Process for the continuous preparation of water-absorbent polymers

The invention relates generally to 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 (±1) and at least one crosslinker (±3); (ii) optionally adding fine particles of a water-absorbent polymer or aqueous salt solutions to the aqueous monomer solution; (iii) adding a polymerization initiator or a 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) crosslinking the surface of the grinded and sized water-absorbent polymer particles; wherein process step (xi) comprises the steps of: (x1a) mixing the particles with an aqueous crosslinker solution; and (x2a) heat-treating the mixture obtained in process step (x1a) in a horizontally operated mixing device, wherein the horizontally operated mixing device is hydraulically driven.