Process for the continuous preparation of water-absorbent polymers

1. A process for the preparation of water-absorbent polymer particles, comprising the process steps of

(i) preparing an aqueous monomer solution comprising at least partially neutralized, monoethylenically unsaturated monomers bearing carboxylic acid groups (α1) and at least one crosslinker (α3);

(ii) optionally adding fine particles of a water-absorbent polymer to the aqueous monomer solution;

(iii) adding a polymerization initiator or at least one component of a polymerization initiator system that comprises two or more components to the aqueous monomer solution;

(iv) decreasing the oxygen content of the aqueous monomer solution;

(v) charging the aqueous monomer solution into a polymerization reactor;

(vi) polymerizing the monomers in the aqueous monomer solution in the polymerization reactor;

(vii) discharging the polymer gel strand out of the polymerization reactor and optionally comminuting the polymer gel thereby obtaining polymer gel particles;

(viii) drying the polymer gel particles;

(ix) grinding the dried polymer gel particles thereby obtaining particulate water-absorbent polymer particles;

(x) sizing the grinded water-absorbent polymer particles; and

(xi) treating the surface of the grinded and sized water-absorbent polymer particles;

wherein in process step (viii) the gel particles obtained in process step (vii) are charged onto the surface of the belt material of a belt dryer at a position L 0 and are subsequently dried on their way through the belt dryer and wherein the belt material has been cooled before coming into contact with the gel particles; and

wherein cooling the surface of the belt material is performed by continuously spraying a cooling liquid onto the surface of the belt material.

2. The process according to claim 1 , wherein the surface of the belt material of the belt dryer at position L 0 has been cooled to a temperature below 160° C. before coming into contact with the gel particles.

3. The process according to claim 1 , wherein the surface of the belt material of the belt dryer at position L 0 has been cooled to a temperature below 140° C. before coming into contact with the gel particles.

4. The process according to claim 1 , wherein the surface of the belt material is cooled by spraying an aqueous additive solution onto the belt material, the additive being a polyalkylene glycol.

5. The process according to claim 4 , wherein the concentration of the additive in the aqueous additive solutions is in the range from 0.01 to 20 wt.-%, based on the weight of the aqueous additive solution.

6. The process according to claim 1 , wherein the carboxylic acid groups in the polymer gel are neutralized to an extend of less than 70 mol-%.

7. The process according to claim 1 , wherein the polymer gel is charged onto the surface of the belt material in process step (viii) by means of a swivel belt.

8. The process according to claim 1 , wherein the thickness of the layer of the polymer particles to be dried on the belt dryer is not less than 2 cm and not more than 20 cm.

9. The process according to claim 8 , wherein the aqueous additive solution is a mixture of water with a polyalkylene glycol.

10. The process according to claim 1 , wherein the belt speed of the belt dryer is not less than 0.005 m/s and not more than 0.05 m/s.

11. The process according to claim 1 , wherein the cooling liquid is water.

12. The process according to claim 1 , wherein the cooling liquid is selected from alcohols, or mixtures of alcohols with water.

13. The process according to claim 1 , wherein the cooling liquid is an aqueous additive solution.

14. The process according to claim 1 , wherein the cooling liquid is sprayed onto the belt at least one position at the charging position L 0 .