DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to a process to treat waste and
more specifically relates to a process to treat the Spent Pot Liner waste of
aluminium industry.
BACKGROUND
[0002] Background description contains information that may be useful in
comprehending the current disclosure. It does not imply that any of the
information provided herein is prior art or relevant to the presently claimed
disclosure, or that any publication specifically or implicitly referred to is prior
art. And the description includes information that may be useful in understanding
the present disclosure.
[0003] The primary aluminium production is the process of converting alumina
(aluminium oxide) to aluminium metal by molten salt electrolysis process. The
most common manufacturing process is the Hall Héroult process which involves
electrolysis of the alumina in
[0004] The cells or pots are constructed in layers contained within an outer
shell made
[0001] from steel. Inside is an inner carbon lining which acts as the cathode
for the electrolytic cell. Between the carbon layer and the steel casing there is a
refractory layer designed to keep heat in the cell. After 3-4 years of service life
the cell or pot is discarded and generates the spent lining materials which are
Spent Pot lining is also popularly called as SPL. SPL
mainly consists of carbon portion and refractory portion. The refractory portion
include a small percentage of Silicon Carbide layer.
[0005] Spent Pot lining (SPL) is considered as a hazardous waste. During the
electrolysis process, a number of substances including aluminium and fluorides
salts besides cyanide are absorbed into the cell linings. The SPL exhibits a
3
many countries:
It contains toxic fluoride & cyanide compounds
It has a high pH due to alkali metals & oxides making it
corrosive
[0006] Because of this SPL is subject to the Basel Convention on the control of
trans-boundary movements of hazardous wastes & their disposal. Land fill is
banned in most countries because of the risks of dissolved fluorides and cyanides
from the SPL leaching into the environment plus there is the risk of the toxic and
explosive gas evolving if the SPL gets wet. SPL is becoming one of the
aluminium industry's most pressing environmental concerns. Most SPL is
currently stored at the aluminium smelter sites or placed in special secured
landfills. Both of these options are expensive and potentially only storing
problems for future generations.
[0007] There are few good options for spent pot lining disposal. Moving the
material around is dangerous and expensive. Secure landfill is very expensive and
may be only delaying an environmental problem. Storage is not a sustainable
solution.
[0008] Out of two components in the SPL, the carbon bearing component is being
utilized as per Central Pollution Control Board (CPCB) guided Standard
Operating Procedure (SOP). However, the spent refractory component is still not
sustainably utilized or recycled.
[0009] Again, with the increase in Aluminium production across the world by
industries, the generation of hazardous wastes have increased substantially and
due to its cyanide content, elevated concentrations of various constituent
compounds, and the significant quantities in which Spent Potliner is generated,
this material poses a potential environmental risk. It also represents a substantial
challenge for Aluminum manufacturers, who hold ultimate responsibility for
appropriately managing Spent Potliner and ensuring its proper disposal.
4
Especially the SPL refractory portion has been accumulated inside the premises
of Alumnium smelter industries for decades in absence of environmentally
friendly method of de-toxification for its reuse. There is a need for treatment of
one of the hazardous waste such as Spent Refractory Lining for reuse in refractory
and other industries.
[0010] There is, therefore, a need to overcome the above drawback, limitations,
and shortcomings associated with the existing process in the present proposal it
is aimed to reduce the fluoride and Cyanide content below the limit, the refractory
material can be gainfully utilized in the refractory and other industries. The details
of the proposed process for removal of fluoride and Cyanide content by leaching
are described in detail description.
OBJECTIVE OF THE INVENTION
[0011] Some of the objects of the present disclosure, which at least one
embodiment herein satisfies are as listed herein below.
[0012] An object of the present disclosure is to overcome the above drawback,
limitations, and shortcomings associated with the existing techniques by
providing a process that uses size reduction, leaching the waste and then through
calcination, which removes the hazardous components like Fluoride and cyanide
completely.
[0013] The main objective of the present disclosure is Detoxification of SPL
Material for re-use.
[0014] Another object of the present disclosure is to Produce Refractory
Material such as Ramming Mass, Mortar, Castables, and Fire Bricks.
[0015] Yet another object of the present disclosure is Utilize the carbon/ fuel
values in the production process and save millions of calories
5
.
[0016] Yet another object of the present disclosure is Utilize Hydro & Pyro
metallurgical process to detoxify and produce refractory material in a closed loop
process plant.
[0017] Yet another object of the present disclosure is to provide a process
which makes the treatment of SPL waste reusable in other industries such as
refactory, cement and other industries to demonstrate the circular economy.
[0018] Yet another object of the present disclosure is to provide a process for
treatment of SPL waste in more economic and efficient way.
[0019] Still, another object of the present disclosure is to provide a process
for providing an environment friendly solution for the treatment and management
of hazardous waste.
SUMMARY OF THE INVENTION
[0020] According to the present invention, the process involves a series of welldefined
steps, including initial collection and storage, size reduction through
