Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.) A. Analysis A.1. Definition A.2. Initial conditions and problem areas

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

A. Analysis

A.1. Definition

The section on analysis includes the determination of the chemical and physical properties of soil, rock and ore samples as well as of concentrates, middlings and tailings from beneficiation processing. The analytical procedure used here consists of the following four steps:

1. sampling,
2. chemical or physical analysis of sample material,
3. classification and statistical analysis of data, and
4. interpretation of the results.

The application of analytical procedures in the small-scale mining industry are particularly significant for prospecting, exploration, quality control during mining, beneficiation, marketing and environmental protection.

A.2. Initial conditions and problem areas

Small-scale mining in developing countries suffers from a lack of knowledge concerning crude ore reserves as well as product composition. The situation is worsened by the fact that ihomogeneous mineralization exists, especially in deposits of sub-volcanic genesis as are characteristic of the Andean region. As a result, variations in mineralization occur within small proximities with regard to both geological relationships and mineralogical and geo-chemical compositions. A good example of this can be seen in Bolivian tin deposits, where the tin source can be Cassiterite (for chemical composition see Table), Cylindrite, Teallite or Frankeite (three Sulfostannates) or Stannite. Knowledge of the entire geological relationship is critical for planning. not only the mining procedure but especially the beneficiation processing.

The composition of concentrates is frequently not known by the small-scale mining operators, which can be disadvantageous for selling the products. Impurities in the concentrates result in lower prices for the product following high penalty deductions assessed by the buyers or the beneficiation plant, and further impairing the marketing of profitable by-products. The Cascabel Mine in Bolivia (Dept. La Paz) serves as an example, which, despite higher lead, silver and tin contents in its concentrates, is only able to market its products with great difficulty, suffering large penalty assessments (price discounts) due to abnormally high levels of mercury contamination. These mineralization problems occur not only in the primary vein ore deposits, but are also present in placer deposits; deficient product knowledge is the reason why valuable platinum contents in alluvial gold deposits (e.g. in Colombia) are not being mined and consequently not being separately marketed.

Another characteristic problem of small-scale mining in developing countries is the questionable credibility of the analyses, which, as a rule, are performed by the buyer himself. Control checks have shown that results of the analyses are being manipulated to the advantage of the buyer and to the disadvantage of the small-scale mine operators. Primarily, the silver contents were given as too low, and the residual moisture levels as too high, which is difficult to prove in the absence of control measures.

The resulting conclusion is that the small-scale mining industry needs to implement its own control program. In addition to quality control during mining (grade control) and beneficiation and marketing planning, analytical procedures suitable for small-scale mining are also important for prospecting and exploration activities.

The use of centralized analytical methods becomes inconvenient or even impossible for small-scale mining due to the location-dependency of stationary analytical techniques, and the lack of infrastructure in the remotely-located, isolated small-scale mining operations.

The need exists within the small-scale mining industry for a simple, portable analyitical procedure. The main criteria should include low cost and quick performance with limited equipment and time requirements while avoiding unnecessary measuring precision. The extent to which the analytical results are representative and are reproducable is determined more through the quality and preciseness of the sampling rather than the application of the most optimal method of analysis. An analysis which is precise to several places behind the decimal point is worthless when an improper sampling procedure results in inaccurate figures in front of the decimal point.

The lack of simple analysing procedures for smallscale mining is not limited just to developing countries; this is an area calling for research-anddevelopment efforts.

A.2.1 SAMPLING

The sampling procedure is of primary importance for the technical planning of mining and beneficiation operations. However, a very precise and exact analysis is of no value if the sample being analyzed is not representative. A sample is representative of its original geologic environment only when the same chemical, mineralogical and physical relationships characteristic of the specific geologic area are exhibited in the sample. These relationships are defined by the mineral or element distribution, humidity, granulation and grain-size distribution, permeability, etc. When a waste dump, mineral deposit or beneficiation product is analyzed, it is not possible to examine the entire dump or deposit, or the total product quantity, but only portions of the whole. Proper conclusions can only be made from these sampled portions when they are representative of the whole.

