Sampling is an important and difficult part of the exploration and evaluation of an iron ore deposit. Iron ore bodies of potential economic value commonly contain from a few million to several hundred million tons of ore material. The gathering of small amounts of material from properly distributed points in an ore body so the composite results of their analysis and research testing accurately represent, in a consistent way, the whole body and its variability can be a formidable undertaking. The accuracy and thoroughness of the sampling methods should be under constant review, with deliberate checks and evaluations to prevent gathering inaccurate samples that can lead to erroneous conclusions.
Iron ores can be conveniently divided into two major types: (1) natural iron ores, which include both merchantable ores and crude ore materials that can be beneficiated by simple methods, and (2) concentrating grade ores that require fine grinding before processing to yield an ore grade product. Each general type has its particular sampling methods and problems.
The sampling of a natural iron ore deposit must determine the distribution of various ore grade materials within the ore body in regard to physical and chemical characteristics and, where needed, the suitability and yield amounts of shipping product obtained from the nonmerchantable materials that require beneficiation. The usual and customary chemical requirements determined by sampling include analysis for Fe, SiO2, Al2O3, P,
Mn, CaO, MgO, S, loss on ignition, and moisture. In ores where other elements are commonly present, analysis for special elements may be required. As an example, an analysis for TiO2 is required for titaniferous magnetite ores. Ores with a high iron
content, low scavenger, slag-forming constituents (SiO2, Al2O3, CaO and MgO) and
very low amounts of deleterious elements are preferred. While scavenger, slag-forming constituents are needed in the blast furnace, it is preferable that these be added at the furnace in controlled additional amounts as required. The slag volume desired varies with blast furnace practice and the nature and amounts of elements such as phosphorus and sulfur that need to be removed in the slag. Most ores do not contain the desired ratio of material used for slag formation so low amounts of these impurities permit easy preparation of the furnace burden.
A considerable list of elements is considered deleterious. The more common, unwanted elements in iron ores include phosphorus, sulfur, and titantium, with a less common to uncommon occurrence of vanadium, copper, zinc, chromium, nickel, arsenic, lead, and tin. Phosphorus is common to most iron ore in amounts that may range from very low (about 0.005%) to very high (over 1% P). Because of the importance of phosphorus in steelmaking, iron ores have long been classified on their phosphorus content, as Bessemer— not over 0.045%; low-phos non-Bessemer between 0.045 and 0.180% P; and high-phos non-Bessemer over 0.180% P. There is a price differential that recognizes the phosphorus grades. With a decline in the production of merchantable ores and the major tonnage production of beneficiated ore and pellets from concentrating grade ores having a low phosphorus content, there has been a lessening of concern with the phosphorus content of ores in steelmaking practice in the United States. There are still major concerns with the phosphorus content in natural ores moving in international trade, as there seems to be a very limited market for high grade iron ore that contains more than 0.07% phosphorus. In the Lake Superior region, iron
ore that contains over 2% manganese is classed as manganiferous ore with a premium paid for ore containing in excess of 5% Mn. There often is some small price recognition for ore with a manganese content below 5%. The iron ore industry also, in past years, has shipped a small tonnage of ore classed as siliceous ore. This ore should be considered a special situation ore and requires a special sales arrangement in order to be recognized as ore in the sampling and evaluation of an iron ore deposit. This fact is apparent from the analysis of the 1971 Tilden silica grade ore, which was 38.96% Fe and 40.41% SiO2.
The physical characteristics of natural ore that influence the evaluation of these ores primarily concern the relative quantities of coarse, plus about 6.35 mm (0.25 in.), and fines. This distinction is important as it determines the amount of the shipping product that can be charged directly into a blast furnace after the ore has been crushed and sized in comparison with the ore that will require agglomeration by sintering or pelletizing before charging. The determination of the physical parameters is often poorly shown by samples obtained by drilling, so trenches, test pits, adits, or shafts are dug to obtain an adequate sample for crushing and screening tests.
The primary sampling of a natural ore deposit during exploration is commonly done by drilling. Drilling iron ore deposits that can be mined by open pit is done either in a grid pattern or along section lines, so cross sections can be prepared to show the distribution of grades of ore. Samples are commonly taken on 1.5 or 3 m (5 or 10 ft) intervals with a recognition in sampling of visible changes in the physical appearance of the ore and of geological boundaries. If core drilling is done, the core and/or sample is split, retaining one half for future study and reference. Samples, properly identified as to drill hole and footage, are sent for chemical analysis. Materials that are not of ore grade are sent for laboratory tests to determine suitability for beneficiation and the analysis of concentrated product. The laboratory personnel commonly determine the proper concentration tests to be used. The analyses and/or test results when received are plotted on maps and sections, and are identified as to ore grades and types for use in ore tonnage determinations and for mine planning. At many mines the drill and analysis results are entered into a computerized data system.
