Hydraulic and earth engineering – Soil remediation – With treatment
Reexamination Certificate
2000-03-28
2002-07-09
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Soil remediation
With treatment
C405S128750, C405S128850, C405S128900, C588S256000
Reexamination Certificate
active
06416251
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the stabilisation of soluble chromium contaminated solids by down draft sintering. The invention also relates to the use of the stabilised solids in and as construction material.
BACKGROUND OF THE INVENTION
Chromium occurring in soluble state is a hazardous pollutant. Soluble chromium containing solids, water residue, sludge, powdery material etc. generated from various industrial processes and mining activities can be treated in the present process for stabilisation of the soluble chromium into insoluble forms. The stabilised solid material can be disposed and utilised as a value added product for constructional use in making building material, road and hydraulic cements.
Hexavalent state of chromium and other chromium compounds are easily soluble in water. It is well known that Cr(VI) is very hazardous and causes cancer and many other diseases. Prevention of water, soil and agricultural land from contamination of Cr(VI) and other soluble chromium chemicals is a serious concern. Many industries and mining sectors dealing with chrome and chrome chemical products generate huge quantity solids, residue and effluents contaminated with metal and chemical compounds of Cr(VI) as byproducts. Disposal and gradual accumulation of these waste materials become the source of Cr(VI) which is released slowly with time, temperature, rain water and weathering effect and contaminate to a great extent the soil agricultural land, vegetation, surface and ground water bodies. Many processes have been developed for the treatment of Cr(VI) and other heavy metal contamination of water and effluents for reuse in agriculture, industry and drinking purposes. At present very limited work had been carried out for the treatment of specific types of waste materials and solids contaminated with Cr(VI) by different techniques for its stabilisation. But there is no such common and flexible process for the treatment of all types of solid or residues containing Cr(VI) and other soluble chromium for their stabilisaton for safe disposal or further utilisation as value added material.
Meegada et al disclose the remediation and reuse of chromium containing solids through cold top ex-vitrification (Meegada, Jey; Librizzi, B.; McKenna, G. F.; Kamolpornwijit, W; Cohen, D; Vaccari, David A; Ezeldin, S; Walden, L; Noval, Barry. A.; et al [New Jersey Institute of Technology, New York, Nj07102, USA] in Hazard. Ind. Waste, 1995, 27, 470-479, (Eng.)).
Yamaoka, Hivoshi; Tsuji, Toru (Chemical Man K. K.), Kokai Tokkyo Koho JP 01, 258, 785 [89258785](Cl B09B3100)), Oct. 16, 1989 discloses the pyrrolysis of waste containing chromium (VI).
Chajduga, Adam;. Kowalski, Zygmunt; Wardas, Adam. M. (Instytut chemii Nieorgaricznej) Pol. PL 135, 181 dated Dec. 11, 1981 discloses the reduction of hexavalent chromium contained in wastes obtained during the production of chromium compounds.
The above-mentioned processes involve different types of high temperature methods such as vitrification, sintering and combustion for the treatment of specific type of chrome(VI) contaminated solids for its stabilisation. The drawbacks of the above processes are the adoption of complicated high temperature systems which are energy intensive and therefore expensive. The prior art processes also require graphite, nitrogen gas, sulphur powder in the process making these processes expensive and hazardous for the environment.
Accordingly, it is necessary to develop a process that is not only inexpensive but also environmentally friendly.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide a process for the stabilisation of soluble chromium contaminated solids by down draft sintering technique which obviates the drawbacks mentioned above.
It is another object of the invention to provide a process for the stabilisation of soluble chromium contaminated solids that results in a value added material useful as construction material.
It is a further object of the invention to provide a process for the stabilisation of soluble chromium contaminated solids that is economical and environmentally friendly.
It is yet another object of the invention to provide a process for the stabilisation of soluble chromium contaminated solids that is useful for all types of Cr(VI) contaminated solids.
It is yet another object of the invention to provide flexibility for the redesigning the mix according to the desired chemistry and the utilisation of various powdery materials.
It is a further object of the invention to provide for the utilisation of other plant solid wastes as additives and carbon as fuel.
It is a further object of the invention to provide a thermally efficient process with low retention of time and faster rate of sintering and cooling.
SUMMARY OF THE INVENTION
Accordingly, the presents invention provides a process for the stabilisation of solid chromium of contaminated solids by-down draft sintering technique, which comprises mixing of SiO
2
, Al
2
O
3
, Fe
2
O
3
, CaO, MgO, SO
4
and solid carbon bearing materials as additives with said chromium contaminated solids in various proportions to maintain the desired ratio of CaO/SiO
2
, SiO
2
/Al
2
O
3
+Fe
2
O
3
and SO
3
/Cr in preparations or raw mixture, homogenisation of the raw mixture by grinding or blending pelletisation of the resultant homogenised in the presence of water, sintering of the pelletised material containing in situ solid carbon by down draft technique for conversion and stabilisation of Cr(VI) and chrome chemical compounds at high temperature, crushing and grinding of the sintered material which is in the form of an aggregate for use in building and road construction and cement making.
In one embodiment of the invention, 5 to 10% solid carbon is required in the raw mix to generate the in situ heat ranging from 800 to 1500° C. for the sintering and complete conversion of soluble chromium compounds into insoluble minerals.
In another embodiment of the invention, the rate of conversion of CaO/SiO
2
, SiO
2
/Al
2
O
3
+Fe
2
O
3
and SO
3
/Cr are in the order of <2.5; 0.5 to 4.0 and 0.2 to 1.0 respectively of the chemical factor of the homogenised raw mix for conversion and complete stabilisation of Cr(VI) and chrome compounds into insoluble minerals at high temperature.
In yet another embodiment of the invention, the particle fineness of the homogenised raw mix is of less than 150 mesh (100 micron) size.
In a further embodiment of the invention, the size of the pellets prepared from the homogenised raw in mix in the presence of water by granulation is less than 15 mm having 8 to 15% water.
In a further embodiment of the invention, the bed height of pelletised charge material in the sintering hearth is 300 to 600 mm, air draft suction pressure is below the grate bar of 200 to 500 mm water guage and vertical sintering speed from top to bottom of the bed is 15 to 20 mm per minute.
In a further embodiment of the invention, the total sintering and cooling period of the entire charge material is between 15 to 30 minutes depending on the charge material particle size, bed height and permeability, air suction pressure, sintering rate and temperature of treatment.
In a further embodiment of the invention, both sintering and cooling operations are within the sintering hearth itself.
In a further embodiment of the invention, depending on the strength and pozzolanic properties, 50 to 90% of the chromium stabilised sintered material is usable as stone aggregate in making building and 20 to 60% as pozzolana for making various types of blended cements as per Indian Standards.
In a further embodiment of the invention, solid waste carbon like cokebreeze, char, coal dust, carbonaceous sludge are used as the fuel source and other solid wastes available from thermal, iron and steel, fertilizer, paper, chemical plants are used as mineralising additives in the process.
DETAILED DESCRIPTION OF THE INVENTION
Solids, residues and sludge form any source contaminated with hexavalent chromium, chromate,
Mohapatra Birendra Kumar
Nayak Bansidhar
Nayak Binod Behari
Ray Hemshankar
Council of Scientific and Industrial Research
Ladas & Parry
Mitchell Katherine W
Shackelford Heather
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