FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION
A froth flotation process for recovering up to 95% unburnt carbon from waste fly ash
APPLICANTS
Hindalco Industries Ltd, an Indian Company, Ahura Centre, 1st Floor, B wing, Mahakali Caves Road, Mumbai-400093, Maharashtra, India.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes this invention and the manner in which it is to be performed:

A froth flotation process for recovering up to 95% unburnt carbon from waste fly ash
FIELD OF THE INVENTION
[01] The present invention relates to a froth flotation process for recovering up to
95% unburnt carbon from waste fly ash.
BACKGROUND OF THE INVENTION
[02] Nowadays, there is a great emphasis on sustainability in most industries.
Utilization of and value recovery from waste constitutes one of the major areas of focus for achieving the goal of sustainability. Fly ash is one of the major wastes generated from coal-based power plants, the utilization of which is a major challenge. The waste fly ash generated from captive power plants contains a certain amount of unburnt carbon, which varies from ~ 6-12% depending upon the coal quality, its mineralogical characteristics and also the combustion efficiency. The benchmark value of unburnt carbon in fly ash is < 2. The unburnt carbon lost with fly ash has a major impact on the overall energy efficiency of a power plant and also on the overall carbon balance.
[03] There are available technologies for recovery of unburnt carbon involving
complex processes such as gravity separation, electrostatic separation, and oil agglomeration as well as sieve-shaking. Most of these methods, except for sieve-shaking are capital and energy intensive technologies. Also, each of these methods leads to another solid / liquid waste being generated which then needs to be disposed suitably. Further, though the sieve-shaking method is the simplest method and most

economical method, the efficiency of separation is very low, making the method
unpopular.
[04] There is, therefore, a need for a simple, sustainable and economical process
for recovering the maximum amount of unburnt carbon having high purity from waste fly ash such that the residual fly ash after said recovery contains less than 2% unburnt carbon.
SUMMARY OF THE INVENTION
[05] According to an embodiment of the invention there is provided a froth
flotation process for recovering up to 95% unburnt carbon from waste fly ash comprising:
a. screening waste fly ash to select fly ash of a predetermined fineness;
b. mixing alkane hydrocarbon and an alcohol-based frother chemical with a
slurry of the fly ash in water;
c. separately collecting and filtering concentrate and tailings during froth
flotation and recycling each filtrate back to the slurry; and
d. collecting the concentrate containing unburnt carbon, having a purity of 50-
60% and a calorific value of 4000-4200 kCal/kg, after filtration of the froth
and collecting a low carbon fly ash residue having less than 2% unburnt
carbon from the tailings after filtration;
provided that the amount of unburnt carbon is 6-12% in the waste fly ash at the outset.
BRIEF DESCRIPTION OF THE DRAWINGS

[06] FIG. 1 is a process flow diagram of the froth flotation process for recovery of
unburnt carbon from waste fly ash, according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[07] In the following detailed description, reference is made to the accompanying
drawings that form a part hereof. The embodiments of the invention are described in sufficient detail to enable those skilled in the art to practice the invention and it is understood that other embodiments may be utilized and that logical processual changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the illustrative embodiments are defined only by the appended claims.
[08] The present invention provides a froth flotation process for recovering up to
95% unburnt carbon from waste fly ash comprising:
a. screening waste fly ash to select fly ash of a predetermined fineness;
b. mixing alkane hydrocarbon and an alcohol-based frother chemical with a
slurry of the fly ash in water;
c. separately collecting and filtering concentrate and tailings during froth
flotation and recycling each filtrate back to the slurry; and
d. collecting the concentrate containing unburnt carbon, having a purity of 50-
60% and a calorific value of 4000-4200 kCal/kg, after filtration of the froth