crushing and pulverization, removal of hazardous components (fluoride and
cyanide) through leaching processes, recovery and further treatment of leached
liquor, and the ultimate transformation of residues into usable refractory products.
The process aims to ensure the safe and effective utilization of the SPL and like
material while adhering to environmental and safety standards and makes the
treatment of hazardous waste like SPL much easier and safer than the existing
process of treatment.
6
BRIEF DESCRIPTION OF DRAWINGS
[0021] The accompanying drawings are included to provide a further
understanding of the present disclosure, and are incorporated in, and constitute a
part of this specification. The drawings illustrate exemplary embodiments of the
present disclosure, and together with the description, serve to explain the
principles of the present disclosure.
[0022] FIG. 1 illustrates Spent Pot Lining source and generation
[0023] FIG. 2 illustrates Process Flow Diagram.
[0024] FIG. 3 illustrates Process Flow Diagram of Plant.
[0025] FIG. 4 illustrates Process Material Balance
DETAILED DESCRIPTION OF THE INVENTION
[0026] The following is a detailed description to clearly communicate the
disclosure. However, the amount of detail offered is not intended to limit the
anticipated variations. On the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit, and scope of the present
disclosure as defined by the appended claims.
[0027] In the following description, numerous specific details are set forth
in order to
provide a thorough understanding of the disclosed process. It will be apparent to
one skilled in the art that process of the present invention may be practiced
without some of these specific details. Process of this disclosure relates to treating
the smelt pot lining coming out of metallurgical industry from smelting process.
7
[0028] Referring to Fig.1, Spent pot lining refers to the used or discarded lining
material that is removed from the inside of aluminium electrolysis pots, which
are used in the production of aluminium through the Hall-Héroult process. The
lining material consists of a mixture of carbon and refractory materials, such as
alumina (Al2O3) and Silica (SiO2). The cells or pots are constructed in layers
contained within an outer shell made from steel. Inside is an inner carbon lining
which acts as the cathode for the electrolytic cell. Between the carbon layer and
the steel casing there is a layer designed to keep heat in the cell. Further the same
portion contains about 5% of Silicon Carbide material as a layer. During the
electrolysis process of alumina dissolved in cryolite (Na3AlF6), the chemical
reactions within the cathode result in the formation of various carbides, nitrides,
cyanides, and others within the pot linings (cathodes, and sidewalls). Due to its
high carbon, fluoride, and cynide content, it is hazardous in nature and has a high
pH value due to alkali metals. Therefore, improper disposal of SPL can lead to
environmental pollution and pose risks to human health.
[0029] As, used herein the disclosure, the SPL composition may vary from one
manufacturing unit to another as various factors are responsible for this variation,
such as cathode materials, operation time, shutdown time, cell design, side
reactions, and electrolyte composition. The majority of the SPL's chemical
components are direct elements of the electrolytic bath that infuses the carbon
cathode and, later, the Spent Pot liner. Out of two components in the SPL, the
carbon-bearing component is being utilised as per the Central Pollution Control
Board (CPCB)-guided Standard Operating Procedure (SOP). However, the SPL
component is still not sustainably utilized or recycled.
8
The typical analysis of SPL Refractory
Sl. No. Parameters Unit Value (Range)
1. Carbon % 10-18
2. Al2O3 % 30-60
3. SiO2 % 10-30
4. Fe2O3 % 1-2
5. Fluoride (F) % 1-3
6. Cyanide (CN) ppm 05-100
[0031] Referring to Fig. 2, the process description in the main critical
process step for
the operation is oxidative alkaline leaching to decompose cyanide ions and leach
out fluoride ions. Process steps involve:
I. Size Reduction using different crushers/pulverizer to
300 micron in closed loop (Enclosures, bag filters,
scrubber, chimney etc)
II. Closed loop leaching in alkaline oxidizing condition
using NaOH
NaOCl with the following operating conditions;
III. Feed Particle Size - <300µm
IV. S/L ratio 20 30 % w/v
9
V. Temperature 50 - 90 C
VI. Residence Time 3-4 h
VII. pH 9.5 12.0 in presence NaOCl
VIII. Filtration and washing of the cake, Fluoride removal
from liquor as CaF2
IX. Cake drying and calcination
X. Products as Ramming mass, Refractory Mortar,
Castables, Fire Bricks (by outside parties)
[0032] The proposed invention discloses a process wherein the 150 250 mmsized
spent refractory material from the pot lining of the aluminium smelter is
carried to the site of the processing facility, whereupon, after segregating the raw
material, it is crushed in a jaw crusher to reduce its size to 25 30 mm. Further
size reduction is then performed in the Pulverizer between 16 and 200 mesh, and
the pulverised fraction is stored in a feed hopper for further processing.
[0033] Referring to Fig. 2 & 3, a process flow diagram and a process flow
of plant has been illustrated, wherein cyanides coming out from aluminium and
like industries are generally in two forms, free cyanides (CN ) and complex
cyanides such as (FeCN ). Free cyanides are generally quite amenable to
destruction through oxidative leaching (i.e. first step of cyanide removal process
in the present disclosure). But, the destruction of complex cyanide depends upon
the complexing ions associated with the cyanide radicals. Therefore, there is need
to have step in the form of calcination at kiln temperature.
10
STEP 1: Oxidative Leaching: The process typically begins with the
preparation of crushed waste with increased surface area and is then
mixed with a leaching solution, which contains an oxidizing agent such
