Claims:WE CLAIM:
1. A process for conversion of inorganic cyanide of aluminium spent pot lining material (SPL) to non-hazardous material; said method comprising the following steps:
a. heating aluminium spent pot lining material (SPL) at a temperature ranging from 300 to 4500C for a time period ranging from 25 to 35 minutes to obtain non-hazardous material mixture; and
b. subjecting said mixture to heat treatment at a temperature ranging from 450 to 600°C for a time period ranging from 30 to 50 minutes to obtain non-hazardous material,
said method characterized in that the step (a) converts soluble cyanide present in the SPL to non-toxic material.; and the step (b) at least partially converts insoluble cyanide to non-hazardous material.

2. The process as claimed in claim 1, wherein the aluminium spent pot lining material (SPL) is 1st cut aluminium spent pot lining material (SPL) which is a hazardous waste of aged and degraded carbonaceous material cell lining separated from refractory lining which is removed from the aluminium smelting cell.

3. The process as claimed in claim 1, wherein the process comprises a pretreatment of SPL material, said pre-treatment consists of crushing the SPL material to less than 50 mm size.

4. The process as claimed in claim 1, wherein the heating is carried out in an apparatus selected from the group consisting of electrically heated rotary tube furnace and kiln furnace in the presence of stagnant/controlled air flow.

5. The process as claimed in claim 1, wherein said process converts at least 98% soluble cyanide and at least 50% of insoluble cyanide into non-hazardous material.

6. A process for recovery of sodium, fluoride and carbon; said process comprising the following steps:
a) subjecting a non-hazardous material obtained from heat treatment of SPL material to leaching with water in order to leach out soluble sodium and fluoride to obtain a leached solid-liquid mixture;
b) decanting said leached solid-liquid mixture to separate out solid fraction and liquid fraction;
c) washing the solid fraction with water to remove adsorbed liquid containing soluble sodium and fluoride to obtain a washed solid-liquid mixture;
d) subjecting the washed solid-liquid mixture to decantation to separate out solid and liquid fraction; and
e) recycling said liquid fraction to leach out the soluble sodium and fluoride.

7. The process as claimed in claim 6, wherein the ratio of non-hazardous material to water is in the range of 1: 10 to 1:20 and the ratio of said solid fraction to water is 1:10.

8. The process as claimed in claim 6, wherein the process further comprises at least one step selected from the group consisting of drying and filtration.

9. The process as claimed in claim 6, wherein the non-hazardous material obtained from heat treatment of SPL material is free from soluble cyanide and at least partially free from insoluble cyanide.

10. The process as claimed in claim 6, wherein said process characterized by recovery of sodium as sodium hydroxide at a concentration in range of 100-160 gpl and leaching is carried out below 60 °C thereby restricting silica in the liquid fraction.

Dated This 8th November 2017

Prashant Patankar
Of Novoip
Applicant’s Patent Agent
, Description:FORM –2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

A PROCESS FOR CONVERSION OF HAZARDOUS ALUMINIUM SPENT POT-LINING (SPL) WASTE INTO NON HAZARDOUS MATERIAL AND RECOVERY OF SODIUM, FLUORIDE AND CARBON VALUE

APPLICANTS:

1. JAWAHARLAL NEHRU ALUMINIUM RESEARCH DEVELOPMENT AND DESIGN CENTRE,
Amravati Road Wadi, Nagpur - 440 023, Maharashtra, India.
AND

2. VEDANTA ALUMINIUM LIMITED
An Indian Organization of Bhurkahamunda PO-Sripura, Dist-Jharsuguda, Odisa, 768202, India.

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH I TIS TO BE PERFORMED.

FIELD OF THE INVENTION:

The invention relates to a process for conversion of inorganic soluble and insoluble cyanide present in the aluminium spent pot-lining (SPL) materials to non-toxic species and recovery of sodium, fluoride and carbon value from the non-toxic SPL material.

BACKGROUND ART:

Worldwide aluminium is produced by electrolysis of alumina at high temperature (~960 °C) in a molten cryolite/AlF3 bath. The pot in which electrolysis carried out called electrolysis pot. The electrolysis pot is provided with electrically conductive carbon linings which act as a cathode in the electrolysis pot.

