Method for removing metal impurities

Abstract

A process for removing metallic impurities from a product mass flow comprising water-absorbing polymer particles by means of bar magnets, wherein the water-absorbing polymer particles comprise a surfactant and have direct contact with the bar magnets.

Claims

1. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer specified under a), and e) optionally one or more water-soluble polymer, metallic impurities being removed from a product mass flow comprising water-absorbing polymer particles by a magnetic separator, the magnetic separator being formed from bar magnets, the bar magnets consisting of an austenitic steel sleeve and a magnetic material present within the sleeve, and the sleeve being bonded to the magnetic material in an undetachable manner, wherein the sleeves have direct contact to the water-absorbing polymer particles and the water-absorbing polymer particles comprise at least one surfactant, wherein the moisture content of the water-absorbing polymer particles is increased upstream of the magnetic separator.

2. The process according to claim 1, wherein the product mass flow has a temperature of 30 to 90° C.

3. The process according to claim 1, wherein the water-absorbing polymer particles in the product mass flow have a moisture content of 1 to 20% by weight.

4. The process according to claim 1, wherein an areal loading of the magnetic separator is 2 to 15 g/cm.sup.2s.

5. The process according to claim 1, wherein the bar magnets have a diameter of 5 to 30 mm.

6. The process according to claim 1, wherein a distance between the bar magnets is from 5 to 30 mm.

7. The process according to claim 1, wherein at least 95% by weight of the water-absorbing polymer particles have a particle size of at least 150 μm.

8. The process according to claim 1, wherein at least 95% by weight of the water-absorbing polymer particles have a particle size of at most 600 μm.

9. The process according to claim 1, wherein the water-absorbing polymer particles have a centrifuge retention capacity of at least 15 g/g.

10. The process according to claim 1, wherein the water-absorbing particles are surface postcrosslinked and the surfactant is added to the water-absorbing particles with a surface postcrosslinker.

11. The process according to claim 10, wherein an amount of the surfactant, based on the water-absorbing polymer particles, is 0.001% to 0.1%, by weight.

12. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer specified under a), and e) optionally one or more water-soluble polymer, metallic impurities being removed from a product mass flow comprising water-absorbing polymer particles by a magnetic separator, the magnetic separator being formed from bar magnets, the bar magnets consisting of an austenitic steel sleeve and a magnetic material present within the sleeve, and the sleeve being bonded to the magnetic material in an undetachable manner, wherein the sleeves have direct contact to the water-absorbing polymer particles and the water-absorbing polymer particles comprise at least one surfactant, wherein an areal loading of the magnetic separator is 2 to 15 g/cm.sup.2s.

Description

EXAMPLES

Example 1 (Comparative Example)

(1) By continuously mixing deionized water, 50% by weight sodium hydroxide solution and acrylic acid, an acrylic acid/sodium acrylate solution was prepared, such that the degree of neutralization corresponded to 71.3 mol %. The solids content of the monomer solution was 38.8% by weight.

(2) The polyethylenically unsaturated crosslinker used was polyethylene glycol-400 diacrylate (diacrylate proceeding from a polyethylene glycol with a mean molar mass of 400 g/mol). The amount used was 2 kg of crosslinker per t of monomer solution.

(3) To initiate the free-radical polymerization, per t of monomer solution, 1.03 kg of a 0.25% by weight aqueous hydrogen peroxide solution, 3.10 kg of a 15% by weight aqueous sodium peroxodisulfate solution and 1.05 kg of a 1% by weight aqueous ascorbic acid solution were used.

(4) The throughput of the monomer solution was 20 t/h. The reaction solution had a temperature of 23.5° C. at the feed.

(5) The individual components were metered in the following amounts continuously into a List Contikneter continuous kneader reactor with a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland):

(6) TABLE-US-00001 20 t/h of monomer solution 40 kg/h of polyethylene glycol-400 diacrylate 82.6 kg/h of hydrogen peroxide solution/sodium peroxodisulfate solution 21 kg/h of ascorbic acid solution

(7) Between the addition point for the crosslinker and the addition sites for the initiators, the monomer solution was inertized with nitrogen.

(8) After approx. 50% of the residence time, a metered addition of fines (1000 kg/h) which were obtained from the production process by grinding and sieving into the reactor additionally took place. The residence time of the reaction mixture in the reactor was 15 minutes.

(9) The resulting polymer gel was placed onto a belt dryer. On the belt dryer, an air/gas mixture flowed continuously around the polymer gel and dried it. The residence time in the belt dryer was 37 minutes.

(10) The dried polymer gel was ground and sieved off to a particle size fraction of 150 to 850 μm. The resulting base polymer was surface postcrosslinked.

(11) In a Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, the Netherlands), the base polymer was coated with a surface postcrosslinker solution and then dried in a NARA paddle dryer (GMF Gouda, Waddinxveen, the Netherlands) at 155° C. for 45 minutes.

(12) The following amounts were metered into the Schugi Flexomix®:

(13) TABLE-US-00002 7.5 t/h of base polymer 308.25 kg/h of surface postcrosslinker solution

(14) The surface postcrosslinker solution comprised 2.7% by weight of Denacol EX-810 (ethylene glycol diglycidyl ether), 24.3% by weight of propylene glycol and deionized water.

(15) After drying, the surface postcrosslinked base polymer was cooled to approx. 60° C. in a NARA paddle cooler (GMF Gouda, Waddinxveen, the Netherlands) and then sieved off again to a particle size fraction of 150 to 850 μm.

(16) The resulting water-absorbing polymer particles had a moisture content of 0.9% by weight, a centrifuge retention capacity (CRC) of 30.1 g/g, an absorption under pressure (AUL0.7 psi) of 23.0 g/g and a saline flow conductivity (SFC) of 45×10.sup.−7 cm.sup.3s/g.

(17) The product mass flow was conducted in free fall through a magnetic separator. The diameter of the product flow line was 30 cm. The magnetic separator consisted of 4 removable cassettes arranged one on top of another. The bar magnets were arranged one on top of another in two offset rows in each cassette. The upper row in each case consisted of four bar magnets, the lower in each case of three. The diameter of the bar magnets was 25 mm, the horizontal gap width between two bar magnets was 30 mm and the vertical gap width between two bar magnets was 30 mm.

(18) The bar magnets were cleanable with a vacuum cleaner only with very great difficulty.

Example 2

(19) The procedure was as in example 1. The surface postcrosslinker solution additionally comprised 0.24% by weight of sorbitan monolaurate (Span® 20, ICI Americas Inc. Wilmington, US).

(20) The bar magnets were cleanable easily with a vacuum cleaner.