Field of the invention
The present invention relates to a process for disposal of hazardous waste acid
sludge. In particular, the present invention relates to a process for disposal of
hazardous waste acid sludge and the like and converting it to a single phase solid
product having calorific value. The process is widely applicable for disposal of
wastes generated in different industries such as coal chemicals section of coke
ovens.
Background of the invention
Generation of acid sludge is a common problem because of its difficulty in
disposal. Few methods are available in art for conversion of wastes into
environmentally acceptable forms for disposal. US 5102556 teaches a method
for rendering ferric hydroxide sludge recyclable. The method comprises
selectively treating flocculated heavy metal contaminated iron based sludges
from an industrial waste water treatment process. The method removes the
heavy metal contaminants with a minimum loss of the iron therein. The method
comprises dissolving the sludge in hydrochloric acid to produce a ferric chloride
solution. The solution is filtered to remove precipitated heavy metal salts, such as
lead chloride. The filtrate containing the remaining heavy metal chlorides and
ferric chloride is then cooled or otherwise treated to remove additional heavy
metal chlorides. The remaining ferric chloride based solution is then reduced
(exposed to iron powder) to a ferrous chloride solution and heavy metals. After
filtration of any additional precipitated heavy metals, the ferrous chloride solution
is ready for recycling. The precipitated/filtered heavy metal salts can be supplied
to one or more conventional recovery processes to produce one or more heavy
metals, instead of disposing of them as hazardous waste.
The above process is not applicable for acid sludges. Furthermore, the sludge is
dissolved in hydraulic acid which has to be purchased and is not cost effective
Disposal practices in some steel plants comprise neutralization of acid sludges
with caustic soda which has to be purchased and loaded in tanker and then
dumped at dump site. In a second process solvent extraction for acid
regeneration is practiced. Such practices have several disadvantages namely
difficulty in handling of a sticky resinous mass. The highly acidic sludge has a pH
2 and is difficult to be neutralized accurately by solid caustic soda. Furthermore,
there is usually no mixing operation and therefore homogenization of the mass is
absent, or inadequate. Raw materials such as caustic soda have to be bought
and are expensive for the plant. The neutralized mass is generally in two distinct
phases, one liquid and the other solid and hence discharge of the entire mass
from tanker is difficult.
Alongwith the above drawbacks, the material of construction for plant and
machinery generally get corroded due to highly corrosive nature of acid sludges
and plant operation is difficult.
None of the known processes lead to product which can be readily used as a
fuel.
Therefore, there is a need for a process for disposal of acid sludges which is cost
effective and leads to a product which has fuel value.
Object of the invention
The main object of the invention is to provide a process of disposal of hazardous
waste acid sludge into a recyclable material which is easy to handle.
Further object of the present invention is to provide a process for disposal of
hazardous waste acid sludge and converting the same to a single solid phase by
alteration of physical characteristics of the neutralized composite.
Another object of the present invention is to provide for a process for disposal of
hazardous waste acid sludge such that the corrosion rate of the material used for
construction of the plant is low.
Yet another object of the present invention to provide for a process for disposal of
hazardous waste acid sludge which produces a composite neutralized mass
which can be readily used as a fuel.
Yet another object of the present invention to provide for a process for disposal of
hazardous waste acid sludge which does not require externally input of material
or energy.
Summary of the invention
Accordingly, the present invention provides for a process for the disposal of
hazardous waste acid sludge and conversion to product having calorific value
comprising:
(i) neutralizing said waste acid sludge with BOF liquor and lime fines;
(ii) optionally, adding non-cookina coal fines to said neutralized waste acid
sludge to form semisolid mass; and
(iii) applying mild pressure to said semisolid mass.
Detailed description of the invention
In the process of the present invention waste acid sludge is first neutralized with
BOF liquor and waste lime fines obtained from lime calcining plants. The BOF
liquor has a pH between 7 to 14, preferably between 8 and 12. The lime fines
have average size of between 1 to 5 mm, preferably 3mm.The heat of reaction
brings the mass to boil and drives off the excess moisture. At the critical
temperature before the mass ceases boiling, non coking coal fines are added and
a semi solid mass is formed from which briquettes are formed through mild
pressure. The briquettes are air-dried and developed sufficient strength for
handling. Alternatively, addition of non coking coal fines is avoided and the acid
sludge is neutralized with lime fines only which produces agglomerates.
BOF liquor is the effluent generated at gas cleaning plants. Steel is refined in the
converter by lancing oxygen at high rate on the surface of molten metal. This
gives rise to gas which contain lime and ore fines. This gas is cleaned by
scrubbing with water resulting in effluent containing solid and liquid, which is
alkaline and has high pH. The solid and liquid phases are separated and the
supernatant liquid is called BOF liquor.
The present invention is ideally used in a plant especially steel plant which
generates acid sludge, lime fines, alkaline water and carbonaceous fines like coal
fines. The best and most economical way of neutralizing the process is to
judiciously mix all the four ingredients of production of aggregates such that
hazardous waste disposal and energy conservation is achieved. The aggregates
having medium calorific value may be utilized in power generation as domestic
fuel.