During the electrolysis process direct current is conducted into the melt through prebaked carbon anode to electrolyze the dissolved alumina. In the electrolysis process the carbon anode get consumed as per the chemical reaction given below:

2 Al2O3 + 3 C = 4 Al + 2 CO2?

Opposite to carbon anode, the carbon cathode does not participate in any reaction and hence do not get consumed and remain intact in operation for longer period of time. But, over a period of electrolysis pot operations, the carbon lining gradually deteriorates due to penetration of elemental sodium and liquid bath materials into the pores of carbon lining material. This results into the cracks or heaving/swelling of cathode which results into the reduction in the integrity of linings. Pot of this type may be used for considerable periods of time, e.g. up to seven years. The pot is said to be fail when iron is detected in molten aluminium, when cell voltage increases or when pot leaks molten metal or electrolytes.

Upon failure of pot, the pot is made empty and cool down. The pot is then removed from pot room to working area for dismantle of linings. By mechanical drilling the steel shell is stripped of carbon lining and refractory lining for re-lining with new materials. Stripped linings separated into two fractions named as 1st cut SPL (carbon portion) and 2nd cut SPL (refractory portion). The 1st cut SPL is hazardous waste as per CPCB (India) due to the presence of soluble inorganic toxic cyanide (according to CPCB if any solid waste contains soluble inorganic toxic cyanide more than 50 mg/kg then it is classified as hazardous waste under class A). The high toxicity of cyanide is well defined; 70 mg KCN has a 50% chance to kill a person of 70 kg within 15 minute (Lygre, 1994). Therefore, the SPL is generally stored with high precautions in rail cars, dumpsters, or piles prior to treatment and disposal.

Levels of cyanides in SPL vary from pot to pot and within a pot. Within a pot, levels can vary between the bottom of the pot liners and its side or end walls. Cyanide is also found in higher concentrations at the side wall where the bottom block carbon is exposed to air. Total cyanide may vary by two orders of magnitude within a single pot and concentration ranges from 100 – 2000 mg/kg or more depends on the life of pot. Total cyanide comprises of soluble cyanide and insoluble cyanide. Soluble cyanides are mostly free cyanides and complex cyanides (WAD). Free cyanides are preferably sodium cyanide and complex cyanide as sodium ferrocyanide. Disposal of such SPL material which contains significant quantity of water soluble/leachable cyanide presents numerous problems to the environment.

Various methods have been disclosed in the prior art documents for the treatment of SPL hazardous waste. For instance, Tabrey et al describes thermal treatment of the linings at high temperature in fluidized beds (Tabrey et al) or rotary kiln. Such methods are intended to destroy the cyanide contents and to convert fluoride values to insoluble salts on account of loss of carbon, aluminum and fluoride. The treated material is suitable only for dumping in land-fill sites. Other known methods suggest leaching of SPL in caustic solutions with or without a preliminary heat treatment. The leaching of heat treated SPL with caustic are exemplified by Snodgrass et al, Rickman et al, Goodes et al, Jenkins et al, and Hittner et al in various patents. Whereas, leaching of SPL (without heat treatment) in alkaline solutions made from lime is exemplified by Gnyra et al and Kaaber et al in their work. Further, Bontron et al describes fixation of SPL into a glassy matrix. This type of process only converts the material into non-hazardous forms which is suitable for safe dumping of treated SPL in landfill sites. Still further, pyrohydrolysis and pyrosulpholysis of SPL have also been disclosed by Bell et al. These types of processes require expensive reactors, high capital and thereby high operating costs.

None of the disclosed methods are capable of recovering the carbon and valuable constituents of the pot-lining material while also dealing safely with the cyanide content, at least in an economical and relatively simple manner. There is therefore still a need for a safe, economical and relatively simple way of recycling spent pot-lining materials in equipment that does not require undue capital outlay.

OBJECT OF THE INVENTION:

It is an object of the present invention to provide a process for conversion of water soluble inorganic toxic cyanide present in 1st cut aluminium spent pot-lining materials (SPL) to non- toxic species by selective thermal treatment.

It is another object of the present invention to provide a process for reduction of insoluble inorganic cyanide concentration by conversion of insoluble cyanide present in 1st cut aluminium spent pot-lining materials (SPL) to non-toxic species by selective heat treatment.

It is still another object of the present invention to provide an economically viable process for conversion of soluble cyanide and partially insoluble cyanide to non toxic species without losing the carbon, sodium and fluoride value present in 1st cut SPL material using the selective heat treatment.
It is yet another object of the present invention to provide a process for safe recovery of carbon, sodium and fluoride values by further processing the heat treated substantially soluble cyanide free non-hazardous 1st cut SPL material.

