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The Environmental History of Silver Production, and Its Impact on the United Nations Minamata Convention on Mercury

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The Environmental History of Silver Production, and Its Impact on the United Nations Minamata Convention on Mercury The environmental history of silver production, and its impact on the United Nations Minamata Convention on mercury. Saul Guerrero, Invited Professor, Universidad Metropolitana, Caracas, Venezuela ([email protected]) Introduction The tragic consequence of the presence of mercury compounds in the waters of Minamata Bay (Japan) on the health of its local population in mid-20c, focused the world’s attention on the global dangers to humans of anthropogenic emissions of mercury and its compounds to the environment. By 2018, 91 countries had ratified the Minamata Convention (2013) sponsored by the United Nations, through its Environmental Program (UNEP). This is a multilateral agreement that seeks, among other aims, to control ‘the anthropogenic releases of mercury throughout its lifecycle’.1 An important part of the work undertaken by environmental scientists in support of this convention, has been the modelling of mercury emissions to the environment, as the sum of natural processes and anthropogenic emissions over time. Mercury is known to persist in the atmosphere for many months, thus contributing to its global dispersion. Historic anthropogenic mercury can be re-issued to the environment over multiple cycles spanning centuries, before it is taken out of circulation within deep ocean sediments or land sinks. The problem lies in that ‘the accumulated burden of legacy anthropogenic Hg [mercury] means that future deposition will increase even if primary anthropogenic emissions are held constant. Aggressive global Hg emission reductions will be necessary just to maintain oceanic Hg concentrations at present levels.’2 It is widely recognized that the main source of this legacy mercury up to the end of the 19c has been the historic refining of silver and gold, thus this topic provides a fertile ground for collaboration between historians and environmental scientists. Current scenarios Several studies have been published proposing ranges for historical anthropogenic mercury emissions from silver refining. The majority are based on two assumptions: 1) 60 to 65% was due to direct losses of volatile mercury to the air, the remainder as losses of liquid mercury to the ground or waterways 2) a conflation of historic silver refining with modern artisanal gold production practice, so that ratios of mercury ‘losses’ to refined metal are deemed to be the same for gold and silver.3 Some later models have not made explicit the distinction between historic production of silver using mercury, and the fraction that has been extracted by smelting.4 Others 1 see http://www.mercuryconvention.org. 2 Helen M Amos et al., "Legacy Impacts of All‐time Anthropogenic Emissions on the Global Mercury Cycle," Global Biogeochemical Cycles 27, no. 2 (2013): 410. Legacy mercury is defined as ‘the re-emitted component of anthropogenic Hg’, Helen M. Amos et al., "Observational and Modeling Constraints on Global Anthropogenic Enrichment of Mercury," Environmental Science & Technology 49, no. 7 (2015): 4036. 3 J. Nriagu, "Legacy of Mercury Pollution," Nature 363 (1993): 589; "Mercury Pollution from the Past Mining of Gold and Silver in the Americas," The Science of the Total Environment 149 (1994): 167-189 . 4 S. Strode, L. Jaeglé, and N.E. Selin, "Impact of Mercury Emissions from Historic Gold and Silver Mining: Global Modeling," Atmospheric Environment 43, no. 12 (2009): 2012-2017; David G Streets et al., "All-time releases of 2 have sidestepped the issue of exactly how much silver was produced using mercury, and based their projections on the estimated amount of mercury used to refine silver.5 In the following sections I will argue that the historical (and chemical) scenario was more complex than current models allow for, and though I will present working figures at an order of magnitude level, to highlight the need for a major review of current models, the more important aim at this stage is to identify those areas where further historical research is required. It will be of great importance for UNEP’s policy decisions on the control of future emissions of anthropogenic mercury, to have a firm knowledge of the historical record on all the processes related to mercury and its use in silver refining.6 Methodology The projections I present of mercury issued to the environment as a result of historic silver production take into account that: 1) only total consumption of mercury, or direct losses to the air of volatile mercury during the heating of the amalgam, the casting of silver bars, or the processing of cinnabar (mercury sulphide) to produce the required mercury, are reported in historical sources. The fraction of liquid mercury lost, or the amount of mercury consumed as calomel (mercurous chloride), have to be estimated, based on historical and chemical evidence, since they were never measured during actual operations 2) results are location and