Claims:We Claim:

1. A process for selective dissolution of reactive silica present in a bauxite sample or a laterite sample; said process comprising treating a crushed and powdered bauxite sample or a laterite sample with water and 3 to 7 % hydrofluoric acid for 5 to 30 minutes at a room temperature in order to dissolve reactive silica.

2. The process as claimed in claim 1, wherein said process comprises the following steps:
- providing the crushed and powdered bauxite sample or laterite sample of 25- 250 mesh size containing 0.25-15 % SiO2;
- mixing said sample with a with double distilled water to form a wet mass;
- subjecting said mass to autogenous dissolution of reactive SiO2 by mixing said mass with 3-7% HF at room temperature for 5 to 30 minutes to obtain a mixture;
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica.

3. The process as claimed in claim 1, wherein said process comprises a pre-step of determining the amount of total silicates in the bauxite sample or laterite sample.

4. The process as claimed in claim 1, wherein said process characterized in that the proportion of hydrofluoric acid corresponds to proportion of total silicates in the bauxite sample or laterite sample.

5. The process as claimed in claim 1, wherein the bauxite sample or laterite sample contains alumina (Al2O3) ranging from 33-50 %.

6. A process for quantitative determination of reactive silica; said process comprises:
- treating a crushed and powdered bauxite sample or laterite sample with water and 3 to 7 % hydrofluoric acid at a room temperature in order to dissolve reactive silica and obtain a mixture; and
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica; and
- measuring the absorbance of said filtrate at λmax 800-810nm.

7. The process as claimed in claim 6, wherein said process further comprises addition of an anionic surfactant to said filtrate.

8. The process as claimed in claim 7, wherein said anionic surfactant is aqueous sodium dodecyl sulphate.

9. The process as claimed in claim 7, wherein the absorbance is measured using a spectrophotometer.

Dated This 28th Day Of June 2018

Applicant’s Patent Agent
 
, Description:FIELD OF THE INVENTION:
The present invention relates to the development of process for selective dissolution of reactive silica (RS=RSiO2) in bauxite and laterite.
The present invention also relates to quantitative determination of reactive silica by visible spectrophotometry for geo-analytical applications.

BACKGROUND OF THE INVENTION:
Bauxite is the most economical ore for commercial production of hydrated aluminium oxide by Bayer chemical process. The nature and concentration of naturally occurring mineralogical phases in bauxite is significant in the formulation of Bayer plant parameters like caustic concentration, digestion pressure, caustic consumption as well as the quantum of red mud (bauxite tailing) generation. Reactive silica (RS) in bauxite is one of the major parameters which often used to assess viability of bauxite of various origins in commercial and industrial means. RS is a measure of the amount of siliceous materials that will react with the caustic soda in Bayer process. RS is an important parameter of a bauxite deposit because the silica that goes into solution is subsequently precipitated as a form of sodium aluminium silicate which results in significant cost because of consumption of caustic soda. Applied analytical methods found extensive use in interdisciplinary analytical needs for both in-situ and ex-situ analysis at R&D and quality testing laboratories for rapid and cost-effective quality check and material characterization. The existing analytical procedure for RS is based on the lengthy procedures involving determination of total silica and reactive silica by treating bauxite with sodium carbonate (Na2CO3).
Accordingly, it is envisaged to explore the possibility of utilizing quick sample preparation procedures for determination of reactive silica in bauxite of all geological origin and laterite so as to enable the rapid analysis within 2-3 hours compared to 28-34 hours for standard wet analysis.

OBJECT OF THE INVENTION:
An object of the present invention is to develop a process for enabling rapid and selective determination of reactive silica present in bauxite of all geological origin of Indian subcontinent and laterite.
Another object of the present invention is to provide selective dissolution of RS in bauxite and laterite at ambient temperature (25-450C) for rapid and selective dissolution of silicate mineral phases.