It is a further object of the present invention to provide to minimize the stockpiling of hazardous SPL waste material of aluminium industry for promotion of greener technology developments.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:

Figure-1 illustrates block diagram of the process for conversion of soluble and insoluble inorganic toxic cyanide present in the first cut aluminium spent pot-lining (SPL) materials to non-toxic species by heat treatment and recovery of sodium, fluoride and carbon value in accordance with one illustrative embodiment of the present invention.

Summary of the Invention
The present invention provides a process for conversion of inorganic cyanide of aluminium spent pot-lining material (SPL) to non-hazardous material; (a) said method comprising (a) heating aluminium spent pot-lining material (SPL) at a temperature ranging from 300 to 4500C for a time period ranging from 25 to 35 minutes to obtain non-hazardous material mixture; and (b) subjecting said mixture to heat treatment at a temperature ranging from 450 to 6000C for a time period ranging from 30 to 50 minutes to non hazardous species.
Said method characterized in that the step (a) converts soluble cyanide present in the SPL to non-toxic material; and the step (b) partially converts insoluble cyanide to non-hazardous material.
In preferred embodiment 1st cut aluminium spent pot-lining materials (SPL) is utilized.

Typically, the aluminium spent pot-lining material (SPL) is a hazardous waste of aged and degraded carbonaceous material cell lining separated from refractory lining which is removed from the aluminium smelting cell.

In one embodiment, the process comprises a pretreatment of SPL material, said pre-treatment consists of crushing the SPL material to less than 50 mm size.

In one embodiment, the heating is carried out in an apparatus selected from the group consisting of electrically heated rotary tube furnace or kiln furnace in the presence of stagnant/controlled air flow.

Typically, said process converts at least 98% soluble cyanide and at least 50% of insoluble cyanide into non-hazardous material.

In accordance with another aspect of the present invention there is providing a process for recovery of sodium, fluoride and carbon; said process comprising the following steps:
- subjecting a non-hazardous material obtained from heat treatment of SPL material to leaching with water in order to leach out soluble sodium and fluoride to obtain a leached solid-liquid mixture;
- decanting said leached solid-liquid mixture to separate out solid fraction and liquid fraction;
- washing the solid fraction with water to remove adsorbed liquid containing soluble sodium and fluoride to obtain a washed solid-liquid mixture;
- subjecting the washed solid-liquid mixture to decantation to separate out solid and liquid fraction; and
- recycling said liquid fraction to leach out the soluble sodium and fluoride.

Typically, the ratio of non-hazardous material to water is in the range of 1:10 to 1:20.

Typically, the ratio of said solid fraction to water is 1:10.

Typically, the process further comprises at least one step selected from the group consisting of drying and filtration.

In one embodiment, the non-hazardous material obtained from heat treatment of SPL material is free from soluble cyanide and at least partially free from insoluble cyanide.

In one embodiment, said process characterized by recovery of sodium as sodium hydroxide at a concentration in range of 100-160 gpl.

In one embodiment, said process comprises recycling cycles ranging from 1 to 25 basis the composition SPL material.

In one embodiment, leaching is carried out below 60 °C thereby restricting silica in the liquid fraction.

DETAILED DISCRIPTION OF THE INVENTION:
The present invention focused on utilization of aluminium spent pot-lining material (SPL), particularly 1st cut SPL which has potentially high carbon thereby making it high calorific value material. Apart from carbon it also contains significant amount of soluble sodium (as NaOH/Na2CO3) and fluoride which are valuable materials after safe recovery.
In the context of the present invention, “first cut aluminium spent pot-lining material (1st cut SPL) refers to the aged and degraded carbonaceous material cell lining separated from refractory lining that is removed from the aluminium smelting cell and is listed as hazardous waste (K088) by EPA, USA and class A by CPCB in India due to the presence of soluble inorganic cyanide and fluoride. Typical composition of 1st cut SPL is listed in the table below:

Table- 1 Typical chemical composition of 1st cut SPL sample
Element or Compound
unit
Carbon portion
C
%
50-70
Al2O3(total)
%
5-13
SiO2 (total)
%
0-1
Na+ (total)
Na+ (leachable)
%
%
12-26
6-11
F- (total)
F- (leachable)
%
%
6-11
2-5
CN- (total)
CN- (leachable)
ppm
ppm
100-2000
90-800