In the testing of a pile of crude ore, for example, it is not sufficient to take only one chunk of ore from the pile, which may be representative of only the country-rock or the mineralization itself. An analysis of this sample alone would result in erroneous conclusions concerning the metal-content of the deposit in general. Several sampling techniques which consistently produce representative results are discussed below:

Bulk sampling is employed for the sampling of loose fine-to-coarse-grained materials, such as in the analysis of tailings, waste dumps, products, and crude ores. Numerous smaller samples are taken from a number of various arbitrary locations throughout the material pile without preference to any particularly richer or poorer regions. The sampling procedure should not only include numerous different sample locations but should also ensure that the grain-size of the samples also vary, in that samples of the finer fractions and the fines are also collected along with the large pieces of ore. In so doing, the sample volume or quantity should always be at least ten times greater than that of the largest individual sample in order to assure that the effects of classification, whether from deposition in the pile or from selectivity during blasting, are statistically compensated.

Channel sampling is a method of sampling exposed in-situ ore-bodies. In this procedure, sample material is obtained from a groove, constant in width and depth, cut into the rock over a specific length, for example from the hanging wall to the foot wall across the width of the face during drifting; for example, a slit 10-cm in height and 5-cm in depth is cut out along the entire stope width with the sample material being collected on a tarp spread on the roadway floor below.

In-situ ore bodies can also be tested by grab sampling. Over the affected sample area, for example the area of the face, numerous equally-sized samples are randomly taken by hammering, digging or prying off loosened chunks without any locational preference for richer or poorer zones of mineralization. This sampling method is considerably easier to carry out than channel sampling, especially in hard ore bodies.

Cuttings-sampling recovers sample material from the washed drill dust or drill cuttings which are produced during drill-and-blast drifting and mining. As a result, the collected sample material originates not just from exposed surfaces, but rather represents a three-dimensional sampling area when an entire drilling-grid is sampled. An additional advantage of this method is that the sample material already exists in a finely-comminuted form.

In order to avoid systematic causes of errors, sampling should always be conducted by only one and the same person.

Fig.: Manual quartering of sample material by mixing, coning, mixing, flattening, quartering, and discarding of two opposite-lying quarters. Source: Schroll

For further treatment larger-sized samples are crushed and subsequently quartered. This is performed by heaping the crushed sample material into a cone, thoroughly mixing it several times via shovelling, and again heaping it into a cone by pouring. The cone is then pressed or stomped flat, and the resulting flat cone base is divided into four equal segments. Two of the quarters, located opposite one another, are analogously processed further, while the remaining two quarters are discarded. This procedure is repeated as often as required until the desired sample quantity has been reached. The conical pouring of the sample material assures the homogeneity of the sample.

The homogeneity of ores, alluvial deposits, tailings or other mined waste, and the degree to which the samples are representative, is particularly important where the element or mineral-content is low. This occurs in discrete aggregates, where the valuable mineral exists as separate grains independent of the mineralized matrix. An important example is gold. Gold analysis demands values of a magnitude of less than 1 g/t. Gold particles can appear as gold glitters or nuggets with individual weights of up to more than 1 9. If, for example, one ton of crude ore which has only a single 1-9 gold nugget is analyzed by using 100kg of sample material which happens to contain this nugget, the results will indicate 10 9 of gold/l, which of course is too high. Statistically, in 9 out of 10 cases the nugget would not be contained in a 100-kg crudeore sample, so that the gold content is then assessed at 0 g/t. This effect is known as the nugget effect and requires, in such cases, multiple samples of large quantities. The more nonhomogeneous the sample material, the higher the number of samples required in order to obtain a statistical median value which approaches the true value for the material as a whole.