Still another object of the present invention is to provide a process for the preparation of analytical sample to have preferable size fractions to ensure maximum/complete leaching of reactive silica in bauxite and laterite at room temperature, optimized contact time, grain size and solid-liquid ratio as well as concentration of leach medium with respect to the range of total silica (TS) and reactive silica (RS) in the sample matrix.

SUMMARY OF THE INVENTION:
Accordingly, the present invention provides a process for selective dissolution of reactive silica present in a bauxite sample or a laterite sample; said process comprising treating a crushed and powdered bauxite sample or a laterite sample with water and 3 to 7 % hydrofluoric acid for 5 to 30 minutes at a room temperature in order to dissolve reactive silica.

In one embodiment of the present invention said process comprises the following steps:
- providing the crushed and powdered bauxite sample or laterite sample of 25- 250 mesh size containing 0.25-15 % SiO2;
- mixing said sample with a with double distilled water to form a wet mass;
- subjecting said mass to autogenous dissolution of reactive SiO2 by mixing said mass with 3-7% HF at room temperature for 5 to 30 minutes to obtain a mixture;
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica.

Typically, said process comprises a pre-step of determining the amount of total silicates in the bauxite sample or laterite sample.
Typically, the proportion of hydrofluoric acid corresponds to proportion of total silicates in the bauxite sample or laterite sample.

Typically, the bauxite sample or laterite sample contains alumina (Al2O3) ranging from 33-50 %.

In accordance with another aspect of the present invention there is provided a process for quantitative determination of reactive silica; said process comprises:
- treating a crushed and powdered bauxite sample or laterite sample with water and 3 to 7 % hydrofluoric acid at a room temperature in order to dissolve reactive silica and obtain a mixture; and
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica; and
- measuring the absorbance of said filtrate at λmax 800-810nm.

Typically, the absorbance is measured using a spectrophotometer.
In one embodiment, said process further comprises addition of an anionic surfactant to said filtrate.
Typically, said anionic surfactant is aqueous sodium dodecyl sulphate.

Detailed Description
In order to facilitate rapid analysis, a chemical process is developed for selective autogenous dissolution of reactive silica in bauxite and laterite of all geological origin. The procedure permits self-sustained autogenous dissolution of reactive silica at ambient temperature (20-450C). Physical state of bauxite and laterite, size fraction of sample, concentration of leaching reagents, solid-liquid ratio of sample and leaching reagent and reaction time are optimized in terms of better accuracy reliability and reproducibility. The selectivity of 2-10 % aqueous hydrofluoric acid in the reaction mixture promotes selective dissolution of free silica (SiO2) in to the leach medium. The single stage dissolution reaction acknowledged 99.99 percent extraction of reactive silica (SiO2) at optimized experimental conditions. The leach solution containing reactive silica is diluted to the preferred concentration range and determined by spectrophotometry procedure. Standard addition of sulphate based anionic surfactant is incorporated in the visible spectrophotometry procedure for reliable and reproducible absorbance measurements for silicon at λmax 800-810nm. It was observed that developed method permits determination of reactive silica (SiO2 reactive) in bauxite and laterite containing total silica (SiO2 total) ranging from 0.50 – 25 % maximum and quartz silica (SiO2 quartz) 0.5-15 % maximum.

According to this invention there is provided a process for selective dissolution of reactive silica in an aqueous reagent medium at a room temperature. The mild agitation of reaction mixture enhanced selective release of mineral bearing reactive silica phases in bauxite and laterite likely; kaolinitic silica (SiO2) at ambient reaction temperature. The presence of dilute aqueous hydrofluoric acid (HF) dissolves the active silicates in the matrix and leaves crystalline silica intact. The invention also provides the option for indirect determination of crystalline (quartz) silica (QS) based on the difference of total silica (TS) and reactive silica (RS) for rapid analytical applications thereof.

In one embodiment, the present invention provides a process for selective dissolution of reactive silica present in a bauxite sample or a laterite sample; said process comprising treating a crushed and powdered bauxite sample or a laterite sample with water and 3 to 7 % hydrofluoric acid for 5 to 30 minutes at a room temperature in order to dissolve reactive silica.

