After some preliminary tests, two leaching stages of fly ash to recover vanadium, molybdenum and nickel were selected. A first stage was an alkaline leaching of fly ash to recover vanadium and molybdenum followed by a second stage using sulfuric acid leaching of the residual ash to recover nickel.The impact of some operational parameters (liquid/solid, leaching temperature, mixing time, and agent leaching concentrations) on the recovery of V, Ni was investigated.Conditions of precipitation of V and Mo from alkaline medium and Ni from sulfuric acid solution were established. IntroductionTwo fundamental reasons underline the interest of metals recovering from waste ash: First, reduction of pollution source. Metals as V, Ni, Mo,. . . etc. were found to be leached out of the waste ash contaminating water and soil [1,2]. Second, the economic value of the recovered metals seems to be very attractive.Many technological process have been proposed to recover V, Ni, Mo,. . . etc. from fly ash, but most literature on this subject is in the form of patents [3,4]. Each ash has its own identity, depending on the characteristics of the burned oil and the place of ash formation, which involves development of an appropriate treatment to recover such metals. Recovery of vanadium and nickel from fly ash can be achieved by acid [5][6][7], alkaline [8][9][10] or water leaching [11,12].A process was developed [5] at Canadian Petrofina Ltd. for large scale production of vanadium from fly ash, the process consists of leaching the fly ash with sulfuric acid to dissolve vanadium and filtrating the resulting slurry. The vanadium was separated from the filtrate by oxidizing to pentavalent state with sodium chlorate and precipitating with ammonia. The hydrated pentoxide was dried, fused and cast into flakes. The author does not indicate any solution purification operation that leads to 99% of pure pentoxide.Other authors [8] consider that acid leaching is inefficient for vanadium dissolution and they propose an alkaline leaching using a high concentration of NaOH that gives about 94% of vanadium recovery. After vanadium recovery, the remaining ashes contain 15% nickel; the latter can be recovered by leaching with 30% H 2 SO 4 .H. Ottartun [13] used H 2 SO 4 coming from the solvent unit extraction to leach the ash. During leaching, vanadium is kept in its tetravalent state by feeding SO 2 to the solution. Solvent extraction of (V +4 ) is performed with 20% di-(2-ethylhexyl) phosphoric acid and 15% tributylphosphate. The raffinate is returned to the leaching stage.Our main objective is to systematically investigate the optimum conditions of V, Mo and Ni recovery from fly ash produced from heavy oil-fired electrical power stations, which gives the best yield of leaching and purity of the final product.

Extraction of vanadium and molybdenum from alkaline leaching solution of boiler ash was investigated. The ash coming from heavy oil-fired electrical power station was leached with 8 mol dm −3 sodium hydroxide at 373 K. The leaching solution was cooled to 278 K; an alkali-precipitate was precipitated. The precipitate was dissolved in sulfuric acid and a selective solvent extraction process was applied. First, almost all the molybdenum was extracted with 1.5% (v/v) tri-ndodecylamine/kerosene, stripped with 0.5 mol dm −3 NaOH and precipitated with CaCl 2 as CaMoO 4 , then acidified with 30% HNO 3 , the resulting H 2 MoO 4 .H 2 O was calcinated at 723 K for 4 hours to give molybdenum oxide, next the vanadium was extracted from the raffinate coming from the first solvent extraction with 25% (v/v) tri-n-dodecylamine/kerosene at pH 1.8, stripped with 0.5 mol dm −3 NaOH, precipitated with (NH 4 ) 2 SO 4 at pH between 7.5-8.5 as ammonium metavanadate and calcinated at 773 K for twenty four hours to yield 99.9% vanadium pentoxide.