Hydrometallurgy 72 (2004) 179–184 www.elsevier.com/locate/hydromet Recovery of platinum from spent catalyst M.A. Barakat, M.H.H. Mahmoud* Central Metallurgical R&D Institute, P.O. Box, 87, Helwan 11421, Cairo, Egypt Received 8 February 2003; received in revised form 22 June 2003; accepted 25 June 2003 Abstract During the manufacture of nitric acid from ammonia there is a slow loss of catalytic platinum gauze. This fine dust, which contains 13.7% Pt, 1.3% Rh and other impurities such as iron oxide and silica, is deposited on the internal reactor walls and cooling coils. The dust was refluxed with aqua regia at a liquid/solid ratio of 7.5 for 1.5 h to solubilise platinum for subsequent recovery. Platinum was separated from the leach liquor by direct precipitation and by solvent extraction using trioctylamine in kerosine. In both cases the platinum was recovered as diammonium hexachloroplatinate by precipitation using ammonium chloride. The precipitate was decomposed by ignition to produce platinum powder with purity of 97.9% and 99.9% for direct and SX routes, respectively. A platinum recovery of 97.5% was achieved by both routes. D 2004 Elsevier B.V. All rights reserved. Keywords: Recovery; Platinum; Catalyst 1. Introduction recovery from spent reforming and isomerization catalysts has been studied by selective dissolution of Platinum gauze is used to catalyse the oxidation of base metals in 50% H2SO4 and treating the insoluble ammonia in the manufacture of nitric acid. During this residue with aqua regia from which platinum was operation, part of the platinum is lost as fine dust due recovered (Jeliyaskova et al., 1982). Platinum was to operation at high temperature and pressure. The also recovered from a spent Pt-aluminosilicate catalyst dust is deposited on the internal reactor walls and by slurrying in sulfuric acid to give an aluminium cooling coils. This dust can be recovered during a sulfate solution and platinum residue. The later was plant shutdown, the dust on the walls was collected leached with concentrated hydrochloric acid and trea- manually while the dust on the coils was washed with ted with chlorine gas at 95 jC to dissolve the platinum dilute hydrochloric acid and the liquor was dried. In (Eugenia et al., 1983, 1984; Okuda, 1989; Ezawa, the Egyptian fertilizer industry, large amounts of such 1989). An aluminum chloride leach process was platinum enriched dust are accumulated. Recovery of developed for recovery of both platinum and palladi- the platinum metal from this dust has a vital economic um from an alumina pellet catalyst (Letowski and impact on the process. Distin, 1985, 1987, 1989). The leachate was an aqua Different authors have investigated the recovery of regia type solution in which part of the hydrochloric platinum group metals from spent catalysts. Platinum acid content was replaced by aluminium chloride to lower the acid consumption. * Corresponding author. Fax: +20-2-5010639. Narita (1998) investigated a method of platinum E-mail address: [email protected] (M.H.H. Mahmoud). recovery from sludge and dust. Base metals were 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0304-386X(03)00141-5 180 M.A. Barakat, M.H.H. Mahmoud / Hydrometallurgy 72 (2004) 179–184 removed from chloroplatinic acid solution by alkaline 2. Experimental precipitation. Ammonium chloride was then added to convert the dissolved platinum to ammonium chlor- A representative sample of 1-kg spent platinum oplatinate, which was subsequently heated to yield dust was obtained from Kima Company for Fertil- platinum sponge. izers, Aswan, Egypt. The dust sample was homoge- Secondary platinum scrap from the glass industry nized by thorough mixing and the < 106-Am fraction also containing iridium, rhodium and palladium was used. Preliminary investigations showed that grind- used to study the refining process. Dissolving this ing to finer sizes had negligible effect on the leach- material in boiling aqua regia, diluting with water or ing of platinum. X-ray diffraction analysis (Cu hydrochloric acid and adding ammonium chloride radiation, Philips PW 1730) indicted the presence gives a Pt precipitate, leaving the accompanying of Pt, quartz and haematite as the major phases platinum group metals in solution (Schreier and Edt- present. maier, 2003). Chemical analysis of the dust and products was Solvent extraction has been practiced for the determined by using both of atomic absorption preparation of highly pure precious metals (Narita, spectrometer type (Perkin Elmer Model 3100) and 1998; Barnes and Edwards, 1982; Cleare and Char- X-ray fluorescence (ARL 9400 Sequential XRF). lesworth, 1979; Reavell and Charlesworth, 1980). The composition of the dust is shown in Table 1. Tributylphosphate (TBP) and trioctylamine (TOA) Fig. 1 shows the procedures followed in this work can extract platinum and have been commercially for platinum separation from the dust. used