A.2.2 MINERALOGICAL EXAMINATION

Under certain conditions, an optical measuring or visual estimation of the ore content underground can serve as a substitute for an analysis. The prerequisites for this are:

- a relatively high proportion of ore minerals in the total material, since only then is the measured or estimated value of sufficient accuracy, and

- high visibility of the ore mineral under the conditions of examination. This requires clean working faces or sampling surfaces, perhaps involving the use of artificial methods to improve visibility, for example with ultra-violet lamps to detect mineral luminescence.

The visual evaluation is significant in lead-zinc mining in hydrothermal deposits with classic vein patterns. As a rule, testing is combined in this case with geological mapping of the hanging or footwalls. In so doing, the total width of the wall and the width of the ore veins are measured onto one profile. The profile must run vertically along the strike and dip of the vein; otherwise the values appear unrealistically high. If the profile is divided into several fragments, the sum of the individual vein thicknesses can be determined (for example, 25 cm galena, 15 cm sphalerite over a thickness of 175 cm). From this information, the volumetric proportion of the various ore minerals can be calculated. When the densities of each ore mineral and host rock are included, the total weight proportion of the ore minerals can then be established. With this information, and the additional knowledge of the metal content in each ore mineral, the metal-content distribution (% by weight) of the sampled profile can be determined. The incorporation of correction factors to account for mineral intergrowths, etc., can increase the accuracy of this method. This form of sampling or testing has proven itself even in highly mechanized operations in industrialized countries where it competes against modern procedures, such as portable X-ray fluorescent analysis.

Another mining sector which employs optical evaluation is scheelite mining, where the mine face is irradiated with an ultra-violet lamp which induces fluorescence of the scheelite.

As is true for sampling procedures, a high degree of accuracy in the optical test results can only be attained through disciplined work procedures and a great deal of experience.

A.2.3 ADVANTAGES OF MINIMIZING ACCURACY

All analysing procedures and evaluation methods exhibit a linear relationship between degree of accuracy and the cost of analysis, or, in other words, the more accurate the analysis, the more complex the equipment and the higher the costs. The lower the detection limit of the analytical method, i.e. the smaller the analyzed value is, the more expensive the analysis will be. Looking at this fact, it is absolutely necessary from an economical standpoint that the small-scale mining industry employs the cheapest method of analysing available within the desired accuracy and metal-content limits.

A.2.4 DETERMINATION OF ELEMENT DISTRIBUTION IN RAW ORE AND CONCENTRATES

Lack of knowledge about the contents of the different elements in raw ore, mine waste and concentrates is frequently the cause for the inefficient or uneconomic performance of small-scale mining operations. As a rule, only the contents of the desired metals in the ore and concentrate are examined. Consequently, the causes for undesired metal contents in the products, and subsequent penalty assessments, are not known by the small-scale miner. Additionally the accounting statements from the ore buyer do not explicitly indicate the reasons for penalty deductions. Commercially marketable byproducts also remain unidentified.

A number of various contaminating metals and elements which may be present in the mine products lead to penalties, assessed by the smelters in the form of price reduction, when the content of these metals exceed a maximum tolerance level. These elements, their maximum tolerance limits, and the penalty amounts are established by the smelting standards, varying according to smelting process, market situation and buyer. Consequently, a definite statement concerning these elements and tolerance limits cannot be made; however, as a general reference, the following table lists some critical elements which are deleterious to non-ferrous metal ore concentrates:

Penalties can lead to a considerable decrease in profit for small-scale mining operations. Therefore, knowledge of the element and trace-element distribution should be obtained, as much as possible, before initiating any mining activities or planning the beneficiation plant in order to establish a marketing strategy.

Similar to the deleterious metals, element contents which would be worth recovering and marketing in the form of by-products are also often overlooked; for example, zircon sand from alluvial deposits, gold-containing pyrite and arsenopyrite from complex sulphide veins. Here, as well, a knowledge of the element distribution prior to the start of any mining activities is crucial in order to formulate an optimal marketing strategy.

