Patent ID: 7300542

Claim:
A method for inhibiting calcium salt scale formation in alkaline chemical pulping processes comprising adding an effective scale inhibiting amount of at least one phosphonate to a black liquor of said chemical pulping process, said composition comprising at least one phosphonate selected from compounds having the formula: M 2 O 3 P—CH 2 —N(R 1 )—(CH 2 ) m —N(R 2 )—CH 2 PO 3 M 2 (I), compounds having the formula: R 3 —C(OH) (PO 3 M 2 ) 2 (II), compounds having the formula: N—(CH 2 PO 3 M 2 ) 3 (III), phosphonates having the formula: amine oxides of phosphonates of formulas (I) and (III), or mixtures thereof; wherein M is independently selected from hydrogen, alkali metal, alkaline earth metal or ammonium, R 1 and R 2 are independently selected from —CH 2 PO 3 M 2 or —(CH 2 ) n —N—(CH 2 PO 3 M 2 ) 2 , m is 2 or 3, n is 2 or 3, and R 3 is an alkyl group having 1 to 17 carbon atoms and R 3 is optionally branched and optionally unsaturated; with the provisos that: (a) said phosphonate is not a blend of a phosphonate of formula (II) with a phosphonate of formula (III), (b) said phosphonate is not a blend of a phosphonate of formula (II) with a phosphonate of the formula (M 2 O 3 P—CH 2 ) 2 —N—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 ) 2 , (c) when said phosphonate is selected from phosphonates of formula (III), phosphonates of the formula (M 2 O 3 P—CH 2 ) 2 —N—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 ) 2 , or phosphonates of the formula (M 2 O 3 P—CH 2 ) 2 —N—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 )—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 ) 2 , said scale inhibiting composition does not contain a nonionic surfactant, (d) when said phosphonate is selected from phosphonates of formula (III), or phosphonates of the formula (M 2 O 3 P—CH 2 ) 2 —N—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 )—(CH 2 ) 2 —N—(CH 2 PO 3 M 2 ) 2 , the amount of said phosphonate on an active acid basis is greater than 25 ppm based on the weight of black liquor recovered from the digester, and (e) when the phosphonate is selected from the phosphonates of formula (IV), the amount of the phosphonate on an active acid basis is greater than 20 ppm based on the weight of black liquor recovered from the digester.