Continuous production process for water-absorbent resin powder and powder surface detector used therefor

A continuous production process for a water-absorbent resin powder, which is a process for continuously producing a surface-modified water-absorbent resin powder comprising a polymerizing step, a drying step, a pulverizing step, a classifying step, and a surface-modifying step, and further, conveying steps of connecting these steps, and is characterized in that:

• the water-absorbent resin powder is obtained by a process including the step of polymerizing an unsaturated monomer and has a crosslinked structure, and has a mass-average particle diameter of 300 to 600 μm, and includes particles having particle diameters of 850 to 150 μm in a ratio of not less than 90 mass % in particle diameter distribution, and has a property of displaying an absorption capacity of not less than 25 g/g without load, and has a water-extractable content of not more than 25 mass %;
• and
• the conveying steps include at least two hoppers for storing and discharging the water-absorbent resin powder after the pulverizing step wherein at least one of the hoppers includes:
• a tank for storing the water-absorbent resin powder;
• a supplying part which is placed in an upper portion of the tank and serves for supplying the water-absorbent resin powder into the tank;
• a discharging part which is placed in a lower portion of the tank and serves for discharging the water-absorbent resin powder from the tank;
• and
• a detector for detecting the quantity of the powder in the tank.