The practice of performing complete analyses on a concentrate sample and on a mixed raw-ore sample, conducted by a competent laboratory for the purpose of determining the contents of all relevant metals, trace-elements and cations, should become standard procedure for the small-scale mining industry. Governmental support of these needs, for example by providing inexpensive analyses, would significantly contribute to promoting the small-scale mining industry.

The performing of mineral analyses also serves an important function from an environmental-protection viewpoint by identifying environmentally-damaging components such as sulfur in coal, residual mercury in gold tailings, cyanide and arsenic contents in mining wastes, etc.

A.2.5 DETERMINATION OF THE VALUABLE-MINERAL SOURCE

In addition to a purely geochemical examination of the raw materials, a mineralogical knowledge, especially of the valuable-mineral sources, is of major priority in small-scale mining. Since beneficiation processing in small-scale mining usually leaves the material components of the minerals unchanged, this identification of the valuable-mineral source is particularly important for planning and marketing. This can be accomplished through microscopic examination of polished sections, which enables experienced microscope analysts to quickly and easily semi-quantitatively recognize segregations, trace element minerals, etc.

The question, for example, of whether silver appears as a silver mineral or as a lattice element of lead or zinc minerals can strongly influence the beneficiation, marketing and profitability of a mining operation.

Equally important is the mineralogical composition of the raw material in primary gold deposits, in which the gold can occur as free gold or bound to pyrite or arsenopyrite as "refractory ore".

Whatever the situation, it is essential that the major ore minerals can be marketed. Some ore deposits produce main valuable minerals which are sellable only with great difficulty, if at all; such as the complex ore deposits with spienles sulfades (antimony and arsenic) as the metal source.

The above example shows that the results of mineralogical studies play an important role in determining whether or not an ore deposit can be mined profitably using the simple mining methods characteristic of small-scale mining.

A.2.6 OTHER RAW MATERIAL STUDIES

In addition to chemical and mineralogical composition, other characteristic data are also important, depending upon the material, for the analysis of raw mineral reserves. Examples are:

- ash content' thermal value, sulfur content, caking capacity, etc. for fossil fuels (coal, peat);
- compressive strength of a cube, cleavability and permeability for construction materials;
- swelling characteristic for certain clays (vermiculite);
- weaving characteristic for asbestos;
- coloration for pigment raw materials (barite, kaolin);
- grain sizes for many raw materials (large grain size for graphite and mica, fine grain size for kaolin;
- hardness for grinding material (corundum, garnet).

The following table presents a list of essential ore minerals including primary physical characteristics and types of veins and host rocks.

Table: Characteristics of Ore Minerals including Vein Types, Gangue or Matrix, Asociated Minerals and Host Rocks:

¹⁾Tenacity characterizes brittleness or breaking characteristics of the mineral

Explanation of tenacity/Remarks:

Table: Characteristics of Ore Minerals including Vein Types, Gangue or Matrix, Associated Minerals and Host Rocks:

HOST ROCK

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.) Technical Chapter 1: Analysis 1.1 Blow pipe assaying 1.2 Pycnometer 1.3 Manual magnetic separator by Dr A. Wilke 1.4 Quick-test-strips merckoquant 1.5 Rifflebox

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

Technical Chapter 1: Analysis

1.1 Blow pipe assaying

General Ore Mining
Analysis

CONDITIONS OF APPLICATION:

Bibliography, Source: Plattner, Wehrle, Kest, Kolbeck, Frick-Dausch

OPERATING PRINCIPLE:

Blowpipe analysis is a multiple-step procedure for qualitatively or semi-quantitatively determining the individual elements contained within a small quantity of sample. The process involves dry thermal procedures, sometimes in combination with wet testing methods. The sample is heated in an open or half-closed pipe, melted to a bead with borax (Na₂B₄O₇ × 10 H₂O) or microcosmic salt (NaNH₄HPO₄ × 4 H₂O) under oxidizing and reducing conditions, burned directly in a flame to determine flame color, or heated with coal under oxidizing and reducing conditions. A small flame torch serves as the energy source, which is intensified by blowing into it through a tapered tube, the blowpipe. The discoloration of smelt, sublimate, corona or flame in each particular assay or experiment, together with any distinct odors and/or reactions which may appear, provide Information on the chemical composition of the sample.