and collecting a low carbon fly ash residue having less than 2% unburnt carbon from the tailings after filtration;
provided that the amount of unburnt carbon is 6-12% in the waste fly ash at the outset.
[09] Preferably, the alkane hydrocarbon is n-hexadecane and the alcohol-based
frother chemical is a mixture of alcohols, heavy aldehydes, esters and glycols. The alcohol-based frother chemical can be OrePrep F-501, Aerofroth 70, Aerofroth 76A, Aerofroth 88, or combinations thereof.
[10] The slurry of fly ash in water is ideally 10 - 30% w/w. Preferably, the fly
ash is ground and screened for a pre-determined fineness of 45 – 150 µ before making the slurry.
[11] Step (a) is preferably performed in a froth flotation cell being mixed at a
constant speed of 600-1000rpm for a pre-determined time of about 30 to 60 mins. In step (b), preferably alkane hydrocarbon in an amount of 0.1% based on weight of the waste fly ash is first mixed into the slurry for a pre-determined time. In step (b), the frother chemical in an amount of 2.5% based on weight of fly ash is preferably mixed into the slurry for a pre-determined time after mixing alkane hydrocarbon into the slurry.
[12] Compressed air at a pressure of 4-6kg/cm2 can be blown into the slurry for
a pre-determined time to facilitate froth flotation. In an embodiment of the invention, each filtrate of step (b) is re-cleaned prior to recycling back to the slurry for higher recovery of unburnt carbon and low carbon fly ash residue.
[13] The present invention involves a simple froth flotation process applying
specific chemicals and bubbling air through the slurry of the waste fly ash in water,

thereby leading to a physical separation of the unburnt carbon and the residual fly ash. The operation is carried out at room temperature and does not involve application of a higher energy source for separation. Also, both the concentrate and froth generated from the present process are used in further applications.
[14] Unburnt carbon generated from the process of the invention can be used as an
adsorbent, and can be recycled and formed as pellets to be re-used in the power plant, while the residual fly ash is recycled back to the process. The residual fly ash product has minimum amount of unburnt carbon and hence can be used in the cement industry or for other applications. The unburnt carbon recovered has a very high calorific value, ~ 4000 kCal/kg, equivalent to indigenous coal, and has a purity of 50 to 60% and can be reused as a fuel in calcination / heating processes for energy recovery.
[15] The process of this invention helps in reducing the overall energy requirement
of the power plants through recycling. Further, the process is simple, economical, and does not generate any additional waste products.
[16] The drawings are described hereafter. FIG. 1 is a process flow diagram of the
process according to an embodiment of the invention. A slurry (105) of fly ash (101) in water (102) is mixed with a collector chemical like alkane hydrocarbon (107) and a frother chemical (109) like OrePrep F-501 in a froth flotation cell (103). Air (111) is bubbled into the cell (103) through a means (115) for the same and the cell is stirred using a stirrer (113). The froth comprises a concentrate (117) of unburnt carbon which is filtered by a means (150) of filtration and the resultant unburnt carbon (119) is used for various applications (160), including as an adsorbent.
[17] The tailings (121) collected from the bottom of the froth flotation cell are also
filtered by a means (150) of filtration to yield residual fly ash (123) which has various

applications (170), including being recycled back for use in a captive power plant.
The filtrate collected from both the means of filtration primarily containing water
(102’) is recycled back to the slurry of fly ash and water to avoid any wastage and to
collect any unburnt carbon and residual fly ash left behind and yet to be recovered.
[18] The following experimental example is illustrative of the invention but not
limitative of the scope thereof:
Example 1:
[19] Carbon content of waste fly ash is generally measured as Loss on Ignition
(LOI). In the present invention too, LOI analysis of fly ash was selected as the method for estimating the unburnt carbon content in fly ash, as it is a simple method and less time consuming. LOI analysis was carried out at a higher temperature of ~ 900-1100oC for a shorter duration of ~ 10-15 mins. Similar procedure was followed for the concentrate and tailings sample generated from the froth flotation process.
[20] The process involved grinding the fly ash and screening the same for a pre-
determined fineness of 45 – 150 µ before making the slurry of fly ash in water of a desired consistency, ~ 10-30 % w/w at room temperature. The slurry was then added to the flotation cell of ~ 3 L and was mixed at a constant speed of ~ 800 rpm for a desired time of ~ 10 mins to make the slurry homogenous. A suitable collector chemical, i.e. n-Hexadecane, was then added at the required dosage of 0.1% (added based on % of fly ash weight taken) and the agitation was continued for a specific time of ~10 mins. Suitable frother chemical, OrePrep F-501 was then added at a specified dosage of 2.5% (added based on % of fly ash weight taken) and agitated for a given time of ~ 10mins. After this, compressed air flow at a pressure of ~ 4 kg/cm2

