NOVELTY - A raw material comprising sludge ash containing phosphorus and iron and/or aluminum is dissolved (205) in liquid comprising hydrochloric acid, and insoluble residues are separated (210) to form primary leach solution. Base is added (220) to primary leach solution, insoluble residues are separated, and phosphate compound is precipitated. The precipitated phosphate compound is removed from primary leach solution, sulfuric acid is added, sulfate is precipitated, and separated from primary leach solution. A portion of leach solution after precipitation of sulfate compound is recycled.    USE - Method for concentrating phosphate compound (claimed) used for fertilizer.    ADVANTAGE - The method provides phosphate compound by economical and environmentally-friendly process. The phosphorus is recovered as concentrated, water-soluble, inorganic product with high quality. The fertilizer containing the phosphorus has high phosphorus availability to plants and reduced heavy metal contamination.    DETAILED DESCRIPTION - A raw material comprising sludge ash containing phosphorus and iron and/or aluminum is dissolved in a liquid comprising hydrochloric acid, and insoluble residues are separated from resulting mixture to form primary leach solution. Base is added to primary leach solution, insoluble residues are separated, and phosphate compound is precipitated from primary leach solution. The precipitated phosphate compound is removed from primary leach solution, sulfuric acid is added, sulfate compound is precipitated, and precipitated sulfate compound is separated from primary leach solution. A portion of leach solution after separation and precipitation of sulfate compound is recycled. The primary leach solution comprises phosphorus and ferric iron and aluminum. The molar ratio of phosphorus with respect to total content of ferric iron and aluminum is greater than 1. INDEPENDENT CLAIMS are included for the following:    (1) method for production of phosphate compound, which involves concentrating phosphate compounds and recovering phosphate compound;    (2) system for concentration of phosphate compound, which consists of ash leaching reactor arranged for dissolving raw material comprising sludge ash, a residue separator connected to leaching reactor, and arranged for separating insoluble residue from ash leaching reactor, primary leach solution, means for controlling mole ratio, a base mixing arrangement connected after residue separator and arranged for adding a base to primary leach solution, phosphate separator, connected to base mixing arrangement, and are arranged for removing precipitated phosphate compounds from primary leach solution, sulfur mixing arrangement connected after phosphate separator and arranged for adding sulfuric acid to primary leach solution, causing precipitation of sulfate compounds, a sulfate separator connected to sulfur mixing arrangement and are arranged for separating precipitated sulfate compounds from primary leach solution, and a recycling arrangement connected between an outlet of sulfate separator and an inlet to ash leaching reactor for recycling portion of leach solution; and    (3) method for recovery of phosphate compound.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart explaining the method for concentrating phosphate compound.    Concentrating phosphate compound (201)    Dissolving sludge ash in hydrochloric acid (205)    Separating insoluble residues (210)    Adding base to precipitated phosphate compound (220)    Recycling hydrochloric acid (248)