The unique feature of the process is that two phase hazardous waste which is
difficult to handle and has no commercial value is converted into a single solid
phase, easy to handle substance of definitive commercial value by combination
of waste materials. The process also uses minimum external energy input. The
process generates its own energy requirements by exothermic nature of the
reaction, the heat of the reaction being sufficient to drive off the excess moisture
and bring the entire mass to boil. This makes the entire mass homogenous,
which then becomes solid and acquires sufficient strength on cooling.
The invention will now be described with reference to a non-limiting example and
figure 1, which shows the process flow sheet.
Examples
Example 1: Preparation of briquettes from acid sludge
Briquettes were prepared from acid sludge as shown in the process flow sheet
(figure 1). 1 litre of acid sludge was mixed with 8 litres of BOF liquor (alkaline
liquor). The acid sludge was found to have a pH of 2.0 and bulk density of 1.8
gm/ml. The acid sludge has two distinct phases, the upper liquid phase which is
40% of total volume and mainly comprises sulphuric acid and a lower solid phase
which is 60% of the total volume and mainly comprises organic polymer. The
average composition of the acid sludge was: SulDhric acid 60%, organic matter
35%, light crude benzol (mainly benzene, toluene, xylene) 3% and moisture 2%.
The BOF liquor was found to have a pH of 9.6 with TSS of 30 mg/l, conductivity
498 us/s. The concentration of Fe was 0.12 mg/l and chlorides 40 mg/l. The BOF
liquor had alkalinity of 10 mili mols/litre and hardness of 12 mg/l as CaCO3, DO 7
mg/l and phenol BDL.
To the mixture, waste lime with the following screen analysis was added and the
mixture was agitated.

The mixture was thoroughly mixed and the pH was periodically determined until
pH of 7 is achieved. The temperature was found to be 100°C. Non coking coal
fines having the following proximity analysis was then added.
Proximity analysis (%) of non coking coal fines
Moisture: 1.3
Volatile matter: 1702
Ash: 60.5
Fixed carbon: 21.0
The mixture was then allowed to cool to 50°C and by hand pressure
approximately 4 inch diameter size briquettes were formed, air cooled and sun
dried.
The material used for the plant was SAPH 45 grade steel.
The properties of the briquettes were tested for Calorific Value (CV) and Ultimate
analysis. The product of the process was tested for hazardous nature as defined
by CPCB by determining: Ignitability, Corrosiveness, Evolution of H2S and HCN
gases and TCLP. The results indicated that the products were in the non
hazardous category. The corrosiveness test was performed on SAPH 45 grade
steel and the rate of corrosiveness was found to be 0.0002 mm/yr. This shows
that this material of construction would not corrode when used with the developed
process and its products. Table 1 shows the properties of the product.
TABLE 1: Properties of Briquettes
Example 2: Preparation of agglomerates from acid sludge
Process similar to example 1 was performed, the only difference being that coal
fines were not added. After neutralization the mixture was allowed to cool to
atmospheric temperature and the agglomerates formed were safely disposed off
in proper dumping grounds.
The properties of agglomerates were tested as above. The results indicated that
the product was in the non hazardous category. The corrosiveness test was
performed on SAPH 45 grade steel and the rate of corrosiveness was found to be
0.0002 mm/yr. This shows that this material of construction would not corrode
when used with the developed process and its products. Table 2 shows the
properties of the product.
WE CLAIM
1. A process for the disposal of hazardous waste acid sludge and conversion to
product having calorific value comprising:
(i) neutralizing said waste acid sludge with basic oxygen furnace (BOF)
liquor and lime fines to form semisolid mass; and
(ii) applying mild pressure to said semisolid mass.
2. A process for the disposal of hazardous waste acid sludge and conversion to
product having calorific value comprising:
(i) neutralizing said waste acid sludge with basic oxygen furnace (BOF)
liquor and lime fines;
(ii) adding non-cooking coal fines having average size of between 1 to 5
mm to said neutralized waste acid sludge to form semisolid mass; and
(iii) applying mild pressure to said semisolid mass.
3. A process as claimed in claims 1 and 2, wherein said BOF liquor has pH of
between 7 to 14, preferably between 8 to 12.
4. A process as claimed in claims 1 to 3, wherein said BOF liquor has pH of 9.6.
5. A process as claimed in claims 1 and 2, wherein said lime fines is obtained
from lime calcining plants.
6. A process according to claims 1 and 2, wherein said lime fines have average
size of 3mm.
7. A process as claimed in claims 1 and 2, wherein said acid sludge has a pH of
at least 2.
8. A process as claimed in claims 1 and 2, wherein said acid sludge has two
phases, solid and liquid.
9. A process as claimed in claims 1 and 2, wherein the temperature of mixture
after neutralization in step (i) is around 100°C.
10. A process as claimed in any preceding claim, wherein said neutralized waste
acid sludge is cooled to around 50°C.
11. A process as claimed in any preceding claim, wherein said semi-solid mass
is optionally air cooled and sun dried.
12. A process as claimed in any preceding claim, wherein the product obtained
has a gross calorific value of between 50 to 3000 Kcal/Kg.

Process for the disposal of hazardous waste acid sludge and conversion to
product having calorific value. The process produces a single phase solid product
having calorific value and is widely applicable in disposing wastes generated in
different industries such as coal chemicals section of coke ovens. The process
comprises neutralizing said waste acid sludge with BOF liquor and lime fines,
optionally adding non-cooking coal fines to the neutralized waste acid sludge to
form semisolid mass and applying mild pressure to the semisolid mass.