DESC:The present disclosure provides a method for recovery of a-alumina from aluminium dross. Particularly, a method of recovery of nano alumina from aluminium dross is disclosed.

Background
Aluminium dross is residue from primary and secondary aluminium production and is generated during aluminium smelting process. Aluminium dross is classified into white and black dross, according to its metal content. White dross is of higher metal content and it is produced from primary and secondary aluminium smelters. White dross may contain 15% to 70% recoverable metallic aluminium. Black dross has a lower metal content and is generated during aluminium recycling. Black dross contains recoverable aluminium in a range of 2% to 15%. Black dross is mainly mixture of aluminium oxide and other compounds such as AlN, Al4C3, MgF2, NaAlCl4, KNaCl4, SiO2, Fe2O3 and MgO etc.
In general, aluminium dross is processed in rotary kilns to recover the aluminium and the resultant salt cake is sent to landfills as waste. Such disposal is increasingly facing environmental problems since salts can leach from the aluminium dross and pass into the soil below. In addition to this when aluminium dross comes in contact with water it emits harmful gases such as NH3, CH4, PH3, H2, H2S, etc. It is known in art that the dross produced by the aluminium industry, although a waste, contain significant amount of valuable element such as aluminium oxide, which can be recovered and reused.
A process for production of G-alumina from waste aluminium dross is reported in Das et al; “Production of G-alumina from Waste Aluminium Dross”, Minerals Engineering 20 (2007) 252–258.This document discloses a process for obtaining G-alumina from aluminium dross and the percent recovery of G-alumina is less than 90%. Furthermore, the G-alumina obtained by the process disclosed in said document does not have nanoscale dimensions.
US7651676 describes a process for preparing aluminium chloride by leaching aluminium dross residues with hydrochloric acid or sulphuric acid to obtain a mixture comprising a solid and a liquid. Further,the liquid obtained is hydrochlorinated to precipitate aluminium chloride. The process further comprises of converting the aluminium chloride into alumina.
The method known in the art either use an additional calcining step or other multiple steps to obtain alumina. There is therefore a need for a simpler and economical process for recovery of alumina from aluminium dross.

Summary
A method for recovery of a-alumina from aluminium dross is disclosed. The method comprises of grinding aluminium dross to obtain particles having average particle size in the range of 80-100 micron, leaching aluminium dross with an acid to obtain a leach liquor having precipitated silica, separating the silica from the leach liquor to obtain a liquid phase, treating the liquid phase with an aqueous ammonia at acidic pH to facilitate precipitation of iron hydroxide, separating the iron hydroxide from the liquid phase to obtain an iron free liquid phase, treating the iron free liquid phase with aqueous ammonia at alkaline pH to facilitate precipitation of aluminium hydroxide; separating and calcining the aluminium hydroxide to obtain a-alumina having size in the range of 200-700 nm.

Brief Description of Drawings
Figure 1: illustrates scanning electron micrograph of a-alumina obtained in accordance with the disclosed method.
Figure 2: illustrates phase analysis using powder X-ray diffraction of a-alumina obtained by the disclosed method.

