Separating Platinum from Gold During the Early Eighteenth Century THE METHODS USED IN SPANISH SOUTH AMERICA By Professor L. F. Capithn-Vallvey Department of Analytical Chemistry, University of Granada, Spain Following the discovery of platinum in the Viceroyalty of New Granada at the beginning of the 18th Century, its use to degrade gold forced the colonial authorities to improve their existing methods of separating and analysing the precious metals. Using information from the Royal Mint of Santa F6 de Bogota, the various ways of separating these metals are now considered, including the little-knuwn method of inquartation. Even before the discovery of the Americas by European explorers, elementary platinum metallurgy was apparently known to some of the indigenous population of the central region of New Granada; an area, shown alongside on a rare 18th century map, which was never fully integrated into the Inca world, and which now constitutes the southern part of Colombia and the northern part of Equador. Following the conquest of the New World by European in- vaders, however, this information appears to have been lost. The somewhat late discovery of platina in the Reproduced by courtesy of the Bibliotecn Nacional, Madrid Chocd area of the New Granada viceroyalty, during the 18th century, was due to several fac- ed. Its presence in the gold obtained from tors. This area had remained relatively isolated alluvial deposits would have been noted by the from Spanish penetration because of the moun- miners, by employees of the Novita and CitarA tainous terrain, high temperatures, heavy rain- foundries, and by officials of the Royal Mints at fall, numerous rivers and, above all, warlike the administrative centres of Popayan, Mari- inhabitants. For these reasons, the extraction of quita and Santa Fc? de BogotB, where the gold Chocdan gold from placer deposits and gold- was melted. bearing veins did not begin until almost the Soon gold was being degraded by the 18th century, so it was only then that the ex- deliberate addition of platina, which was dif- istence of platina among the gold was recognis- ficult to detect because of its similar density. ed. Indeed the presence of platina in such an Indeed the first written reference to platina important gold mining region constituted a pro- noted that the mixing of platina and gold was blem for both the miners and the colonial banned (I). authorities. This contamination required the colonial However, platina could have been en- authorities to improve the existing methods of countered as early as 1690, when the identifying platina and of separating it from the platiniferous area around Tad6 in the upper gold with which it occurred. Three methods part of the San Juan river valley was first work- were available: amalgamation, inquartation and Platinum Metals Rev., 1989, 33, (2), 73-80 73 This is the first known account of the separation of gold from platina by the amalgamation process, and it forms part of a Tad0 judge’s inquiry, dated 1721 melting. The inquartation method which was higher figures have been suggested for used at the beginning of the 18th century is not platinum: 0. I and 0.2 weight per cent at 24 and generally known today, although people are 54OC, respectively (4). The platinum group more aware of the other two processes. metals with the higher melting points, namely The first information about platina to reach osmium, iridium and ruthenium, do not form Europe was communicated by Antonio de Ulloa intermetallic compounds with mercury as their (1716-1799, in the famous “Relaci6n solubility is very small (<10-~weight per cent Hist6rica” (2). This was written as a result of at 50oOC). Finally, rhodium shows a slightly the French-Spanish geodesic expedition, whose higher solubility than these (6x IO-~weight objective was to determine the shape of the per cent at 500°C), but it does not form com- earth. Following this event, scientific investiga- pounds with mercury (3). tion of platinum started in Europe in 1748, at At the end of the amalgamation process, the least half a century after some technical amalgams and any remaining mercury are knowledge of it had existed in New Granada. separated from the insoluble waste by filtration The occurrence of platina in the Viceroyalty through leather or various types of cloth, the of New Granada at the beginning of the 18th degree of separation being determined by the century, the legal and technical problems that pore size of the fitering medium (5). Thus, the it caused and the solutions provided at that time amalgamation process would be better for are not well known today. Therefore in this ar- separating osmium, iridium, ruthenium and ticle we will examine the methods used at that rhodium than for platinum and palladium. time for the separation of platina from gold in However, all of them would, to some slight ex- Spanish America. tent, form an amalgam with mercury and