Accordingly, the present invention provides a process for conversion of soluble and partially insoluble inorganic cyanide of aluminium spent pot-lining material (SPL) to non-hazardous material. The method involves the following steps:
Step (a) : Aluminium spent pot lining material (SPL) is heated at a temperature ranging from 300 to 4500C for a time period ranging from 25 to 35 minutes to convert soluble cyanide present in SPL to non hazardous species to obtain non-hazardous material mixture.
Step (b): The mixture obtained from step (a) is subjected to heat treatment at a temperature ranging from 450 to 6000C for a time period ranging from 30 to 50 minutes for partial conversion of insoluble cyanide present in SPL to non hazardous species to obtain non-hazardous material.
The step (a) converts soluble cyanide present in the SPL to non-toxic material and the step (b) partially converts insoluble cyanide to non-hazardous material.

The detailed process is further described herein below:
Initially, a feed preparation is carried out by primarily crushing of 1st cut SPL material collected from aluminium plant to less than 50 mm size. The primarily crushed 1st cut SPL material subjected to the step of heat treatment in a specially fabricated electrically heated rotary tube/kiln furnace in the presence of stagnant/controlled-flow air for conversion of toxic soluble and insoluble inorganic cyanide to non-hazardous species.

In another aspect of the present invention, the heat treated 1st cut SPL material is subjected to leaching with water for leaching of soluble sodium and fluoride. The obtained leached solid-liquid mixture is subjected to decantation to separate out a solid fraction and a liquid fraction. The solid fraction of decantation is washed with water to remove adsorbed liquid containing soluble sodium and fluoride.

In the next step, the washed solid-liquid mixture is subjected for decantation to separate out solid and liquid fraction. Solid fraction is a washed SPL residue and may be used in cement industry as a raw material or used for generation of heat in furnaces/boilers. Liquid fraction is a washed filtrate and used in recycling back for washing of next batch of solid fraction of leaching process.

The liquid fraction obtained from above step subjected for recycling to leach out the soluble sodium and fluoride from the next batch of heat treated 1st cut SPL material and this is termed as 1st recycling of liquid fraction. The process of recycling of the liquid fraction continues till liquid fraction obtained after successive leaching cycle achieves its saturation limit in terms of fluoride. In the next step, fluoride is removed at saturation limit of liquid fraction by reacting with calcium oxide. This is called treatment of recycled liquid fraction and the liquid fraction called treated recycled liquid.

The treated recycled liquid is recycled to leaching tank to leach next batch of heat treated 1st cut SPL material.

In one embodiment of the present invention there is provided a process for recovery of sodium, fluoride and carbon; said process comprising the following steps:
i. subjecting a non-hazardous material obtained from heat treatment of SPL material to leaching with water in order to leach out soluble sodium and fluoride to obtain a leached solid-liquid mixture;
ii. decanting said leached solid-liquid mixture to separate out solid fraction and liquid fraction;
iii. washing the solid fraction with water to remove adsorbed liquid containing soluble sodium and fluoride to obtain a washed solid-liquid mixture;
iv. subjecting the washed solid-liquid mixture to decantation to separate out solid and liquid fraction; and
v. recycling said liquid fraction to leach out the soluble sodium and fluoride.

In accordance with the present invention, crushing can be done by any conventional methods which include but not limited to jaw crusher, hammering and the like.

Depending on the composition of cyanide the heat treatment temperature should be maintained. If 1st cut SPL contain less insoluble cyanide than permissible limit (600 ppm) decided by pollution controlled authorities, then heat treatment is carried out at temperature between 300 - 450 °C with residence time 25-35 minutes. At these parameters more than 98% conversion of potentially dangerous soluble cyanide in SPL to non hazardous species (such as sodium cyanide to sodium carbonate) can be attained. It is understood that the residence time may vary with different size of crushed SPL and different types of SPL material based on composition. The advantage of maintaining temperature below 450 °C is to avoid the unnecessary oxidation of carbon value in presence of air (measured ignition temperature of 1st cut SPL is around 560 °C).