Detailed Description
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the disclosed process, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
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 invention and are not intended to be restrictive thereof.
Reference throughout this specification to “one embodiment” “an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
The term "nano alumina" as referred to herein means alumina particles having dimensions of nanoscale (1-1000nm).
The present disclosure provides a method for recovery of a-alumina from aluminium dross. Particularly, a method of recovery of nano alumina from aluminium dross is disclosed. Nano alumina has better properties including reactivity as compared to a-alumina not having dimensions of nanoscale. The method comprises of grinding of aluminium dross to obtain an average particle size in a range of 80-100 micron. The ground aluminium dross is subjected to leaching at high temperature with an acid to obtain leach liquor having aluminium and iron in a dissolved state and a precipitate of silica. The silica thus obtained is separated from the leach liquor. The liquid phase obtained after separation of silica is treated with aqueous ammonia at an acidic pH to facilitate precipitation of iron as iron hydroxide [Fe(OH)3]. The iron hydroxide is separated to obtain iron free liquid phase. Thus obtained iron free liquid phase is treated with aqueous ammonia at an alkaline pH to facilitate precipitation of aluminium hydroxide. The method further comprises of separating and calcining the aluminium hydroxide to obtain a-alumina having size in a range of 200 nm to 700 nm.
In accordance with an embodiment, separation of silica from the leach liquor is carried out by filtration.
In accordance with an embodiment, the acidic pH at which the liquid phase is treated with aqueous ammonia is in a range of 4-6.
In accordance with an embodiment, separation of iron hydroxide from the liquid phase is carried out by filtration.
In accordance with an embodiment, the alkaline pH at which the iron free liquid phase is treated with aqueous ammonia is in a range of 7.5-8.5.
In accordance with an embodiment, separation of aluminium hydroxide is carried out by any known methods such as filtration. The separated aluminium hydroxide is subjected to washing and drying. Thus obtained dried aluminium hydroxide is calcined at a temperature in a range of 1100ºC to 1200ºC to obtain a-alumina.
In accordance with an embodiment, the aluminium dross may be a black dross comprising Al2O3, AlN, SiO2 and Fe2O3. In accordance with an embodiment, the black dross comprises of Al2O3 in a range of 60-80%, AlN in a range of 15-30%, SiO2in a range of 0-10% and Fe2O3in a range of 0-5%. By way of specific example, the black dross comprises of 70% of Al2O3, 25% of AlN, 3.5% of SiO2 and 1.5% of Fe2O3.
In accordance with an embodiment, the grinding of aluminium dross is carried out to obtain particle size in a range of 7-1540 micron, with average particle size of 80-100 micron, preferably 90 micron.
In accordance with an embodiment, acid utilized for leaching of aluminium dross is sulphuric acid having concentration in a range of 30%-50% (v/v).
In accordance with an aspect, leaching of aluminium dross with sulphuric acid is carried out at a high temperature in a range of 170 ºC -230 ºC for a time period in a range of 10- 15 hours. Leaching of aluminium dross at such a temperature for prolonged duration of time (i.e. 10-15 hrs) facilitates production of highly pure a-alumina with high percentage yield.
In accordance with an embodiment, aqueous ammonia used for precipitation of iron hydroxide and aluminium hydroxide has a concentration in a ratio of 1:1.
In accordance with an embodiment, drying of aluminium hydroxide precipitate prior to calcination is carried out at a temperature in a range of 100 ºC to 200 ºC.
In accordance with an embodiment, calcination of dried aluminium hydroxide precipitate is carried out for a time period in a range of 15 minutes to 1 hour to obtain a-alumina having size in the range of 200 nm to 700 nm.
In accordance with an embodiment, percentage yield of a-alumina by the method disclosed herein is >90.
In accordance with an embodiment, the a-alumina obtained by the method disclosed herein is highly crystalline single phase a-alumina (as illustrated in Figure 2) having purity of 99.5%.

The following example(s) of method for recovery of a-alumina from aluminium dross and/or analysis of alumina obtained from said method are exemplary and should not be understood to be in any way limiting.

Example 1:
The raw aluminium dross used in this study contained 80% alumina, (quantitative XRD study using Rietveld analysis (70% Al2O3 and 20% AlN, i.e 10% Al2O3). 10 gms batch of the aluminium dross was taken and subjected to leaching, precipitation followed by calcination at 1200?C. White powder obtained after calcination was subjected to powder X-ray diffraction which revealed the single phase nature of the material (presence of highly crystalline alumina). The weight of the powder was found to be 5.6 gms indicating 92% alumina extraction efficiency (yield).

Whiteness of the powder sample was found to be 92 when measured using reflectance spectrophotometer.

