Phosphate recovery from phosphorus-rich solution obtained from chicken manure incineration ash

Abstract

The recovery of phosphorus from waste is very important for Japan because Japan has no natural phosphorus resources. In order to recover phosphorus from incineration ash of chicken manure, an acid dissolution–alkali precipitation method was investigated. Phosphorus content in the ash was 8%. The ash was treated with hydrochloric acid to obtain phosphorus-rich solution. Phosphorus could then be recovered as a precipitant by adding sodium hydroxide solution into the phosphorus-rich solution and gradually changing the pH in the solution to 3, 4, and 8. At pH 3, a small amount of phosphorus was precipitated to remove iron, which would cause coloring of subsequent precipitants. At pH 4, 84% of the phosphorus in the original solution could be recovered as CaHPO₄ · 2H₂O with a purity of 92%. At pH 8, 8% of the phosphorus in the phosphorus-rich solution could be recovered as identified hydroxyapatite. A recovery rate of 92% phosphorus as CaHPO₄ · 2H₂O and identified as hydroxyapatite was achieved.

Introduction

At Miyazaki in the Kyusyu area in Japan, active agriculture is concentrated and an abundant amount of chicken is produced. Chicken manure is used as a fertilizer. However, such use often causes environmental problems such as nitrate pollution of groundwater because excessive chicken manure must be spread on fields in order to use all of the chicken manure generated in the area. In addition, over-application of manures to agricultural land has led to a build-up of phosphorus on many farms (Greaves et al., 1999). In order to solve manure disposal problems, incineration of the chicken manure has been applied in commercial operations. However, incineration ash prevents the economics of wide-spread incineration use, as the final management of the manure due to the high cost of ash disposal. Therefore, it is advantageous to use the incineration ash as a resource (Nzihou and Sharrock, 2002, Merino et al., 2005).

As described later, chicken manure incineration ash (CMIA) contains a high concentration of phosphorus (Table 1). Since Japan has no natural resources of phosphorus, all of the phosphate rocks used in Japan are imported. Because a shortage of phosphorus in Japan would be serious, it is necessary to develop technologies for reclaiming phosphorus from other sources.

There are two types of recovery methods of phosphorus from incineration ash: a dry process and a wet process. For the wet process, phosphorus is extracted by acid or an organic solvent and subsequently recovered from the solution. For the dry process, phosphorus is recovered by melting the ash. The wet process would be more economical in terms of energy than the dry process. Many recovery methods of phosphorus from solution have been reported. The chemical synthesis of hydroxyapatite by mixing phosphate solution and calcium salt solution has been reported (Pang and Bao, 2003, Wenjian and Baptista, 1998). Recently, recovery of phosphorus as magnesium ammonium phosphate (struvite) has attracted much attention to phosphorus recovery from waste and swine wastewater (Li et al., 1999, Nelson et al., 2003, Zhang et al., 2004, Suzuki et al., 2005). Recovery of phosphorus from incineration ash of sewage sludge using solvent extraction has been researched (Hino et al., 1998). Initially, ash from sewage sludge incineration is leached with hydrochloric acid. Ammonium solution is then added to the leachate to precipitate aluminum phosphate. The aluminum phosphate is dissolved in hydrochloric acid, from which phosphoric acid is extracted with tri-n-butyl phosphate (TBP). The extraction is successfully carried out, since the extraction of phosphoric acid by TBP is enhanced by the presence of hydrochloric acid.

The purpose of this paper is to examine the recovery of phosphorus from CMIA. In order to recover phosphorus, an acid dissolution–alkali precipitation method was employed: dissolution of the CMIA using hydrochloric acid to obtain phosphorus-rich solution and precipitation using sodium hydroxide to separate phosphorus. Due to the lack of ammonium in the ash, the struvite method was not attempted. Precipitation with alkali seems to be a more promising technology than solvent extraction from the viewpoint of environmental protection. No study focused on the recovery of phosphorus from incineration ash of waste using precipitation processes has been reported. The present work focuses on the design of unique precipitation processes for phosphorus recovery from CMIA.