REMARKS:

Of importance is a waterbag, which is an extension of the pipe for collecting condensed water, to prevent It being expelled during blowing.

Lamps with cotton wick and rape-oil, paraffin, tree oil and mixtures of alcohol (spirits) with gasoline (benzine) or oil of turpentine are suitable.

Polished pieces of charcoal of approx. 30 × 30 × 40 mm are employed as a base. If charcoal is not available, the foundation or base can be prepared using coal dust and starch paste.

In 1670, Erasmus Bartholin conducted the first scientific research on the use of the blowpipe. A homogeneous air current can be achieved during blowing by connecting the blowpipe to an available compressed air line. If this is not possible, a pumped-up tire, for example from a wheel barrow or automobile, can be used as a compressed air tank.

The advantages of blowpipe analysis are the simplicity of both the determination and the required equipment. Samples can be analyzed very quickly and comparatively accurately, which is particularly important in an operating mine. Special sample preparation, such as extensive crushing, etc. are not necessary.

The analysis methods, which appear complex in their description, can be greatly simplified when standard assays are conducted on known metals.

SUITABILITY FOR SMALL-SCALE MINING:

Semi-quantitative and qualitative analytical technique requiring simple and low-cost equipment; demands, however, a highly-experienced operator. In metal mining, the blowpipe analysis is suitable for grade control and for assaying during prospecting and exploration.

Fig.: View of blowpipe with changeable precious-metal nozzle (above left). Source: Frick-Dausch

Table: The primary chemical reactions of blowpipe analysis. Source: Frick-Dausch

A. Heating of the substance in a half-closed pipe

1. A distillate develops: water.

2.
A pure white sublimate develops:
Salts of ammonia: simultaneous occurrence of NH3 odor.
Mercury chloride: melts, evaporates and condenses in a needle-like form.
Mercury all-chloride: sublimates without melting; hot sublimate is yellow, cold is white.
Arsenic tri-oxide: fine-crystalline white sublimate.
Antimony tri-oxide: melts to a yellow liquid and sublimates at higher temperatures.

3.
A coloured sublimate develops:
Arsenic and high-grade arsenic ores: mirror of arsenic, garlic odor.
Antimony: mirror of antimony.
Arsenic sulphides, arsenopyrite: hot sublimate is dark, cold varies from yellow to red.
Antimony sulphides: at higher temperatures; hot sublimate is black, cold is red-brown.
Sulphur: melts easily, condenses as a yellow sublimate.
Mercury sulphide: black sublimate which when rubbed with a match changes only very slowly into a red modification.
Mercury grey sublimate from metallic mercury.

4.
A gas develops:
Oxygen: from chlorates and peroxides.
Carbon all-oxide: from carbonates and bi-carbonates. CO₂-gas put into lime water produces a white precipitate, which then dissipates when acidified with HCI, contrary to CaSO₄.
Ammonia: from salts of ammonia
Hydrogen sulphide: from water-bearing sulphides.

B. Heating in a half-closed pipe with potassium-bisulphate
Nitrate and nitrite form NO₂.
Bromides emit red-brown bromine vapors.
Iodides release violet-colored iodine vapors.
Chlorides form hydrogen chloride.

C. Heating in a pipe open on both sides (calcination test)
Free sulphur and metal sulphides form SO₂.
Tellurium emits white smoke, which partially condenses.
Selenium sublimates black, on the upper edge often reddish, Selenium odor.
Arsenic substances emit white, volatile, cristalline arsenic tri-oxide.