as oxygen, hydrogen peroxide, or chlorine. Further, the oxidizing agent
reacts with the metal in the presence of water, leading to the formation
of metal ions or complexes and the leaching solution is agitated to
enhance the contact between the metal particles and the oxidizing agent,
facilitating the dissolution of the metal. Moreover, the reaction kinetics
can be controlled by adjusting factors like temperature, pH, and the
concentration of the oxidizing agent. Further, any traces of cyanide will
be removed by activated carbon filter.
Basic Chemical reactions involved are:
EXAMPLE-1
The cyanide present in the SPL Reefractory is decomposed to nitrogen
in the operating conditions and fluoride is leached out as soluble
fluoride.
CN- + OCl- + H2 (aq) + 2OHa)
CNCl(aq) + 2OH- - + Cl- + H2O
b) 2OCN- + 3OCl- + H2 2(g) + 2CO2(g) + 3Cl- + 2OHEXAMPLE-
2
The water-soluble fluorides present in the refractory material react with
alkali (NaOH) to form NaF and leached out.
F- -
c) 2NaF + Ca(OH)2 2 + 2NaOH
The fluoride is precipitated out as insoluble CaF2.
11
STEP 2: Calcination: Mainly cyanide is removed by chemical treatment
followed by activated carbon filter. But for precautionary measure the
product is passed through a rotary calciner to convert cyanide
compounds into less toxic forms by subjecting them to high
temperatures and simultaneously dry or calcine the material to get the
product. Calcination involves heating the cyanide-containing material
in an oxygen-rich environment, such as in a furnace or kiln, to induce
chemical reactions that break down the cyanide. During the calcination
process, the high temperatures in the range of 500 to 700 oC, cause the
decomposition of cyanide compounds, resulting in the release of
gaseous products, such as carbon dioxide (CO2), nitrogen gas (N2),
and water vapor (H2O). The exact reactions that take place depend on
the specific cyanide compound being treated. Like, Hydrogen cyanide
(HCN) can be decomposed
through calcination according to the following reaction:
2HCN + O2 2 + N2 + H2O
Similarly, other cyanide compounds, such as metal cyanides (e.g.,
sodium cyanide, potassium cyanide), can also undergo
decomposition during calcination, leading to the formation of less
toxic carbonates or oxides.
[0034] Once the calcination is complete, the calcined residues are allowed to cool
down to room temperature. This is done gradually to prevent thermal shock and
maintain the integrity of the refractory materials. The cooled and transformed
residues undergo further processing steps, including grinding, blending, forming,
curing, drying, firing, and quality control, to produce high-quality refractory
materials. These materials find extensive use in industries that require durable
and heat-resistant linings and structures to withstand demanding operating
conditions, thereby making the treatment of the SPL waste a complete zero waste
process, economic as well as environment friendly.
12
[0035] Referring to Fig. 4, the process of material balance has been
illustrated, where,
in an exemplary embodiment, material balancing involves a holistic approach to
optimize the input materials, process pathways, and process conditions to achieve
the highest possible yield
of desired end products while minimizing the generation of unwanted byproducts.
The amount mentioned herein are only indicative and do not represent the
accurate amount as under different conditions, the amount obtained may vary to
± 5%.
[0036] The process will use ground water and whole process water is treated in
effluent treatment plant so that it can be reused and no water is wasted in this
process and thereby making it a zero liquid discharge process. This can further
be illustrated through a table:
Applications Unit Input Recycling
Process water
Requirement
m3/Tonne of
SPL processed 1.5 Complete
Non-Process m3/day 5 0
Applications
consumed
Pollution Control m3/day 30 Complete
measures(scrubbing)
[0037] While the foregoing disclosure describes various steps of the invention,
other and further steps of the invention may be devised without departing from
the basic scope thereof. The scope of the invention is determined by the claims
that follow. The invention is not limited to the described steps, versions, or
examples, which are comprised to enable a person having ordinary skill in the art
to make and use the invention when combined with information and knowledge
available to the person having ordinary skill in the art.
13
ADVANTAGES OF THE INVENTION
[0038] The present disclosure provides an environment friendly solution to deal
with the hazardous wastes coming out from the smelting industry.
[0039] The present disclosure provides a solution through which treated waste
can be used in other industries like cement industries, which makes it zero waste
process. Thereby, promotes Circular Economy by treating SPL waste as a
valuable resource rather than a burden. Recycling and reusing SPL waste in
various industrial processes not only reduces waste generation but also fosters a
closed-loop system where materials are continuously circulated and utilized. This
approach reduces the demand for new materials, minimizes waste production,
and promotes a more sustainable and efficient industrial ecosystem.
[0040] The present disclosure provides a process where emission of hazardous
gas is minimized and therefore, makes it safe environment for workers of the
industry as well as the local population around the industry.
[0041] The present disclosure provides an economic solution for the hazardous
waste treatment. Proper disposal of SPL waste saves soil and water from
contamination, posing risks to ecosystems and human health and thereby
minimize the risk to the ecology.
[0042] The present disclosure provides a process which is economic and efficient.
Since SPL waste contains valuable materials such as carbon and various metals
like aluminum. By appropriately handling and recycling SPL waste, we can
recover these resources and thereby making the process efficient and economic. ,CLAIMS:CLAIM:
We claim:

1. A process for processing spent pot refractory lining of aluminum smelter comprising:

a. Size Reduction using crushers/pulverizers;

b. Closed loop leaching using closed vibro Screen;

c. Separation of Solid and Liquid waste; and

d. Drying and calcination of waste;

wherein the size reduction is done by use of different crushers and/or pulverizers to 300 microns in closed loop;

wherein closed loop leaching in alkaline oxidizing conditions is done using NaOH and NaOCl with feed particle size not exceeding 300 micrometers, S/L ratio between 20 to 30 percent weight by volume, maintaining a temperature between 50 to 90 centigrade within a residence time of 3 to 4 hours, while maintaining the pH of 9.5 to 12.0;

wherein filtration and washing of the cake obtained from collection tank is done for Fluoride removal from liquor as CaF2; and

wherein drying and calcination of the cake is done to obtain the final product.

2. The process for processing spent pot refractory lining of aluminum smelter for treatment of leach water obtained from reaction vessel, for reuse, ensuring zero water waste comprising:

a. Collecting of leach water into a collection vessel;

b. Separation of fluorides and cyanides from the leach water; and

c. Neutralization of water obtained from leach water through proper chemical treatment in an Effluent Treatment System.

3. The process as claimed in claim 2, wherein the leach water obtained from the reaction vessel and collected in a collection tank while decreasing the pH from 11.2 to 7.2.

4. The process as claimed in claim 2, wherein lime treatment of the collected material is done and it is left in a settling pit where calcium fluoride is separated and the left water is sent into a neutralization pit.

5. The process as claimed in claim 2, wherein the water obtained from settling pit to neutralization pits cleaned and the pH of such water is brought close to pH of clean water.

6. The process as claimed in claim 2, wherein the pH, fluoride concentration and cyanide content of wastewater separated from Spent Pot Lining material obtained in reaction vessel is treated in an Effluent Treatment Plant with proper dosing of chemical to reduce the toxic content to safer limits to make the water reusable in the process.