Sampling of concentrating grade, hard rock type ores, such as magnetite taconite, jaspilite, OXIBIF, and contact replacement ore, is commonly done by core drilling using diamond bits. In deposits such as Mesabi Range magnetite taconite having large lateral extent, the drilling is often done as a phased program with a relatively wide spacing of drill holes in the initial phase to quickly determine the ore occurrence and the size of the deposit with progressively closer drill hole and section spacing as the exploration drilling passes into development drilling. Often there is no marked demarcation between exploration work and development work in the sampling of an ore body, although there may be a modification of drill procedure. Splitting of the drill core is strongly recommended during the preliminary exploration stage, with one half of the core retained for later study during the exploration, development, and mining stages. After a competent general knowledge of the ore body has been obtained, and when the purpose of continued drilling is to give adequate knowledge of ore grade distribution for detailed mine planning and ore grading for mine production, the core size may be reduced with all ore grade core sent for analysis and testing. Drill core recovered should be logged by a competent geologist, geological boundaries marked, and sample units determined. The core footage included in a single sample for analysis or for laboratory
tests should be varied as needed to show variations in the ore occurrence so maximum use can be made of the sample data in mine and production planning.
Bulk Sampling
Bulk sampling is an essential part of exploration and evaluation of iron ores. Large samples are required to check the accuracy of the drill samples and to obtain the needed information on the crushing and grinding characteristics of the ore as well as for use in detailed and large-scale metallurgical tests. In natural ores, bulk samples give information regarding the nature and quantities of various sized ore products that can be shipped. Herkenoff (1968) in a discussion of the need for large samples of natural iron ores points out the importance of information on how the ore fractures in blasting and breaks into fine sizes during mining, crushing, and in all subsequent handling steps, such as trucking, stockpiling, ship loading, and ship unloading. He states, “On the Mesabi Range where jig and heavy-media ores occur, it is always critically important to determine what percent weights would be in the 64 mm (2.5 in.) plus 6.35 mm (0.25 in.), minus 6.35 mm (0.25 in.) plus 0.21 mm (65 mesh), and minus 0.21 mm (65 mesh) fractions because these splits directly affect capacity required for the respective heavy- media separation cyclone (or jig) and spiral plants which are to work in concert when processing the run-of-mine crude. Because HMS is more efficient than other processes, it is advantageous to keep as much material as possible in that size range. So the problem in sampling ores is to obtain material at least 102 mm (4 in.) in top size and to try to duplicate a typical plant feed structure.”
“On the other hand, if the ore is found to require complete crushing and grinding to, say, minus 0.21 mm (65 mesh) for beneficiation by flotation, spirals, etc., there is not much use in striving to obtain material coarser than diamond drill core size…” As Herkenhoff indicates, even in iron ore districts as well known as the Mesabi Range, bulk samples of some ores may be required but for other ore bodies high quality drill samples may be sufficient to determine the beneficiation characteristics. In areas where the accumulated knowledge regarding the expected natural ore characteristics is not well known, bulk sampling may be required for all ore deposits.
Bulk sampling is recommended for concentrating grade iron ore deposits that require fine grinding to attain iron mineral liberation before concentration. Accurate data are needed concerning the various types and/or grades of ore present and all aspects of the mining and processing of the ore materials to the final agglomerated product. These data should include information on drilling, blasting, fragmentation, crushing, grinding, concentration, and agglomeration for use in mine pit design, mine production schedules and to permit the development of a flow pattern for the correct correlation of product mix and feed rates to each successive process unit. Needed information includes ore type classification, iron content, mineralogy, liberation characteristics, feed size distribution, grindability indices for each ore type, and the optimum feed rate relationships for each step of processing to final product. It is critically important that specific information be available concerning the various types of ore that must be blended to obtain the normal mill feed grade. This commonly requires the selection of bulk samples from each ore type so a blended mill feed can be prepared and tested in the laboratory and pilot plant. Much valuable data will be available as a result of analysis and bench scale testing of drill core samples, but reliable information for mine and plant design can only be adequately known from the mining and pilot scale
processing of the various ore materials. As an example, before the Atlantic City taconite mine and mill were built, two 1360-t (1500-st) bulk samples were obtained from a 222- m (728-ft) long edit. The two samples represented major types of taconite ore that were recognized during test drilling. The bulk samples were shipped from Wyoming to the Pilotac plant located at Mountain Iron, MN, for plant tests. The bulk samples confirmed the drill results and laboratory tests and furnished needed information for final design plans (Cohlmeyer, et al., 1962). Whether bulk samples are sent to an existing plant for testing or a pilot plant is built at the ore deposit depends on the availability of a plant where the bulk sample can be sent and other factors. When the iron ore body at the Jeannine Lake deposit, Quebec, was tested, a pilot plant was erected on site.
Bulk samples can be obtained in a number of ways, such as from the surface area by test pits, trenches, or small open cut mine pits or from underground openings such as adits, shafts, tunnels, and drifts. In a number of iron ore deposits there may be a marked zone of oxidation and leaching at or near the surface, so surface samples may not be representative of the main body of ore. The validity of the bulk sample in regard to the purpose for which it is taken and the expected ore type is of paramount importance. The sample must be representative of the desired material. Because of the importance of the large-scale testing to several aspects of mine and plant design, there is small latitude for compromise or error. There often is a temptation to use easily available surface material even though it is somewhat altered. This approach should be firmly rejected. The ability to examine and to test bulk sample material from each ore type present in an iron deposit may be essential to the development of a final mine plan, plant flowsheet, and mill design. The several ore type bulk samples can be blended to a number of simulated mill feeds for process and flowsheet development planning.