If 1st cut SPL contain more insoluble cyanide than permissible limit (600 ppm) decided by pollution controlled authorities, then heat treatment is carried out at temperature between 450 - 600 °C with residence time 30-50 minutes. At these parameters more than 98% conversion of soluble cyanide and more than 50% insoluble cyanide in 1st cut SPL to non hazardous species can be attained. For conversion of insoluble cyanide a limited flow of air could be maintained to avoid the unnecessary oxidation of carbon value (measured ignition temperature of 1st cut SPL is around 560 °C). In this process temperature is selective for conversion of soluble and insoluble cyanides and conservation of fuel value of carbon in 1st cut SPL for commercial exploitation.

During the heat treatment at 450 °C outlet gases were subjected for detection of cyanide and fluorides. It confirmed that no cyanide or fluoride from 1st cut SPL vaporize below 450 °C eliminates the health environmental hazards.

The present invention process is illustrated with the help of accompanying drawing. Crushed SPL (100) is selectively heat treated by any suitable method such as oil, gas fired or electrically heated rotary tube/kiln furnace or fluidized bed furnace (1) with/without passing air for conversion of soluble or insoluble inorganic cyanide.
The numbers represented in figure-1 for various unit operations, inputs and outputs streams are described as follows.
Unit Operation equipment
1 :Rotary Kiln which is an electrical, gas/oil fired for heating 1st cut SPL material
2 :Leaching Tank which is a solid/liquid contacting device for leaching of soluble sodium and fluoride from heat treated 1st cut SPL material
3 :Decantation Unit which is a solid/liquid separation unit for separation of solid and liquid
4 :Treatment Tank which is a solid/liquid contacting device for carry out reaction with calcium oxide
5 :Washing Tank which is a solid/liquid contacting device for washing of solid residue
6 :Decantation Unit which is solid/liquid separation unit for separation of solid and liquid
7 :Filtration Unit which is a solid/liquid separation unit for separation of fine solid particles from liquid mixture

Input to the process
100 : Stream of crushed 1st cut SPL material
101 : Stream of leaching medium (water)
102 : Stream of washing medium (water)
103 : Stream of calcium oxide

Intermediate steps in the process
11 : Stream of heat treated 1st cut SPL material to leaching tank (2)
12 : Stream of leached solid-liquid mixture to decantation unit (3)
13 : Stream of recycling liquid (filtrate) from decantation unit (3) to leaching tank (2) along with makeup water to maintain solid liquid ratio for leaching next batch of heat treated 1st cut SPL material
14 : Stream of solid residue from decantation unit (3) to washing tank (5) for washing of solid residue with water
15 : Stream of washed solid-liquid mixture for decantation unit (6)
16 : Stream of recycling washed liquid (washed filtrate) from decantation unit (6) to washing tank (5) to wash next batch of solid residue from decantation unit (3)
17 : Stream of recycling liquid (filtrate) at saturation limit in terms of fluoride to treatment tank (4) to remove fluoride from recycling liquid
18 : Stream of treated solid-liquid mixture to filtration unit (7)
19 : Stream of treated recycling liquid from filtration unit (7) to leaching tank (2) along with makeup water to maintain solid liquid ratio for leaching next batch of heat treated 1st cut SPL material
Output of the process
200 : Product as a washed solid residue reached in carbon and substantially free from soluble cyanide, fluoride and sodium.
201 : Product as insoluble calcium fluoride precipitate
202 : Product as concentrated caustic liquor (NaOH)

