FIELD OF INVENTION:
The invention relates to a process of treatment of gas cleaning Plant (GCP) water
of a steel making process (LD) by means of removal of total suspended solid
(TSS) and total dissolved solid (TDS). The process involves in-situ formation of
coagulants with simultaneous removal of TSS and TDS from LD GCP water by
sequential addition of slag quenching effluent followed by gas purging. The
process has three step i.e. in-situ coagulant formation, nucleation and
coagulation-precipitation.
BACKGROUND OF THE INVENTION:
There are two major water circuits in LD steel making process-
A) GCP WATER CIRCUIT
Gases are produced during steel making process. Large volume of water is
required for cleaning of LD gas in gas cleaning plant (GCP). Total suspended
solids (TSS) and total dissolved solids (TDS) are the major pollutants of GCP
water. The GCP water includes the following contaminants:
Ca: 150-180 ppm,
Mg: 16-28 ppm,
S04: 4-9 ppm,
TDS: 400-900 ppm
TSS: 550-700 ppm
The specified ranges of concentration of the contaminants may vary depending
upon plant operations. In a conventional process, the polluted water from GCP is
first channelized in to clarifier followed by thickener as shown in figure 1.
Thickeners and clarifiers are both used to separate liquids and solids by settling.
Thickeners are used to concentrate solids, while clarifiers are used to purify
liquids. Most solids are removed in thickener with the solids being recovered as

slurry or sludge. Water with very fine suspended solids and dissolved solids with
densities close to the density of water flows to clarifier. Synthetic flocculate are
then used to enhance the settling of fine suspended particles in thickener.
Finally, the overflow from the thickener containing dissolved solids is then
reused.
Scale may form on the surface of plant equipment and pipeline due to unwanted
deposition of chemically generated materials due to the presence of high
concentration of TDS. Formation of concentrated alkaline earth metal sulfates
and carbonates are the harmful constituents that deposits on the pipeline and
reduce the effective diameter due to scaling. The flow chart of the conventional
GCP water treatment process is given in Figure 1.
B) SLAG QUENCHING WATER CIRCUIT
Molten slag is generated from blast oxygen furnace (BOF) during steel making
process. Cooling of the molten slag is another step in steel making process which
requires high volume of water. The molten slag is cooled through a high-
pressure water spray. After cooling, the polluted water is collected in a pit. The
suspended solid presents in the polluted water is allowed to settle in the pit. Part
of the overflow is reused for slag quenching and remaining is discharged through
drainage system without any further treatment. The constituents of discharged
slag quenching water contain high concentration of dissolved alkaline earth metal
salts and sulfur compounds. The important characteristics of discharged slag
quenching water are as follows:
Ca: 1500-2000 ppm
Mg: 30-55 ppm
S04: 40-65 ppm
TDS: 2500-3600 ppm
TSS: 20-45 ppm

The flow chart of the conventional slag quenching process is shown in Figure 2.
The said slag quenching water is drained without any further treatment and
creates environmental hazards as it contains dissolved alkaline earth metal salts
and some sulfur compounds.
Slag quenching water environmentally hazardous contaminants and on other
hand, GCP water contains high amount of TDS which results in scale formation in
pipeline and hence, may halts the operations. In light of the above, there is
need of developing a process that can enable reuse of slag quenching water and
GCP water. Further, the process should be feasible from implementation
perspective without making any changes in the LD steel making process.
OBJECTS OF THE INVENTION
An object of the invention is to propose a process for removing total suspended
solids (TSS) from gas cleaning plant (GCP) water.
Another object of the invention is to develop a process for removing total
suspended solids (TSS) from Gas cleaning Plant (GCP) thickener water without
addition of a surfactant.
Yet another object of the invention is to develop a unit which leads to "zero
discharge LD process".
Another object of the invention is to propose a process that make use of slag
quenching water for the treatment of gas cleaning plant.
Still another object of the invention is to develop a process that utilize slag
quenching water.

SUMMARY OF THE INVENTION
The present invention provides a process of removing total suspended solids
(TSS) and total dissolved solids (TDS) from a steel gas cleaning plant (GCP)
water. The process utilize drained slag quenching water to achieve the objective
of the process and in turn ensures a zero discharge LD process. The process of
the current invention involves mixing GCP water with slag quenching water in the
volume ratio of 5:1-9:1 followed by CO2 purging.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Flow diagram of conventional GCP water circuit.
Figure 2: Flow diagram of conventional slag quenching water circuit.
Figure 3: Proposed closed loop water circuit for simultaneous removal of TSS and
TDS in Gas Cleaning Plant of Steel Making Process.
DETAILED DESCRIPTION OF THE INVENTION
Figure 3 shows the proposed process flow diagram of the entire process. The
entire operation is comprised of five steps:
1. GCP Thickener (Collection of GCP blow down water)
2. Addition of Slag quenching water
3. Precipitation Stage I (Precipitation of suspended solid)
4. Precipitation Stage II (Precipitation of alkaline earth metals)
5. Recycle of water
Step 1: Collection of GCP blow down water In steel making process, GCP
water is collected in thickener to separate out heavy particle. The clarifier
overflow is then channelized to thickener. Slag quenching water is mixed in the