In one embodiment of the present invention said process comprises the following steps:
- providing the crushed and powdered bauxite sample or laterite sample of 25- 250 mesh size containing 0.25-15 % SiO2;
- mixing said sample with a with double distilled water to form a wet mass;
- subjecting said mass to autogenous dissolution of reactive SiO2 by mixing said mass with 3-7% HF at room temperature for 5 to 30 minutes to obtain a mixture;
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica.

Typically, said process comprises a pre-step of determining the amount of total silicates in the bauxite sample or laterite sample.
Typically, the proportion of hydrofluoric acid corresponds to proportion of total silicates in the bauxite sample or laterite sample.

Typically, the bauxite sample or laterite sample contains alumina (Al2O3) ranging from 33-50 %.

In accordance with another aspect of the present invention there is provided a process for quantitative determination of reactive silica; said process comprises:
- treating a crushed and powdered bauxite sample or laterite sample with water and 3 to 7 % hydrofluoric acid at a room temperature in order to dissolve reactive silica and obtain a mixture; and
- subjecting said mixture to filtration to obtain a residue and a filtrate containing reactive silica; and
- measuring the absorbance of said filtrate at λmax 800-810nm.

Typically, the absorbance is measured using a spectrophotometer.
In one embodiment, said process further comprises addition of an anionic surfactant to said filtrate.
Typically, said anionic surfactant is aqueous sodium dodecyl sulphate.

The experimental procedures start with collection of bauxite and laterite samples at the site (bauxite mines), mixing, grinding and homogenizing to generate representative sample. Subsequently, the sample is ground in case it appeared as lump and sieved in to size fractions preferably in the range 25 to 200 mesh for leaching for dissolution of amorphous silica (RS). 0.50 to 100.00g bauxite sample is taken from 25-200 mesh fractions in standard weighing cups ranging from 0.50-250g capacity and transferred in a 500mL beaker.
The sample is made wet with 1-20mL double distilled water and mixed with 20-50 mL dilute hydrofluoric acid (HF) of 1-10% concentration in cold condition.

The mixture is subjected to shaking intermittently in cold condition and allowed to stand for 20- 30 minutes in ice-tray to ensure complete dissolution of amorphous silicates. The beaker containing sample-reagent mixture is filtered by Whatman No.40 filter paper. The filtrate is collected and subjected for colour complex formation in the presences of dilute aqueous solution of sodium dodecyl sulpahte (SDS). Silicates in the filtrate are determined by visible spectrophotometry.

PROCESS STEPS AND METHODOLOGY:
According to this invention there is provided process details for preparation of bauxite and laterite which involve collection of lump sample (1 - 2 kg) at various locations followed by crushing ( lump dry residue) in to smaller fractions (1 - 2 cm) in laboratory crusher. Further the process steps lead homogenization of the fraction prior to braking into smaller fraction (0.5 - 1.0cm). Crushed smaller fractions were pulverized to get sample fractions in the range 25 - 100 mesh. The lower fractions are further ground to finer fractions in the range 100 - 200 mesh. Subsequently the sample is sieved in to separate group that contains 100 - 150 mesh and 200 mesh for the selective leaching of SiO2 bearing mineral phases (RS).