The heat treated 1st cut SPL material (11) is passed below 60 °C in a leaching tank (2) and water is added (101) as a leaching medium to leach out soluble salts of sodium and fluoride, namely sodium hydroxide, sodium carbonate and sodium fluoride. A leaching tank can be of any type of stirred tank reactor. During leaching solid liquid ratio maintained in between 1:10 to 1:20 by weight and leaching time between 1.5-2.0 hours, for maximum extraction of leachable material from detoxified heat treated 1st cut SPL. The leached mixture (12) is transferred to decantation unit (3) to separate out the solid from liquid. Decantation unit can be of any type of settler or wet sieving assembly having 100 mesh sieves. The separated liquid is called as leached liquid or leached filtrate (13) and solid is called as unwashed SPL residue (14). The unwashed SPL residue (14) to washing tank (5) to wash with water(102) which helps in removal of adsorbed leached filtrate from unwashed SPL residue. In washing tank, the solid liquid ratio is maintained at 1:10 and washing time in between 15-20 minutes. The washed mixture (15) is transferred to decantation unit (6) to separate solid and liquid fraction. Decantation unit can be of any type of settler or wet sieving assembly having 100 mesh sieves. The separated liquid fraction is called as washed filtrate (16) and solid fraction is called as washed SPL residue (200). The washed SPL residue is one of the products of the present process which is high in carbon value and free from hazardous materials such as soluble cyanide, sodium and fluoride. These washed SPL residues may be used in cement industry as a raw material or used a high calorific value fuel for generation of heat in furnaces/boilers. The washed filtrate is used for recycling for subsequent washing of unwashed residue of next washing cycle. At the end washed filtrate can be used as a leaching medium at start of new leaching cycle. The leached filtrate (13) from decantation unit (3) is recycled to leaching tank (2) to leach out new batch of heat treated 1st cut SPL. The recycling of filtrate helps to build up the concentration of soluble sodium and fluoride in filtrate. With each recycling of leached filtrate, there is gradual increase in concentration of soluble sodium and fluoride in leached filtrate. After 8-10 recycling cycles the fluoride in filtrate reaches to its saturation point. At this point stop the recycling and saturated filtrate (17) is send to reaction tank (4) for removal of fluoride.
The saturated filtrate (17) obtained from decantation unit (3) is transferred to the reaction tank (4) along with the calculated amount of calcium oxide (103). Reaction mixture is stirred for two hours. In the reaction tank sodium fluoride gets converted to insoluble calcium fluoride and sodium carbonate to insoluble calcium carbonate and there is formation of sodium hydroxide.

The reaction mixture (18) is transferred to filtration unit (7) to separate out solid and liquid fraction. Solid fraction mainly consisting of mixture of calcium fluoride, calcium carbonate and un-reacted calcium hydroxide. During this reaction more than 98% soluble fluoride gets converted into insoluble calcium fluoride. Analytical study shows that the solid product contains 65-70% calcium fluoride. Liquid fraction is mainly consisting of sodium hydroxide and is called treated recycled filtrate and can be used again for recycling to leach out new batch of heat treated 1st cut SPL. The advantage of recycling of treated recycled filtrate is to increase the concentration of caustic in filtrate with minimum use of water.
The treated recycled filtrate (19) is transferred to leaching tank (2) to leach out next batch of heat treated 1st cut SPL. Makeup water is added (101) to leaching tank to maintain solid liquid ratio.

Process is continued as leaching (2) – decantation (3) – recycling of leached filtrate (13) till next saturation point of fluoride reaches. At saturation point saturated filtrate is diverted to reaction tank (4) and treated with calculated amount of calcium oxide (103). Treated recycled filtrate is again recycled for leaching. In similar way process is continued till treated recycled filtrate could not achieve the NaOH concentration near to 100-160 gpl. Total number of recycling depends upon initial concentration of soluble sodium and fluoride present in 1st cut SPL sample and solid liquid ratio used during process.

Solubility of sodium fluoride in liquor (g/100ml) is defined by Y
Y=1.673 e-0.198*X
Where X = concentration of Na+ in liquor (g/lit)
Analytical results show that not more than 90 ppm soluble fluoride remains in treated recycled filtrate. This can further be reduced depending on the requirement. Obtained product can be used in Bayer process for extraction of alumina. The other by-product that is solid mixture of calcium fluoride+calcium carbonate+calcium hydroxide can be used as a raw material for manufacturing of Aluminium fluoride.

The invention is illustrated with the help of following non-limited examples

EXAMPLE-1
A series of experiments were carried out to arrive at the process of present invention. These experiments help in developing the process flow chart as shown in figure-1. The composition 1st cut SPL used for experiments is shown in table 2. During the experimentations, soluble and insoluble Cyanide determination is carried out by distillation followed by silver nitrate titration. Soluble and insoluble sodium determination is carried out by flame photometry. Soluble and insoluble fluoride determination is carried out by selective ion analyser. Carbon determination is carried out thermo-gravimetric analysis.

Total cyanide present in 1st cut SPL was well below the limit prescribed by pollution controlled board; hence the selective conversion of cyanide by heat treatment was carried below 450 °C.

