Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Patent
1994-05-04
1996-02-27
Suchfield, George A.
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
405129, 588257, A62D 300
Patent
active
054950643
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the immobilization of metal contaminants from a liquid to a solid medium and in particular to a method for the immobilization of heavy metal contaminants from a liquid medium containing acids of phosphorus.
The present invention will be described herein with particular reference to the immobilization of heavy metal contaminants from acidic raffinates generated during the commercial production of phosphoric acid, but is not to be construed as being limited thereto.
Phosphoric acid is used in the manufacture of neutral phosphates and polyphosphates, which in turn find wide application in the food and detergent industries.
Phosphoric acid is obtained from naturally-occurring phosphate-containing rock. The rock is mixed with concentrated sulfuric acid and the crude phosphoric acid thus produced is removed, leaving behind a gypsum slurry waste stream. The crude phosphoric acid is then extracted into an organic solvent, from which it is subsequently stripped to produce purified phosphoric acid.
The purification process leads to generation of a secondary phosphoric acid stream (the underflow), which contains a large proportion of the various contaminants, including heavy metals, originally present in the phosphate rock.
The secondary phosphoric acid stream is treated in a second extraction plant, where residual phosphoric acid is extracted for re-use. The heavy metal contaminants, together with some residual phosphorus (present as phosphoric acid), remain in a sulfuric acid-rich stream (raffinate).
Hitherto, the raffinate has been treated by partial neutralization as a slurry followed by discharge to the sea. Although the level of heavy metal contaminants in the discharge has been kept within the prescribed limits laid down by the appropriate authority, there is pressure to reduce such sea discharge, necessitating alternative methods of disposal.
It has now been found that neutralization of the raffinate with an alkaline earth metal oxide to form agglomerates, followed by treatment of the agglomerates with water, results in the formation of a solid material of generally cementitious nature, which isolates the heavy metal contaminants within the solid material. Chemical leaching tests on the solid material have shown that the heavy metal contaminants (including cadmium, chromium, lead and members of the uranium radioactive decay series) cannot be leached out from the solid cementitious material, which may suitably be used for Landfill or similar purposes.
Accordingly, the present invention provides a method for the immobilization of heavy metal contaminants from a liquid medium containing sulfuric acid and one or more acids of phosphorus, said method comprising: alkaline earth metal oxide to form agglomerates; material wherein the heavy metal contaminants are substantially chemically isolated.
The present invention also provides a solid cementitious material made by the method described in the immediately-preceding paragraph.
The liquid medium is suitably an acidic raffinate generated during the purification of phosphoric acid. This raffinate contains approximately 40% by weight sulfuric acid and approximately 8% by weight phosphoric acid, together with metal sulfates and various heavy metal contaminants.
The preferred alkaline earth metal oxide for neutralization of the raffinate is calcium oxide (which may suitably be added as quicklime). The required stoichiometric excess of calcium oxide may be achieved by maintaining the weight ratio of raffinate to calcium oxide within the range 0.1:1 to 3.5:1, preferably 2.6:1 to 3.1:1.
Neutralization of the acidic raffinate with quicklime leads to the formation of agglomerates consisting essentially of calcium sulfate hemihydrate and calcium sulfate anhydrite. Addition of water to the agglomerates converts the calcium sulfate to calcium sulfate dihydrate (gypsum). The heat of reaction is more than sufficient to evaporate off the water present in the raffinate.
Suitably, both the neutralization and the water-treatment are carried out in a single ro
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patent: 4457848 (1984-07-01), Schimmel et al.
patent: 4671882 (1987-06-01), Douglas et al.
patent: 4737356 (1988-04-01), O'Hara et al.
James Michael L.
Rose Mark A.
Albright & Wilson Limited
Suchfield George A.
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