Circular 58 Economic Recovery of Selenium by Flotation From Sandstone Ores of New Mexico by Roshan B. Bhappu NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY E J. Workman, President STATE BUREAU OF MINES AND MINERAL RESOURCES A. J. Thompson, Director Circular 58 ECONOMIC RECOVERY OF SELENIUM BY FLOTATION FROM SANDSTONE ORES OF NEW MEXICO by ROSHAN B. BHAPPU 1961 STATE BUREAU OF MINES & MINERAL RESOURCES NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY CAMPUS STATION SOCORRO, NEW MEXICO ABSTRACT Investigation has shown that the selenium-bearing sandstone ores from the Morrison and Galisteo formations of New Mexico are amenable to simple froth-flotation procedures. The common flotation reagents used are soda ash, sodium silicate, a neutral oil (kerosene), a xanthate, and a frother. Conditioning of the pulp prior to flotation plays an important role in the proc- ess, especially if the sandstone contains excessive amounts of slimy clays and shale s. Under optimum conditions, the selenium flotation is rapid and selective; selenium recovery above 85 percent is possible; and an acceptable grade of concentrates above 4 percent is obtainable. From the economic viewpoint, flotation recovery of selenium ap- pears attractive. The cutoff grades for the profitable recovery of selenium as the major ore constituent are 0.12 and 0. 04 percent for underground and open-pit mining operations respectively. On the other hand, when selenium is recovered as a byproduct of some other mining operation, such as mining for uranium, the cutoff grade is only 0. 01 percent. INTRODUCTION If an element could be said to have "magical properties," it is prob- able that high-purity selenium would come closest to fitting this description. Despite this tribute, selenium remains a metal with an unimpressive back- ground and an unknown and unpredictable future. Although selenium is widely distributed in the earth's crust, the known deposits are either so small or of such low grade that they cannot be processed economically for selenium alone. Many years ago the sole source of selenium was thought to be the flue dusts from metallurgical processes uti- lizing sulfur ores; today, however, recovery from this source is virtually nonexistent. The anode slimes from electrolytic copper refineries are now providing the source of most of the world's selenium, and its production is centered at such refineries in the industrialized nations of the world. The most serious problem confronting the selenium industry is its dependence on the refining of electrolytic copper, for this byproduct source is not sufficiently flexible to permit a normal supply-demand balance. The remedy, it is apparent, lies in the development of alternate sources of selenium on an economic and commercial scale. Efforts, however, to find such sources, even under a price inducement nearly double that in effect today, have not been successful. 1 Most of the growth in selenium output has been within the last dec- ade. In view of the relative immaturity of this segment of the mining industry, it is not surprising that so little is known of the geology of selenium deposits and that only limited progress has been made in developing techniques in mineral beneficiation and metallurgy especially adapted to the production of this metal. Although the chemical and physical properties of selenium have been studied extensively and are fully covered in the literature, numerous problems of application to potential new uses remain unsolved. The apparent lack of knowledge about selenium goes back to a his- torical attitude of aloofness. The metal had few applications at first and was looked upon as a poisonous contaminant. A program of educational promotion, with a direct appeal to those interested in both the exploitation and utilization of selenium, could eradicate much of the cautious indifference that has ham- pered growth in the use of this metal. Since selenium is a strategic metal in short supply, new sources are being sought by both government and private interests. Although considerable attention has been given to this effort during the last few years, hardly any research has been published that is devoted to the recovery of the small amounts of selenium present in low-grade deposits in which the metal occurs as native selenium or as selenides of copper, silver, lead, mercury, bismuth, thallium, and iron. In recent years, some native selenium (in both the red and gray forms) and selenides, especially iron selenide (ferroselite), reportedly have been found in the sandstones of the Morrison formation in New Mexico. •Sun and Weege (1959) encountered a sample containing considerable native sele- nium, in the course of working on a calibration curve for the determination of selenium by an X-ray spectroscopic method. This sample had been collected from the Marquez mine, operated by Calumet and Hecla, Inc. , near Grants, New Mexico. Similar occurrences in the same area have been reported by other writers and