Table- 2 chemical composition of 1st cut SPL sample
Element or Compound
unit
Composition
C
%
70.14
Al2O3(total)
%
8.2
Na+ (total)
Na+ (leachable)
%
%
9.52
4.25
F- (total)
F- (leachable)
%
%
6.90
3.01
CN- (total)
CN- (leachable)
ppm
ppm
129.84
118.47

During the experimentations 2000 gram homogenized 1st cut SPL sample was taken and heat treated at 440 °C in the rotary kiln furnace for 30 minutes in the presence of stagnant air. After heat treatment, sample were collected from the furnace and cooled down to room temperature. Heat treated sample were analyzed for total weight loss, total and soluble cyanide, fluoride and sodium. Analytical results are shown in table 3.

Table- 3 chemical composition of Heat treated 1st cut SPL sample
Element or Compound
unit
Composition
C
%
70.96
Al2O3(total)
%
8.30
Na+ (total)
Na+ (leachable)
%
%
9.62
4.33
F- (total)
F- (leachable)
%
%
7.06
3.04
CN- (total)
CN- (leachable)
ppm
ppm
11.97
Not detected

Total weight loss measured by actual weighing was found to be less than 2%. Soluble cyanide is not detected in the heat treated sample. It is also found that there were no major changes in concentration of carbon, sodium and fluoride. It is therefore inferred that sodium or fluoride does not get volatilized or decomposed below 450 °C. Carbon also not gets burned or oxidized below 450 °C. The heat treated sample now said to be non-hazardous due to the presence of cyanide below its prescribed limit. The gases liberated during the heat treatment trials were also trapped in NaOH solution and analyzed for fluoride and cyanide. It is found that no fluoride or cyanide was entrapped in NaOH solution.

For recovery of soluble sodium (as NaOH) and fluoride (as insoluble calcium fluoride), a leaching procedure was used. 100 g heat treated sample was taken into the leaching tank. To this was added 1000 ml of fresh water and stirred continuously for 2 hours maintaining the temperature at 60 °C. After 2 hours the sample was sent to decantation unit to separate out solid and liquid fraction. Solid fraction (unwashed residue) was sent to washing tank and washed with 1000 ml fresh water for 20 minutes to wash off adhere soluble materials from leached solid fraction. After 20 minutes, washed mixture was sent to decantation unit to separate out solid (washed residue) and liquid (washed filtrate) fraction. Washed residue was then sent to sundry. This washed solid residue is a product which is free from soluble materials and enhanced in carbon percentage as given in table 4. Liquid fraction (leached filtrate) of leaching process and liquid fraction (washed filtrate) of washing process were kept for recycling to leach next batch of heat treated SPL and wash leached solid residue respectively.

Table- 4 chemical composition of washed residue of 1st cycle
Element or Compound
unit
Composition
C
%
79.16
Na+ (leachable)
ppm
25.04
F- (leachable)
ppm
19.14
CN- (total)
CN- (leachable)
ppm
ppm
11.97
Not detected

For processing the next batch, 100 g heat treated sample was taken and to this 1000 ml liquid fraction of previous leaching was added (water was added for makeup which is lost with unwashed residue to maintain solid liquid ratio) and the steps as above were repeated. In this way numbers of batches were performed until fluoride reaches to its saturation point in recycled leached filtrate. At fluoride saturation point recycled filtrate was treated with calcium oxide. Study showed that after 7th, 11th, 14th, 17th and 18th cycles treatment with calcium oxide is needed. The 18th treated filtrate becomes the product of process which is substantially pure sodium hydroxide (caustic) solution as shown in table 5.

Table- 5 Composition of treated recycled filtrate of 18th cycle
Element or Compound
Unit
Composition
Al2O3(Leachable)
gpl
< 1
Na+ (leachable) as NaOH
gpl
101
F- (leachable)
ppm
90
CN- (leachable)
ppm
Not detected

Insoluble precipitate obtained after treatment of recycled filtrate becomes a byproduct of the process which mainly consists of 65.63 % calcium fluoride and rest is expected to be as calcium carbonate, un-reacted calcium oxide and impurities present in calcium oxide.

EXAMPLE-2
1st cut SPL used for experiments has the composition shown in table 6. Total cyanide present in 1st cut SPL is well above the limit prescribed by pollution controlled board; hence the selective heat treatment was carried at 600 °C for complete conversion of soluble as well as partial conversion of insoluble cyanide.

