provide a simple proces for obtaining amorphous silica from a natural rock, wherein most of the chemical reactants are co-produced.

BRIEF DESCRIPTION OF THE INVENTION.

The invention relates to a process for the manufacture of pure amorphous silica from rocks containing non-crystalline silica which comprises the steps of: (a) dissolution of said rock in an alkaline solution comprising sodium hydroxide and sodium carbonate, the concentration thereof being in the range of between 5% to 25% by weight, expressed as Na20 obtaining a solution of sodium silicate and (b) precipitation of pure amorphous silica by adding a stream of carbon dioxide or a compound which generates carbon dioxide into said sodium silicate solution and co-producing sodium carbonate, being characterized by the fact that the weight ratio of Si02 to Na∑0 in said dissolution step, is maintained in the range of between 1.2 to 1.58. It was unexpectedly found that outside this range the efficiency of silica dissolution is significantly decreased. According to a most preferred embdiment the concentration of sodium hydroxide in said alkaline solution is in the range of between 5% to 25% by weight.

The silica produced is of a very high purity being substantially free of foreign cations which are present in the starting rock.

BRIEF DESCRIPTION OF THE FIGURES.

Figure 1. represents the correlation between the efficiency of silica dissolution and the concentration of alkalinity exoressed as percentage of Na20.

Figure 2. represents a schematic flowsheet of the process according to the present invention.

DETAILED DESCRIPTION OF THE FIGURE AND THE PROCESS. In Figure 1, it can be noticed that the extent of silica dissolution depends on the weight ratio between silica (expressed as Si02> to the alkalinity (expressed as Na20) present during said dissolution, reaching a maximum at a ratio of 1.58. It was srprisingly found that it is possible to exploit more than one equivalent of Si02 per unit of a20. Also below the weight ratio of 1.1 Si02 to Na20, only a small amount of silica will dissolve. The preferred concentration of the sodium hydroxide to be used in the process, is between 5% to 14% (by weight) and most preferable between 7% to 14%. Below 5% large amounts of solution have to be handled, while above 14%, the resultant mass is very viscous being difficult to be handled due to various undesired reactions which occur in the system.

In Figure 2, a schematic flowsheet of the process is presented comprising two main steps: In the first step, sodium carbonate (13), which is co-produced from the last step, is reacted with a slurry of calcium hydroxide (1) in a vessel (A) , generati ng a slurry (2) comprising sodium hydroxide and calcium carbonate. The reaction involved in this stage is as follows: Na2C03 + Ca(OH)2 = CaCOs + 2NaOH The calcium carbonate obtained has a beneficial effect since it assists the filtration and also adsorbs some of the organic matter and other impurities present in the dissolution step. This slurry is conveyed into the vessel (B) where it reacts with the rock containing non-crystalline silica (3), ground to a particle size in the range of between 50 to 350 mesh (Tyler scale), producing a mixture of sodium silicate and calcium carbonate. This mixture is filtered (C) , the solids comprising' the calcium carbonate with the impurities after washing by tap water (5) being discarded (6).

In the second step, the slurry as obtained above is conveyed into vessel (B) where it reacts with porcellanite (3) ground to a particles size in the range of between 50 to 350 mesh (Tyler scale) producing a solution of sodium silicate and calcium carbonate. The resulted mixture is filtered (C) and the solids comprising the calcium carbonate after washing by tap water (5) are discarded (6). In the last step, the sodium silicate (7) and the wash solution (8) are conveyed into the vessel (D), By introducing a stream of carbon dioxide (9) or a compound which generates the carbon dioxide, such as sodium bicarbonate (10), in the vessel (D) the excess of sodium hydroxide is transformed into sodium carbonate while the sodium silicate is transformed into silica. One may also prefer to use carbon dioxide and a generator thereof for this pu pose .

The reactions which are involved in this stage are as fol lows : 4NaOH + porcellanite Na2Si03 + 2NaOH.

Na2Si03 + CO2 = Na2C03 + Si02 NaOH + CO2 = 2Na2(X>3 + H20 In a similar manner, in case that sodium bicarbonate, as a compound generating the carbon dioxide is used, the reaction involved will be as follows: Na2Si03 + 2 NaHC03 = 2Na2C03 + Si02 + H2O The slurry containing the solution of sodium carbonate and silica (11) is filtered (E), obtaining the solid silica product (12) and a solution of sodium carbonate (13) which contains less then 0.1% by weight silica. The cake of silica is washed by tap water (14), and the resulting solution (15) is introduced to the reactor (D).

In this manner, the solution of sodium carbonate will contain less than 0.1% by weight Si02.

In addition to the production of a valuable pure silica product from the porcellanite rock, the invention imparts a significant bonus to the mining of phosphate rock from the Nahal Zin deposits without creating any ecological problem which will required an adequate but costly solution.

The invention will be hereafter illustrated by the following Examples, being understood that these Examples are presented only for a better understanding of the invention without limiting its scope. A person skilled in the art, after reading the present specification will be in a position to insert slight modif cations without being outside the invention as covered by the appended Claims. It should be pointed out that Example 3 does not illustrate the invention, being presented only for comparison purposes.

In. the Examples, the concentrations are given by weight percent unless otherwise stated.

EXAMPLE 1.

A solution of 500 ml of sodium carbonate (17.5% Na20) heated to about 70°C, was thoroughly mixed with 197 g of fines of calcium oxide obtained from the calcination of phosphate rock (as accumulated in the cyclones). To the resulted slurry, it was added an amount of of 129.3 g of porcellanite containing 69% Si02 (from Nahal Zin) having a particle size of 50 mesh. After agitation, the mixture for 30 minutes, the resulted slurry was filtered (on a vacuum filter) and the solids were washed by an amount of 294 g of water, thus obtaining 696 g of a solution and 409 g of solids.

