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 (α1) and at least one crosslinker (α3)(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 particleswherein 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.