Table- 6 chemical composition of 1st cut SPL sample
Element or Compound
Unit
Composition
C
%
52.12
Al2O3(total)
%
11.58
Na+ (total)
Na+ (leachable)
%
%
23.95
10.02
F- (total)
F- (leachable)
%
%
7.60
3.51
CN- (total)
CN- (leachable)
Ppm
ppm
1280
611

During the experimentations 3000 gram homogenized 1st cut SPL sample was taken and heat treated at 600 °C in the rotary kiln furnace for 40 minutes in the presence of controlled atmosphere of air. After heat treatment sample were collected from the furnace and cooled down to room temperature. Heat treated sample were analyzed for total weight loss, total and soluble cyanide, fluoride and sodium. Analytical results are shown in table 7.

Table- 7 chemical composition of Heat treated 1st cut SPL sample
Element or Compound
unit
Composition
C
%
52.31
Al2O3(total)
%
12.06
Na+ (total)
Na+ (leachable)
%
%
25.01
10.45
F- (total)
F- (leachable)
%
%
7.87
3.66
CN- (total)
CN- (leachable)
ppm
ppm
310
Not detected

Total weight loss measured by actual weighing was found to be less than 4%. Soluble cyanide was not detected in the heat treated sample where as more than 50% insoluble cyanide gets converted to non hazardous species. It is also found that there are no major changes in concentration of sodium and fluoride. It is therefore inferred that sodium or fluoride does not get volatilized or decomposed at 600 °C. The heat treated sample now said to be non hazardous due to presence of cyanide below it prescribe limit. The gases liberated during the heat treatment trials were also trapped in NaOH solution and analyzed for fluoride and cyanide. It is found that no fluoride or cyanide was entrapped in NaOH solution.

For recovery of soluble sodium (as NaOH) and fluoride (as insoluble calcium fluoride), a leaching procedure was used. 100 g heat treated sample was taken into leaching tank. To this was added 2000 ml fresh water (solid:liquid::1:20) and stirred continuously for 2 hours maintaining the temperature at 60 °C. After 2 hours sample was sent to decantation unit to separate out solid and liquid. Solid fraction sent to washing tank and washed with 1000 ml fresh water for 20 minutes to wash off adhere soluble materials from solid fraction. After 20 minutes, washed mixture was send to decantation unit to separate out washed solid residue and washed liquid (washed filtrate). Washed solid residue was then sent to sundry. This washed solid residue is product which is free from soluble materials and enhanced in carbon percentage as given in table 8. Liquid fraction (leached filtrate) of leaching process and liquid fraction (washed filtrate) of washing process were kept for leaching next batch of heat treated sample and washing of leached solid residue respectively.

Table- 8 chemical composition of washed residue of 1st cycle
Element or Compound
unit
Composition
C
%
60.75
Na+ (leachable)
ppm
22.16
F- (leachable)
ppm
17.24
CN- (total)
CN- (leachable)
ppm
ppm
310
Not detected

For processing the next batch, 100 g heat treated sample was taken and to which 2000 ml liquid fraction of previous leaching was added (water was added for makeup which is lost with unwashed residue to maintain solid liquid ratio) and the steps as above were repeated. In this way numbers of batches were performed until fluoride reaches to its saturation point in recycled leached filtrate. At fluoride saturation point recycled leached filtrate was treated with calcium oxide. Study shows that after 7th ,11th, 14th, 16th,18thand 20th cycles calcium oxide treatment is needed. The20th treated recycled filtrate becomes the product of process which is substantially pure sodium hydroxide (caustic) solution as shown in table 9.

Table- 9 Composition of treated recycled filtrate of 20th cycle
Element or Compound
unit
Composition
Al2O3(Leachable)
gpl
< 1
Na+ (leachable) as NaOH
gpl
158
F- (leachable)
ppm
75
CN- (leachable)
ppm
Not detected

Insoluble precipitate obtained during the treatment of recycled filtrate becomes a byproduct of the process which mainly consists of 65% calcium fluoride and rest is expected to be as calcium carbonate, un-reacted calcium oxide and impurities present in calcium oxide.

The analytical studies show that:
1. Washed residue is not corrosive compared to untreated SPL. Therefore, this high calorific treated carbon portion could be used as a high grade fuel in furnaces or boilers for generation heat or electricity.
2. Hardgrove Grindability Index (HIG) studies show that the washed SPL residue have higher HIG value (HIG =55.45) compared to unwashed SPL (HIG =43.78).
3. Washed residue tested for its hazardousness in term of soluble cyanide and fluoride by TCLP procedure. It found that treated sample clears the TCLP test.

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.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

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 specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure 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.