Process for the removal of oxides of sulfur from a gaseous stream

Claim 2

IN A LIME SCRUBBING PROCESS FOR REMOVAL OF OXIDES OF SULFUR FROM A GASEOUS STREAM, WHEREIN SAID GASEOUS STREAM IS CONTACTED IN A SCRUBBING UNIT AN AQUEOUS LIQUOR CONTAINING CALCIUM SALTS AND WHEREIN AQUEOUS EFFLUENT FROM THE SCRBBING UNIT IS CLARIFIED, WITH REMOVAL OF A SOLIDS SLURRY CONTAINING PREDOMINANTLY SOLID CALCIUM SALTS AS REACTION PRODUCT, AS CLARIFIER UNDERFLOW, AND WHERIN AQUEOUS LIQUOR IS RECYCLED TO THE SCRUBBING UNIT, THE IMPROVEMENT COMPISING PREVENTING THE FORMATION OF SCRUBBING WITHIN THE SCRUBBING UNIT BY ADDING TO SAID CALCIUM SALT-CONTAINING LIQUOR SUFFICIENT MAGNESIUM OXIDE TO PROVIDE A MAGNESIUM ION CONTENT OF 250-5000 PARTS PER MILLION BY WEIGHT IN SAID AQUEOUS WITHIN SAID SCRUBBING UNIT, SO AS TO PREVENT SATURATION OF THE LIQUOR WITH MAGNESIUM SALTS SIGNIFICANT RESULTANT PRECIPITATE AND LOSS OF MAGNESIUM IN THE CLARIFIER UNDERFLOW, WITH THE PH OF SAID AQUEOUS LIQUOR BEING BETWEEN 5.5 AND 10.0 UPON INTRODUCTION OF THE SCRUBBING UNIT, TO INCREASE THE ALKALINIY OF THE LIQUOR AND THE REMOVAL OF OXIDES OF SULFUR FROM THE GASEOUS STREAM.

Claim 3

In the process defined in claim 1, the improvement wherein said magnesium oxide is added to said calcium salt containing liquor in the form of a burnt lime composition containing calcium oxide and between 1.5 to 7 per cent by weight of magnesium oxide.

Claim 4

In the process defined in claim 2, the improvement wherein said burnt lime composition is formed by calcining limestone containing magnesium and calcium in a ratio of between 1:60 to 1: 9.

Claim 5

In the process defined in claim 2, the improvement wherein said burnt lime composition is formed by blending a high calcium content limestone and a high magnesium content limestone and calcining said blend.

Claim 6

In the process defined in claim 1, the improvement wherein said aqueous effluent from said scrubbing unit, that is recycled thereto, contains the magnesium salts, magnesium carbonate, magnesium bicarbonate and magnesium sulfite in addition to magnesium hydroxide and calcium salts, which magnesium salts react with sulfur dioxide within said scrubbing unit.

Claim 7

In the process defined in claim 5, the improvement wherein said magnesium salts are replenished by containing said effluent for a period of time and adding thereto, during said period, an aqueous mixture of calcium oxide and magnesium oxide.

Claim 8

In the process defined in claim 1, the improvement wherein aqueous effluent from the scrubbing unit is passed to a clarifying unit and held therein and wherein, following holding of said effluent in said clarifYing unit, clarified effluent is returned to the scrubbing unit.

Claim 9

In the process defined in claim 7, the improvement wherein a calcium compound selected from calcium oxide and calcium hydroxide is added to clarifying unit underflow to increase precipitation of solids from the underflow.

Claim 10

In the process defined in claim 1, the improvement wherein aqueous effluent from the scrubbing unit is contained for a period of time and following said containment is passed to a clarifying unit and held therein to separate solids therefrom and wherein, following holding of said effluent in said clarifying unit, at least a portion of clarified effluent is added to said scrubber effluent so contained.

Claim 11

In the process defined in claim 9, the improvement wherein there is also added to said scrubber effluent so contained, a mixture of calcium and magnesium compounds selected from calcium oxide and hydroxide and magnesium oxide and hydroxide.

Claim 12

In a lime scrubbing process for removal of oxides of sulfur from a gaseous stream resulting from the combustion of a sulfur-containg fuel, wherein said gaseous stream is contacted in a scrubbing unit with an aqueous liquor containing calcium salts and wherein aqueous effluent from the scrubbing unit is clarified, with removal of a solids slurry containing predominantly solid calcium salts as reaction product, as clarifier underflow and wherein aqueous liquor is recycled to the scrubbing unit; the improvement comprising preventing the formation of scaling within the scrubbing unit by adding to said aqueous effluent, prior to recycle to the scrubbing unit, calcium oxide and magnesium oxide, the magnesium oxide being added in an amount sufficient to provide a magnesium ion content of 250-5000 parts per million by weight in said aqueous liquor within said scrubbing unit, so as to prevent saturation of the liquor with magnesium salts and significant resultant precipitation and loss of magnesium in the clarifier underflow, with the pH of said aqueous liquor being between 5.5 and 10.0 upon introduction to the scrubbing unit, to increase the alkalinity of the liquor and the removal of oxides of sulfur from the gaseous stream.

Claim 13

In a lime scrubbing process for removal of oxides of sulfur from a gaseous stream resulting from the combustion of a sulfur-containg fuel, wherein said gaseous stream is contacted in a scrubbing unit with an aqueous liquor containing calcium salts and wherein aqueous effluent from the scrubbing unit is clarified, with removal of a solids slurry containing predominantly solid calcium salts as reaction product, as clarifier underflow and wherein aqueous liquor is recycled to the scrubbing unit; the improvement comprising preventing the formation of scaling within the scrubbing unit by adding to said aqueous effluent, prior to recycle to the scrubbing unit, calcium oxide and magnesium oxide, the magnesium oxide being added in an amount sufficient to provide a magnesium ion content of 250-5000 parts per million by weight in said aqueous liquor within said scrubbing unit, so as to prevent saturation of the liquor with magnesium salts and significant resultant precipitation and loss of magnesium in the clarifier underflow, with the pH of said aqueous liquor being between 5.5 and 10.0 upon introduction to the scrubbing unit, to increase the alkalinity of the liquor and the removal of oxides of sulfur from the gaseous stream.