The analyses of said filtrate and solids were as follows: Solids (on dry basis): Fi 1 trate: Si02 3.3% Si02 10.62% Na20 1.4 % Na2C03 3.61% Humidity 40% Si02/Na20 1.55% The filtrate was introduced into a vessel provided with an agitator in which a stream of carbon dioxide (at a rate of 210 ml/min) was introduced for about 90 minutes. Together with the carbon dioxide, an amount of 37 g of sodium bicarbonate was also introduced, the reaction mixture being maintained at a temperature of about 70°C. Silica prec pitation was noticed, its concentration being 0.13%; the slurry was filtered obtaining a cake and a filtrate. The cake was washed by an amount of 87 ml of warm water (about 60°C) obtaining 122 g of pure silica and 94 g of a washing solution containing 0.08% silica and 6% sodium. The analysis of the impurities in the silica product as obtained was as follows: Fe : 6 ppm Cd : 0.3 ppm g : 4 ppm Ba: 4 ppm Ni : 0.5 ppm Ti : 0.5 ppm Zn: 1.5 ppm Ca: 5 ppm Cr 3 ppm The filtrate which resulted, in amount of 722 g contained 14.1% sodium carbonate and 0.13% silica, was reused for an additional dissolution of silica from porcellanite, after the causti f ication step.

EXAMPLE 2.

An amount of 500 g of a solution of sodium carbonate (10%) heated to about 90°C,was mixed for about 90 minutes with 115 g of fines of calcium oxide as obtained from the calcination plant of phosphate rock.

To the alkaline solution obtained, it was added an amount of 50.9g of porcellanite (69% Si02) having a particle size of 50 mesh (Tyler scale) and the agitation was continued for about 30 minutes.

The resulted slurry was filtered on a vacuum filter and the cake was washed with an amount of 132 g of tap water, obtaining 533 g of a filtrate containing 7.1% Si02 and 2.5% of sodium carbonate. The ratio Si02/Na20 was 1.20. The cake obtained in amount of 185 g, consists of calcium carbonate (40% humidity), 2.7% Si02 and 1.2% a20 along other cations impurities.

The filtrate as obtained above, was introduced into a vessel wherein a stream of carbon dioxide (210 ml/hr) was introduced for about 1 hour together with 21 g of sodium bicarbonate, thus precipitating the silica. The silica was filtered out and washed with 34 g of warm water (60°C), obtaining 46.8 g of pure precipitated silica (40% humidity). The filtrate and wash water were recycled for an additional stage of porcellanite precipitation.

EXAMPLE 3. (comparative) .

An amount of 500 g of a sodium carbonate solution (4%) was mixed with 46.5 g of fines of calcium oxide from the valcination of phosphate rock.

The resulted alkaline solution, comprising a mixture of sodium hydroxide and sodium carbonate, was mixed with 29 g of the same porcellanite as used in Example 1 under the same reaction conditions obtaining a slurry, which after filtration gave a cake of 97 g of calcium carbonate (43% humidity) and 545 g of filtrate containing 2.88% Si02 carbonate .

The filtrate was treated exactly as in Example 1 by a stream of carbon dioxide and sodium bicarbonate. A gel was noticed which could not be filtered in order to obtain the desired silica precipitated product.

Claims (11)

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1. C L A I M S :- . A process for the manufacture of a very pure amorphous silica (above 99%) from rocks containing noncrystalline silica, such as porcellanite, which comprises the steps of: (a) dissolution of said rock in an alkaline solution comprising sodium hydroxide and sodium carbonate, at a concentrat on in the range of between 5% to 25% expressed as Na20 obtaining a solution of sodium silicate, and (b) precipitation of pure amorphous silica by adding sodium bicarbonate into said sodium silicate and co-producing sodium carbonate, the weight ratio between Si02 and Na20 in said dissolution step being in the range of between 1.2 to 2.58.

2. The process according to Claim 1, wherein said alkaline solution is obtained by the causti f ication of sodium carbonate with calcium hydroxide.

3. The process according to Claim 1, wherein the concentration of said alkaline solution in said dissolution step, expressed as Na20, is between 7% to 14% by weight.

4. The process according to Claim 1, wherein the cake of silica obtained is first washed by water, followed by a further washing with a dilute solution of hydrochloric acid . - 12 - 104,584/2

5. The process according to Claim 1, wherein the concentration of sodium hydroxide in said alkaline solution is in the range of between 3% to 20% by weight.

6. The process according to Claim 1, wherein the silica-containing rock used in the dissolution step has a particle size in the range of between 50 to 350 mesh (of the Tyler scale) .

7. The process according to Claim 2, wherein the sodium carbonate used in the causti f ication step, is co-produced in the step of silica separation.

8. The process according to Claims 1 to 7, wherein some calcium carbonate is present during the dissolution of silica-conta ning rock by the alkaline solution.

9. The process according to Claim 8, wherein said calcium carbonate assists the filtration of the silica product and retains most of the impurities present in said rock containing non-crystalli e silica.

10. The process according to Claim 1, wherein said rock containing non-crystalline silica is selected from porcellanite, diatomite and amorphic silica.

11. A process for the manufacture of a very pure silica (around 99%) from a rock containing non-crystalline silica, such as porcellanite, substantially as described in the specification and in any one of Claims 1 to 10. For the Applicant,