for platinum refining. However, the drawback Preliminary leaching of the dust sample with of using TBP is the coextraction of iron with plati- concentrated hydrochloric acid showed that although num at a wide range of acid concentration. The the haematite dissolved, so did some of the platinum. stripping efficiency of platinum from the organic This can be attributed to the oxidative nature of the phase is known to be rather difficult. Precipitation dust sample that was collected as fine particles from stripping was recently investigated to overcome this the highly oxidizing nitrogen oxide atmosphere. difficulty. Pure diammonium hexachloroplatinate Thus, a preliminary stage to dissolve iron is not powder was efficiently prepared from platinum load- appropriate due to the losses of platinum. ed alamine 336 and TBP using aqueous mixed Leaching experiments were performed in a 250-mL solutions of ammonium chloride and ammonium glass container immersed in a thermostatically con- hydroxide or ammonium chloride and perchloric acid trolled water bath equipped with magnetic stirrer. (Sumiko, 1999). Aqua regia was freshly prepared by mixing concen- The composition of the dust sample under inves- trated hydrochloric acid (36%) and concentrated nitric tigation in this work is different from those cited in the acid (65%) by the volume ratio of 3 HCl:1 HNO3 and literature as it is contaminated with several impurities used as leaching agent for the dust. A reflux system such as excessive amounts of iron oxide from the (glass condenser) was used to recover HCl and NOx surroundings and other base metal oxides. The aim of gases and any escaped gases were scrubbed in water. this study is investigation of the leaching and separa- The reaction mixture was filtered and washed to tion conditions for recovery of platinum from the dust of the spent catalyst arising from a nitric acid produc- Table 1 tion plant. Acid leaching followed by either chemical Chemical analysis of the platinum dust precipitation or solvent extraction is examined. Element Weight Element Weight Parameters affecting leaching and separation process- percent percent es such as time, temperature, acid concentration and Pt 13.7 Cu 0.15 liquid/solid ratio were studied. Rhodium was also Rh 1.3 Zn 0.17 found to be partially leached with aqua regia. The Fe 16.9 Pb 0.17 leaching residue and the filtrate after platinum sepa- Ni 2.46 As 0.62 ration are used for Rh recovery and the results will Ca 1.15 Re 0.84 appear in a subsequent paper. Mg 1.35 Si 8.25 M.A. Barakat, M.H.H. Mahmoud / Hydrometallurgy 72 (2004) 179–184 181 Fig. 1. Process flowsheet for platinum recovery from the dust. separate the undissolved materials from the pregnant In both cases, the precipitate was filtered washed platinum solution. with saturated ammonium chloride solution and dried. Two alternative methods were investigated for The dried precipitate was ignited to produce sponge separation of both commercial and pure grade plati- platinum powder. num from the solution. In the first method, platinum was precipitated as diammonium hexachloroplatinate, (NH4)2PtCl6, by adding ammonium chloride to the 3. Results and discussion leach solution. Solvent extraction experiments used 10% trioctyl- The leaching results of platinum dust with aqua amine (TOA) in kerosine and were performed in 250- regia at different times, temperatures and liquid/solid mL cylindrical glass vessels with equal volumes of the (acid/dust) ratios are shown in Fig. 2. Fig. 2a shows organic and aqueous phases. The vessels were shaken the effect of leaching time on the recovery of platinum in a thermostatted shaker (GFL Model 1083) for the at a liquid/solid ratio of 25, and 109 jC. It can be seen required time. After phase separation a sample from that even after 15 min, the extent of platinum recovery the aqueous phase was withdrawn for chemical anal- was relatively high with a recovery of 72%. The ysis. An aqueous solution of ammonium hydroxide recovery increases with increasing time, reaching a and ammonium chloride was used as the stripping maximum and constant value of 98% after 1.5 h. The agent with diammonium hexachloroplatinate immedi- effect of liquid/solid ratios at 109 jC for 2 h is shown ately precipitated. Precipitation stripping was sug- in Fig. 2b. At a liquid/solid ratio of 2.5, about 77% of gested (Sumiko, 1999) to enhance the stripping of the platinum was recovered; the maximum recovery platinum from Pt-loaded TOA. of 98% was attained with a liquid/solid ratio of 10. 182 M.A. Barakat, M.H.H. Mahmoud / Hydrometallurgy 72 (2004) 179–184 Saturated ammonium chloride solution was added to the leach liquor (as-prepared) that contains hexa- chloroplatinic acid to precipitate diammonium hexa- chloroplatinate complex (Loewen, 1995; Schreier and Edtmaier, 2003). The effect of temperature on the precipitation of this salt using ammonium chloride was investigated.