was started for generation of froth. The air flow was stopped after a specific time of ~ 20mins, which is the average time for flotation, indicated by a drop in the froth generation. After flotation, the concentrate and the tailings were filtered, and the water was recycled back into the process to reduce freshwater consumption. The concentrate and the tailings obtained after filtration were analyzed for LOI and XRF (X-ray fluorescence). Also, the tailings were analyzed for GCV (Gross calorific value), while the concentrate was analyzed for purity of the unburnt carbon.
[21] The unburnt carbon recovered was found to have a very high calorific value,
~ 4000 kCal/kg, and a purity of 50 to 60%. Also, the amount of unburnt carbon recovered was up to 95% of the unburnt carbon in the waste fly ash. Starting out with an amount of unburnt carbon of 6-12% in the waste fly ash, the amount of unburnt carbon reduced to less than 2% in the residual fly ash.
[22] The above example is non-limiting. The invention is defined by the claims
that follow.

We claim:
1. A froth flotation process for recovering up to 95% unburnt carbon from waste fly ash
comprising:
a. screening waste fly ash to select fly ash of a predetermined fineness;
b. mixing alkane hydrocarbon and an alcohol-based frother chemical with a
slurry of the fly ash in water;
c. separately collecting and filtering concentrate and tailings during froth
flotation and recycling each filtrate back to the slurry; and
d. collecting the concentrate containing unburnt carbon, having a purity of 50-
60% and a calorific value of 4000-4200 kCal/kg, after filtration of the froth
and collecting a low carbon fly ash residue having less than 2% unburnt
carbon from the tailings after filtration;
provided that the amount of unburnt carbon is 6-12% in the waste fly ash at the outset.
2. The process as claimed in claim 1, wherein the alkane hydrocarbon is n-hexadecane and the alcohol-based frother chemical is a mixture of alcohols, heavy aldehydes, esters and glycols.
3. The process as claimed in claim 1, wherein the alcohol-based frother chemical is OrePrep F-501, Aerofroth 70, Aerofroth 76A, Aerofroth 88, or combinations thereof.
4. The process as claimed in claim 1, wherein the slurry of fly ash in water is 10 - 30% w/w.
5. The process as claimed in claim 1, wherein the fly ash is ground and screened for a pre-determined fineness of 45 – 150 µ before making the slurry.

6. The process as claimed in claim 1, wherein step (a) is performed in a froth flotation cell being mixed at a constant speed of 600-1000rpm for a pre-determined time.
7. The process as claimed in claim 1, wherein, in step (b), alkane hydrocarbon in an amount of 0.1% based on weight of the waste fly ash is first mixed into the slurry for a pre-determined time.
8. The process as claimed in claim 1, wherein, in step (b), the frother chemical in an amount of 2.5% based on weight of fly ash is mixed into the slurry for a pre-determined time after mixing alkane hydrocarbon into the slurry.
9. The process as claimed in claim 1, wherein compressed air at a pressure of 4-6kg/cm2 is blown into the slurry for a pre-determined time to facilitate froth flotation.
10. The process as claimed in claim 1, wherein each filtrate of step (b) is re-cleaned prior to recycling back to the slurry for higher recovery of unburnt carbon and low carbon fly ash residue.