mine operators. Elsewhere, Wilson (1960) and the Dial Exploration Co. , of Albuquerque, New Mexico have discovered an ore body containing some selenium in the Hagan coal basin, in Sandoval County, New Mexico. Samples from New Mexico ores submitted to the State Bureau of Mines and Mineral Resources, and assayed in the Bureau's laboratory, have contained from a trace amount to as high as 0.425 percent selenium. Some of these samples have come from uranium mines in the Grants area and repre- sent mined ore and mill feed for uranium reduction plants. In such cases, the selenium usually is reported as an undesirable impurity and is lost in the tailings. At present, no attempt is being made to recover this waste metal. Owing to recent advances in mining and milling technology, many valuable minor metals and mineralscan be recovered economically even though their total content in the ore may appear to be very low and unworthy of ex- ploitation. At a price of $7 per pound for commercial-grade selenium, it would appear justifiable to investigate the possibility of recovering small amounts of 2 this metal contained in such ore, especially if it occurs along with other valu- able metals and can be recovered as a byproduct. In view of the economic importance of selenium and its presence in some ores from New Mexico, the metallurgical staff of the State Bureau of Mines and MineralResources undertook the present investigation on the recovery of selenium from such ores by froth flotation. This report covers the experimental work carried out on the flotation recovery of selenium from sandstone ore samples from the Morrison formation, near Grants, New Mexico (especially Poison Canyon sandstone), and from samples of selenium ores submitted by the Dial Exploration Co. from its development work on the We Hope mining claims, in Sandoval County, New Mexico. Since many of these ores contain. uranium and molybdenum besides selenium, efforts have been made to develop procedures and flowsheets to in- corporate the recovery of these associated metals as well. Realizing, moreover, the need for rapid, reliable methods for the qualitative (including field tests) and quantitative determination of selenium in ores, the Bureau has spent considerable time on developing such methods in its analytical laboratory. These methods are included in the report for the benefit of prospectors and mine -mill operator s. Since knowledge of the economics of a metal is essential for obtain- ing a clear overall picture of the metal under investigation, efforts were made to include in the first part of the report a brief discussion of topics included under the broad classification of mineral economics. Selected references have been given at the end of the report for the use of those interested in obtaining more detailed information on the subject. It is hoped that the present investigation will provide the basis for more detailed study on the recovery of selenium contained in New Mexico ores. ECONOMICS OF SELENIUM General Selenium was discovered in 1817 by John Jacob Berzelius while looking for a new source of the then rare element tellurium in the flue dust from the lead chambers of the sulfuric acid plant in Gripsholm, Sweden. So closely akin were these two elements that Berzelius decided to call the former selenium, from the Greek word Selene (moon), tellurium having been derived from the Latin word tellus (the earth). Although discovered in 1817, selenium remained but a laboratory curiosity for a little over half a century. Finally, in 1873, selenium was brought to public attention by the work of Willoughby Smith, who discovered the 3 particular characteristics of the electrical conductivity of selenium that helped in the development of the photoelectric cell. Thereafter, a multitude of appli- cations developed, so that this metal now plays a definite part in everyday life. Physical and Chemical Properties Selenium might be described as a paradoxical element, being either a metal or a nonmetal, a conductor or a nonconductor, a colorant or a decol- orant, and a hydrogenator or a dehydrogenator, as well as being either amor- phous or crystalline. The properties of selenium are understood more easily when its po- sition in the periodic table of the elements is examined. It stands in the sixth group (VIA), between sulfur and tellurium, both of which it resembles. Like sulfur, selenium occurs in a number of allotropic forms. However, the various forms of selenium are not so well defined as those of sulfur. Selenium boils at 684. 8°C, giving off dark-red vapors; at higher temperatures it sublimes, emitting yellowish vapors. Optically, metallic selenium is pleochroic and anisotropic, having an "0" index of 3. 0 and an "E" index of 4. 04. Electrically, selenium is a photoelectric semiconductor, a property that has contributed substantially to its great value in the modern electronic world. The chemical behavior of selenium is intermediate between that of sulfur and that of tellurium.