Example 2:
Purity of synthesised alumina was evaluated using EDTA back titration. For the titration, the sample was prepared by dissolving alumina obtained from the method disclosed in a mixture of acids. In conical flask sample solution was pipetted out along with buffer solution (pH=10) and EDTA (0.01M). Subsequently, the solution was boiled on a hot plate for 5 mins to speed up the formation of Al-EDTA complex. 3-4 drops of Erichrome black T indicator was added and mixed so that the solution appears pure blue in colour. Excess of EDTA was titrated with ZnSO4 solution (0.01M) until the colour changed to purple blue at its endpoint. The purity of alumina was calculated and found to be 99.5%.

Specific Embodiments are Described Below
A method for recovery of a-alumina from aluminium dross comprising grinding aluminium dross to obtain particles having average particle size in the range of 80-100 micron, leaching aluminium dross with an acid to obtain a leach liquor having precipitated silica, separating the silica from the leach liquor to obtain a liquid phase, treating the liquid phase with an aqueous ammonia at acidic pH to facilitate precipitation of iron hydroxide, separating the iron hydroxide from the liquid phase to obtain an iron free liquid phase, treating the iron free liquid phase with aqueous ammonia at alkaline pH to facilitate precipitation of aluminium hydroxide; and
separating and calcining the aluminium hydroxide to obtain a-alumina having size in the range of 200-700 nm.
Such method(s), wherein leaching of aluminium dross is carried out at a temperature in the range of 170?C – 230?C for a time period in the range of 10-15 hrs.
Such method(s), wherein the acid is suphuric acid having concentration in the range of 30%-50% (v/v).
Such method(s), wherein the acidic pH at which the liquid phase is treated with aqueous ammonia is in the range of 4-6.
Such method(s), wherein the alkaline pH at which the iron free liquid phase is treated with aqueous ammonia is in the range of 7.5 to 8.5.
Such method(s), wherein calcining is carried out at a temperature in the range of 1100?C – 1200?C.

Industrial Application
The method for recovery of a-alumina from aluminium dross described above is easy to perform, economical and utilizes a waste generated during aluminium smelting process as a starting material. The a-alumina obtained from the method described above is highly crystalline single phase a-alumina having percentage purity of 99.5. Further, a-alumina obtained has a size in the range of 200 nm to 700 nm. Nano alumina even if used in small amounts significantly improves properties of a desired product.
,CLAIMS:We Claim:

1. A method for recovery of a-alumina from aluminium dross comprising:
grinding aluminium dross to obtain particles having average particle size in the range of 80-100 micron;
leaching aluminium dross with an acid to obtain a leach liquor having precipitated silica;
separating the silica from the leach liquor to obtain a liquid phase;
treating the liquid phase with an aqueous ammonia at acidic pH to facilitate precipitation of iron hydroxide;
separating the iron hydroxide from the liquid phase to obtain an iron free liquid phase;
treating the iron free liquid phase with aqueous ammonia at alkaline pH to facilitate precipitation of aluminium hydroxide; and
separating and calcining the aluminium hydroxide to obtain a-alumina having size in the range of 200-700 nm.

2. A method for recovery of a-alumina from aluminium dross as claimed in claim 1, wherein leaching of aluminium dross is carried out at a temperature in the range of 170?C – 230?C for a time period in the range of 10-15 hrs.

3. A method for recovery of a-alumina from aluminium dross as claimed in claim 1, wherein the acid is suphuric acid having concentration in the range of 30%-50% (v/v).

4. A method for recovery of a-alumina from aluminium dross as claimed in claim 1, wherein the acidic pH at which the liquid phase is treated with aqueous ammonia is in the range of 4-6.

5. A method for recovery of a-alumina from aluminium dross as claimed in claim 1, wherein the alkaline pH at which the iron free liquid phase is treated with aqueous ammonia is in the range of 7.5 to 8.5.

6. A method for recovery of a-alumina from aluminium dross as claimed in claim 1, wherein calcining is carried out at a temperature in the range of 1100?C – 1200?C.

Dated this 22nd day of April, 2013

Sneha Agarwal
Of Obhan & Associates
Agent for the Applicant
Patent Agent No. 1969