In accordance with the above method, for leaching of SiO2 bearing mineral phases in bauxite (BXT) and overburden laterite (OBL), the sample is made wet with 1-10mL double distilled water and mixed with 20-50 mL dilute hydrofluoric acid (HF) of 1- 10% concentration in cold condition. The reaction mixture is allowed to stand for 20-30 minutes after proper shaking and mixing to complete the dissociation of HF at ambient temperature. In the subsequent stage the mixture is filtered through Whatman No.40 filter paper. Residue on the filter paper is washed analytically with distilled water to ensure complete transfer of RS in the filtrate.
To achieve the optimum experimental conditions in the preparation of BXT and OBL samples suitable for determination of RS specific experimental parameter are defined. The major parameters such as size fractions for liberation of major mineral phases, composition of total silica, reactive silica and quartz silica, solid liquid ratio, order of component addition and contact time of solid-liquid are also assessed:
a) Crushed and powdered BXT and OBL (100% as dry basis, containing 0.50 – 2.00 % moisture at room temperature and 25-200 mesh size fraction) and double distilled water are hand mixed in a beaker to form a wet paste.
b) The homogenized wet mix of red BXT and OBL are further mixed with 20-50 mL aqueous HF of concentration 1-10% and allowed to stand for 10-50 minutes at room temperature (20-45 0C) for dissolution of silica (RS) bearing mineral phases.
c) The reaction mixture is covered with watch glass and kept in cold water bath at ice-cold condition to prevent vaporization of fluoro-silisic acid formed during the dissolution of RS. The reaction mixture is slowly homogenized with a teflon rod
d) The reaction mixture is allowed to stand for 20– 50 minutes.
e) The reaction mixture is filtered and the filtrate along with washings was collected in polythene volumetric flask. The solution is made up to 100 mL with double distilled water and marked as stock for analysis of silicate minerals by standard analytical procedures.
f) Typical characteristics (chemical and mineralogical) of few BXT samples used for the studies are shown in Table 1. The optimized experimental conditions identified for autogenous dissolution of RS minerals in BXT and OBL at room temperature is given in Table- 2.
g) The repeatability and reproducibility of autogenous dissolution of RS in BXT and
OBL at ambient temperature is summarized in Table 3.
h) Details about indirect determination of quarts silica (QS) based on the difference of reactive silica (RS) and total silica (TS) is shown in Table 4.
i) The range of sample quantity (weight of sample in g) required for determination of RS with respect to the range of total silica (%) in BXT or OBL is illustrated in Table 5.

Table 1: Chemical and mineralogy of some BXT used for the studies
Mineral
Phases Bauxite
A B C D E F G
Al2O3 as gibbsite 35.95 45.75 33.33 43.14 50.33 47.71 49.67
Al-goethite 01.99 01.04 01.90 00.68 00.57 01.04 01.13
Kaolinite 00.59 01.38 03.95 02.37 02.57 00.99 00.59
Bohemite - - - 03.82 00.85 00.42 -
Diaspore - - - - 00.42 - -
Total 38.53 48.17 38.37 50.01 54.73 50.16 51.40
SiO2 as Kaolinite 00.70 01.63 04.65 02.79 03.03 01.16 00.70
Quartz 03.50 01.50 08.00 - 00.50 - -
Total 04.20 03.13 12.66 02.79 03.53 01.16 00.70
Fe2O3 as Al-goethite 17.67 09.23 09.64 06.02 05.02 09.23 10.40
Hematite 09.50 08.00 11.50 05.50 03.00 10.00 07.00
Limonite 00.53 - - - - - -
Total 27.69 17.23 21.14 11.52 08.02 19.23 17.04
TiO2 as Anatase - 02.50 00.50 08.50 02.00 01.50 01.50
Rutile - - - 00.50 - - -
Limonite 00.53 - - 09.00 -
Total 00.53 02.50 00.50 02.00 01.50 01.50
CaO as calcite - - - - 01.12 - -
Total - - - - 01.12 - -

