Recovery of Calcium Phosphates from Composted Chicken Manure

Abstract

To recover phosphorus from composted chicken manure, a batch method with aqueous HNO₃, HCl and H₂SO₄ was used to examine the elution behavior of the aqueous calcium and phosphate contained in the manure. Since the main components in manure are Ca²⁺ and K⁺ along with PO₄³⁻ and those ions can be dissolved using an acidic eluate, it was expected that most of the aqueous Ca²⁺, K⁺ and PO₄³⁻ could be obtained via the elution. Therefore, it seemed plausible that the removal of the aqueous K⁺ obtained by the elution of composted chicken manure would result in the formation of calcium phosphates. If calcium phosphates are formed, they can be used for phosphate rock, which also consists of various calcium phosphates. When using 0.1 mol/L HNO₃, HCl or H₂SO₄, the elution behavior of the PO₄³⁻ was not dependent on the acids. However, 0.1 mol/L H₂SO₄ was not sufficient for the elution of Ca²⁺, probably due to the precipitation of the calcium sulfate. The eluted amount of K⁺ using 0.1 mol/L HNO₃ was lower than that using 0.1 mol/L of either HCl or H₂SO₄. Since the poor elution of K⁺ should enrich the concentrations of Ca²⁺ and PO₄³⁻ in the acidic aqueous solution after the elution, it was suggested that aqueous HNO₃ would be suitable as an eluate in the present system. After the elution of the composted chicken manure, when 0.1 mol/L HNO₃ was used to adjust the solution pH of the acidic aqueous solution to greater than 6, Ca²⁺ and PO₄³⁻ were precipitated, but K⁺ was not. The precipitate was calcium hydroxyapatite, one of the typical components of phosphate rock, which showed that composted chicken manure could be replaced phosphate rock as a new source of phosphorus.