period specific, since recipes, processes and equipment went through major changes between the 16c and 19c 3) historical silver refining with mercury and modern artisanal gold refining practice are two quite different historical and technical processes, and there is no theoretical or empirical basis to justify extrapolating values between them 4) the fraction of silver produced by smelting has to be subtracted from total silver production data. Direct air emissions of volatile mercury Operational data measured in the 19c in Mexico, of direct losses of mercury to the air during the recycling of mercury from the amalgam, ranged from 0.06 to 0.6%.7 In the U.S.A. ‘when the mercury to the atmosphere from human activities," Environmental science & technology 45, no. 24 (2011): 104585- 10491; David G Streets et al., "Total mercury released to the environment by human activities," Environmental Science & Technology 51, no. 11 (2017): 5969-5977. 5 Nicola Pirrone et al., "Global Mercury Emissions to the Atmosphere from Anthropogenic and Natural Sources," Atmospheric Chemistry and Physics 10, no. 13 (2010): 5951-5964; Julio A. Camargo, "Contribution of Spanish– American Silver Mines (1570–1820) to the Present High Mercury Concentrations in the Global Environment : A Review," Chemosphere 48 (2002): 51-57. 6 At present there is a debate within the field of environmental science whether the high level of legacy mercury predicted by some models is not consistent with global field measurements of mercury. See for example C.H. Lamborg et al., “Modern and Historic Atmospheric Mercury Fluxes in Both Hemispheres: Global and Regional Mercury Cycling Implications,” Global Biogeochemical Cycles 16, no. 4, 1104; Colin A. Cooke et al., "Over Three Millennia of Mercury Pollution in the Peruvian Andes," Proceedings of the National Academy of Sciences 106, no. 22 (2009): 8830-8834; Daniel R. Engstrom et al., "Atmospheric Hg Emissions from Preindustrial Gold and Silver Extraction in the Americas: A Reevaluation from Lake-Sediment Archives," Environmental Science & Technology 48, no. 12 (2014): 6533-6543; Yanxu Zhang et al., "Six Centuries of Changing Oceanic Mercury," Global Biogeochemical Cycles 28, no. 11 (2014):1251-1261. 7 Saint Clair Duport, De la production des métaux précieux au Mexique, considérée dans ses rapports avec la géologie, la métallurgie et l'économie politique (Paris: Firmin Didot Frères, 1843), 117 ; M.P. Laur, "De la metallurgie de 3 retorting is properly done the loss of quicksilver by distillation is merely nominal. At the California mill about one half a pound of quicksilver is lost per 1,000 pounds of amalgam retorted [less than 0.1% of the mercury in the amalgam]’.8 The heating stage of the amalgam never removed all the mercury, for fear of compromising the long-term physical integrity of the iron vessels employed, so those percentages had to include the amount of residual mercury left with the silver. Other data indicate a 1 to 2% of weight loss (with respect to silver) during the final casting of silver bars, that was attributed to loss of residual mercury.9 I will apply the highest scenario of the historical data, of an average loss of 5% of volatile mercury, with respect to the weight of silver refined. The chemistry of silver refining with mercury ‘In the patio process of amalgamation a very large amount of quicksilver is converted into calomel and lost’.10 I have argued at length elsewhere on the importance of recognizing the formation of calomel as a major by-product of the chemical reaction between mercury and silver chloride, the last of several chemical stages in the refining process that allowed metallic silver to be extracted from its chemical compounds (halides, sulphides) present in an ore. I have proposed a simple mathematical model that can explain the narrow range of historical values for the ratio of mercury consumed/silver refined in Hispanic America, on the basis of 85% of mercury consumed (not ‘lost’) by conversion to calomel, and 15% lost by physical pathways (liquid mercury to the soil, or to waterways, and direct volatile losses to the air), for ores with little native silver.11 Calomel is an insoluble solid, and would have been buried within millions of tons of fine mineral silt, either in landfills close to the refining units, or washed away in waterways close to refining centres. In the patio process it would have functioned as a chemical sink for anthropogenic mercury emissions from silver refining. The pan process utilized in the latter part of the 19c in the U.S.A. introduced a major change in the refining process. It incorporated a very large ratio of iron to mercury consumed (up to 15), compared to ratios of zero to less than unity in the patio process or its precursor in Peru. In addition, the ore was subject to significant attrition in an iron vessel (the pan) and steam, or indirect heat, was applied to the slurry together with the addition of mercury.12 Iron was known since the 16c to l'argent au Mexique," Annales des Mines, 6th series, 20 (1871): 163, 176-177 ; Thomas Egleston, The Metallurgy of Silver, Gold, and Mercury in the United States (London; New York: J.
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