Table 2: Conditions for autogenous dissolution of RS in BXT and OBL
Item Condition
Sample
• Dry powder 100-200 mesh fraction
• Nature of reaction medium Aqueous, acidic, chemical dissociation
• Aprox. range of dissociation in % RS : 95-99
Reagents
• HF RS : 1-10 % [v/v aqueous]
• Volume HF RS: 0.125 :10 Solid to Liquid
[1-10 % aqueous]
• Water DM or Double distilled
• Volume 30-150 mL
Physical parameters
• Temperature Room temperature (20-45 0C)
• Reaction time RS: 15-20 minutes
Table 3 :Autogenous dissolution of RS in some BXT and OBL samples
% Reactive silica
Sample Spectrophotometry*
2-3 hours Gravimetry
3 days Difference
1 06.64 06.83 00.19
2 02.13 02.53 00.24
3 01.72 01.75 00.03
4 01.01 00.56 00.45
5 01.31 01.09 00.22
6 01.92 01.80 00.12
7 05.97 05.75 00.22
8 04.05 04.04 00.01
Standard BXT 13.16 13.50 00.34
*With surfactant addition; All values are average of three experimental trials

Table 4: Quarts silica from weight difference of Total silica and Reactive silica

Sample fraction
(Mesh) % Total silica
(A) % Reactive silica (B) % Quartz silica
(A-B)
A3( -200#) 04.06 01.59 02.47
B3( -200#) 03.08 01.92 01.16
C3( -200#) 11.00 05.80 05.20
% of quarts silica = % of total silica - % of reactive silica

Table 5: Sample weight according to total silica content IN BXT and OBL

Sr. No Weight of sample % of total silica contain
1 0.5gm 1-5%
2 0.2gm 5-10%
3 0.1gm 10-15%
4 0.05gm 15% & above

The following examples illustrate the exact process of preparation of bauxite samples and autogenous dissolution of alumina and silica at room temperature.
EXAMPLE-1
0.25-10 gm of dry bauxite [BXT] or overburden laterite [OBL]of 25 – 200 mesh fractions was mixed with 1-30 mL double distilled water to make a wet paste in Teflon beaker. The wet paste was mixed further with 10-40 mL aqueous hydrofluoric acid [HF] of 1.0-10% and allowed to stand for 10-40 minutes at room temperature (20-450C). After completion of chemical reaction, the mixture was filtered through Whatman No 40 filter paper in polythene volumetric flask for separation of residue and filtrate in ice-cold condition. Dilute the sample to 100 mL with DD. Take 1-5 mL sample in 100ml volumetric flask for dilution and keep in cold condition for about 20-40 minutes prior to spectrophotometric determination that involve addition of aqueous sodium dodecyl sulfate (SDS) after addition of sodium molybdate.

EXAMPLE-2
0.50-50 gm of dry bauxite [BXT] or overburden laterite [OBL]of 50 – 150 mesh fractions was mixed with 1-30 mL double distilled water to make a wet paste in Teflon beaker. The wet paste was mixed further with 05-30 mL aqueous hydrofluoric acid [HF] of 0.50-15% and allowed to stand for 15-50 minutes at room temperature (20-45 0C). After completion of chemical reaction the mixture was filtered through Whatman No 40 filter paper in polythene volumetric flask for separation of residue and filtrate in ice-cold condition. Dilute the sample to 100 mL with DD. Take 1-5 mL sample in 100ml volumetric flask for dilution and keep in cold condition for about 10-30 minutes prior to spectrophotometric determination that involve addition of aqueous sodium dodecyl sulfate (SDS) after addition of sodium molybdate.

EXAMPLE-3
1.0-20 gm of dry bauxite [BXT] or overburden laterite [OBL]of 100 – 200 mesh fractions was mixed with 1-20 mL double distilled water to make a wet paste in Teflon beaker. The wet paste was mixed further with 05-30 mL aqueous hydrofluoric acid [HF] of 1.00-35% and allowed to stand for 10-40 minutes at room temperature (20-45 0C). After completion of chemical reaction, the mixture was filtered through Whatman No 40 filter paper in polythene volumetric flask for separation of residue and filtrate in ice-cold condition. Dilute the sample to 100 mL with DD. Take 1-5 mL sample in 100mlvolumetric flask for dilution and keep in cold condition for about 5-40 minutes prior to spectrophotometric determination that involve addition of aqueous sodium dodecyl sulfate (SDS) after addition of sodium molybdate.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.