launder of the thickener. The formation of instantaneous coagulant occurs due to
the mixing. The characteristics of coagulant may be varied depending upon the
pH value of the slag quenching water. The temperature of the process is kept
between 60 to 80°C. Feed slag quenching water contains 32% TDS which
comprise approximate 18% calcium and 4-5% sulfur.
Step 1: Precipitation
The specific gravity of the sulfur based compound is in the range of 1.21-1.29.
The sulfur based compounds is formed due to the mixing of GCP water and slag
quenching water. This specific gravity corresponds to the formation of calcium
polysulfide. Once calcium polysulfide is formed, this initiates the nucleation
which helps the precipitation of suspended solid from GCP water.
1. H2O+ LD Slag → Calcium polysulfide (CaSx, H2O )+
Insoluble matter + H2O
2. Suspended solid (carbon/iron oxide) + (CaSx, H2O )→H2O
H2O + Insoluble ( bonded material with Calcium polysulfide)
Step 2: CO2 Capture /Purging
After step 1, the GCP thickener water contains unreacted Ca (OH)2 Mg(OH)2,
aluminates, silicates and a few complex iron compounds. All the complex
compounds of calcium and magnesium have to be precipitated first. Therefore,
purging of CO2 into the system is required. A constant source of CO2 is required
to run process operation continuously and accordingly, pH value of thickener
overflow needs to be 10-11. Steel industry flue gas can be used as a source of
CO2. Steel industry flue gas contains O2 , and N2 along with CO2. Alternatively,
industrial grade CO2 is used to run the process.

Second Precipitation
The GCP blow down water coming out from launder of thickener is first exposed
to CO2 for several minutes. The temperature of the process is around 60°C and
the pressure of CO2 injection is around 1.3 Kg. After CO2 injection, a white
gelatinous precipitation is formed which agglomerates further with the other
soluble metal of thickener water. The precipitate contains calcium, magnesium
and iron. Table 1 gives the composition of the precipitate.
The precipitation reactions are:
3. Ca (OH) 2 + CO2 → CaCO3 + (Fe2O3 + Others Insoluble)
4. Mg (OH)2 + C02 → MgCO3+ (Fe2O3 + Others Insoluble)
Example:
GCP water with following concentration of contaminants was treated as per the
current process with slag quenching water. The mixing ratio of GCP water and
slag quenching water was 8:1 and the hydraulic residence time of the thickener
was about 5 hrs. Table 1 below shows that treated water had significant removal
of the TDS and TSS.

Yield: more than 0.2% of TDS thickener overflows water with pH rang of 9.82 to
10.5

The slag quenching water used here is getting recycled. The impact of the
process is more profound as it produces desire quality of GCP thickener over flow
water. A major byproduct of the process is carbonate of calcium, magnesium and
iron oxide mixture which can be used for sinter application. CO2 is used here for
precipitation of CaCO3 /MgCO3. A substantial amount of emitted CO2 from the
steel making process can be used and thus the entire process is a green process.

WE CLAIM:
1. A process for treating gas cleaning water (GCP water) in a LD steel
making process, the process comprising:
collecting GCP water in a thickener via a clarifier;
mixing the GCP water with slag quenching water in the thickener in a
volume ratio of 5:1 to 9:1;
purging carbon dioxide in to the thickener after GCP water is mixed with
the slag quenching water, the pH of thickener overflow being maintained
in the range of 10-11 and temperature in the range of 60 to 80°C; and
separating a sludge and recycling the treated GCP water.
2. The process as claimed in claim 1, wherein total dissolved solids and total
suspended solids of the GCP water is in the range of 400-900 ppm and
550-700 ppm respectively.
3. The process as claimed in claim 1, wherein concentration of Calcium,
Magnesium and Sulfate in the GCP water is in the range of 150-180 ppm,
16-28 ppm, and 4-9 ppm.
4. The process as claimed in claim 1, wherein total dissolved solids and total
suspended solids of the slag quenching water is in the range of 2500-3600
ppm and 20-45 ppm respectively.
5. The process as claimed in claim 1, wherein concentration of Calcium,
Magnesium and Sulfate in the slag quenching water is in the range of
1500-2000 ppm, 30-55 ppm and 40-65 ppm respectively.

6. The process as claimed in claim 1, wherein pH of the GCP water is in the
range of 8.5-11.
7. The process as claimed in claim 1, wherein pH of the slag quenching
water is in the range of 10.5-13.
8. The process of claim 1, wherein the sludge contains carbonate of calcium,
magnesium and iron oxide mixture.
9. A system for treating gas cleaning water (GCP water) in a LD steel making
process as per the claim 1, wherein the thickener is provided with a mean
for collecting slag quenching water, GCP water and continuous supply of
Carbon dioxide.

ABSTRACT

A process for simultaneous removal of total suspended solid (TSS) and total
dissolved solids (TDS) from gas cleaning plant (GCP) water of steel making
process is disclosed. The process involves mixing of GCP water with slag
quenching water followed by CO2 purging. The invention provides a process for
substantial removal of dissolved and un-dissolved elements to precipitate out and
formation of an insoluble metal compounds without leaving unwanted ions in the
thickener water. The treated water is reused by adjustment of pH with sequential
purging of carbon dioxide gas.