would therefore be recovered with the gold. Separation by Amalgamation In Spanish America treatment with mercury The amalgamation method of separation is was the usual means of isolating gold mined based upon the fact that the solubility of the from mineral veins. The process was not platinum group metals in mercury is low, while necessary for gold recovered from placer the solubility of gold is high. Platinum and deposits, as Bernabe Cobo (1582-1657) noted palladium, the lower melting points metals in in his “Historia del Nuevo Mundo” (6): the platinum group, form intermetallic com- “Gold is present in two forms: the first, pure and pounds with mercury. Both show slight perfect, does not need to be melted nor treated by solubility in mercury: 0.0008 and 0.2 weight heat or mercury, and the second, in veins, like per cent for platinum and palladium, respec- silver, is firmly embedded in the rock.” tively, at a temperature of 3moC (3); although It was, however, necessary to demonstrate that Platinum Metals Rev., 1989, 33, (2) 74 alluvial gold which contained platina, could be degree of dispersion in other metals, as well as purified by amalgamation. by the formation of intermetallic compounds. It In the Choc6 area the amalgamation process also depends on the presence and type of im- was carried out in shallow wooden or clay purities, particle size, metallurgical history and washing pans or trays (bateas), where the finely dissolution technique employed. divided alloy was treated with mercury. After Therefore, one way of breaking up compact that, it was separated by passing the amalgam metals before dissolution is to alloy them with through filter cloths. a more active metal. In this way the dispersion The recovery of gold from the amalgam was of the platinum group metals throughout the carried out by distillation in an improvised fur- metal, or the formation of intermetallic com- nace containing two different sized bateas, pounds which are soluble in mineral acids, arranged one above the other. A very hot helps the dissolution process (II , I 2). To assist flagstone with the amalgam upon it, was placed in the dissolution of the platinum group metals, inside the furnace and then the volatilised mer- zinc (13-16), tin (12, 17), bismuth (12) and cury condensed on the upper batea and col- silver (18-26) have been used. lected in the lower one, while the resulting gold In the case being considered here, the separa- remained on the stone (7). tion is based on a ternary silver-gold-platinum The first known reference to the use of the alloy. In suitable proportions, this alloy is par- amalgamation process to separate gold from tially soluble in nitric acid which, of course, platina appears in a report dated 1721 (8). It oc- leaves gold undissolved. In consequence, the curs in an account of a Tad6 judge’s inquiry, in addition of the correct quantity of silver follow- which he points out that he had been present ed by treatment with nitric acid, will permit the during a separation operation: separation of the gold-platinum alloy. “The gold was washed and cleaned in my This method of separation, from now on pencewith mercury and gave two hundred and referred to as the inquartation process, is very fifty pounds and what appeared to be seven or similar to the apartado method, which, during eight pounds of platina. The latter took place in the presence of Captain Francisco Perea and the time we are referring to, was used to witnesses who hereby confirm that it came from separate gold and silver, and was based upon the San Juan river”. the selective dissolution of silver in nitric acid. On many occasions this amalgamation pro- cess has been quoted as being the only method The Inquartation Method used for the separation of platina before a scien- in Europe tific interest in the platinum metals developed The inquartation method of separating gold in Europe during the second half of the 18th and platinum was investigated in Europe by century (9). However, this was erroneous. several scientists. The first of these was the Frenchman Mathieu Tillet, (1714-1791), the A Separation Method Based Royal Commissioner for Assays and Refining at on Alloying the Paris Mint, who in 1779 published three ar- In the same way as gold, the platinum group ticles about this method (23, 24, 25). In these metals are very resistant to chemical reagents, he said that a good separation will always be including acids, bases and halogens. This is due achieved if the metals are properly alloyed to the thermodynamic stability of their (which can be obtained by cupellation), if the crystalline structures and also to the formation proportion of platinum is low (0.I to 0.05 of the of monoatomic layers of some oxides on the sur- gold), if the proportion of silver is high (two or face of the metals during the dissolution pro- three times the amount of the remainder), and cess, which renders them passive (10).