D. Bead test

a) Borax bead: borax Na₂B₄O₇ 10 H₂O

Borax bead

¹⁾ black when solution is too strong
²⁾ from finely-divided metallic nickel
³⁾ easy to produce with tin, highly characteristic
⁴⁾ when saturation is too strong and by whirling, greenish-black and muddy
⁵⁾ only in a very pure oxidizing flame completely free of reducing components

b) Phosphor salt test; phosphor salt (= microcosmic salt) NaNH₄HPO₄ × 4H₂O

Phosphor salt bead

¹⁾ with the help of tin
²⁾ only in a very pure oxidizing flame completely free of reducing components
³⁾ calcined with a trace of ferro sulphate, blood red; very sensitive!
⁴⁾ with a trace of ferro sulphate, blood red; also very sensitive (e.g. wolframite!). With SnCl₂ and without Fe-additive, dark green.

E. Flame coloration

Differentiation between Li and Sr:
LiCl is more volatile than SrCI₂. LiCI develops at once and does not last.
Green flame: barium: yellowish-red lasting coloration.

Boric acid: very sensitive when sample is mixed with CaF₂ and H₂SO₄; evaporates as BF₃
Copper nitrate: pure green (copper chloride: blue).
Phosphoric acid: light bluish-green, especially after moistening the sample with H₂SO₄.

Selenium: selenium odor.

Separation of Na and K: viewed through a cobalt glass, the light from Na fades, and the potassium flame appears purple-violet.

The sample is soaked with a cobalt solution (1:10) and heated on a magnesia stick in the oxidizing flame.
Blue coloration: silicic acid and silicates: light blue.

alumina: dark blue (Thenard's-blue).

Tin oxide: blue-green.

G. Soda-saltpeter-melt

Light yellow melt: chrome.
Light reddish-yellow melt: uranium.

Vanadium produces a very pale-yellowish-colored melt; colorless when cold.
Ferro oxide does not go into solution.

The soda-salpeter-melt is rubbed with water in a flask filtered, and the filtrate is acidified with H₂SO₄. A piece of metallic zinc is soaked in the solution for a longer period of time.

H. Testing on Coal

1. Sublimates

Yellow sublimate: hot - dark yellow lead, bismuth (often bead).
White sublimate: hot - yellow zinc; when moistened with cobalt nitrate and strongly annealed: green.Blue sublimate: cadmium.
White sublimate, adhering to sample: tin (involatile).
White sublimate: hot - yellow. molybdenum. When a reducing flame is briefly held over a molybdenum sublimate, perpendicular to longitudinal direction of coal, a dark blue band of Mo₃O₂ develops in the middle of the white sublimate. Highly characteristic.
Brownish sublimate: silver (silver bead).
Grey sublimate and odorous fumes: selenium.
White sublimate and arsenic odor: arsenic.
White sublimate, slightly volatile and thick fumes: antimony.

2. Reduction with soda

White bead: silver, lead, bismuth, antimony, tin.

Colored bead: copper, gold.

Grey metallic spangle: iron, nickel, cobalt (magnetic) and platinum metals (non-magnetic).
Important special samples: sulphur (Hepar test). The substance is melted with soda under reducing conditions and placed on a thin sheet of silver. After moistening, a brownish-black coating develops on the silver sheet in the presence of sulphur.

Flourine: heating of the sample substance in a lead crucible with SiO₂ and H₂SO₄ (Browning test, see below).

Tellurium: when tellurium ores are slightly warmed with concentrated H₂SO₄, the sulphuric acid turns red.

Uranium: the sample substance is first melted with soda, then with saltpeter; the melt is mixed with water to a pulp, which is then placed on a filter; acetic acid and solution of ferro potassium cyanide are added, which produces a brownish-red spot in the presence of uranium.

Silicic acid and flourine: Browning Test: the sample is mixed with calcium chloride and sulphuric acid to a pulp in a thimble-shaped lead crucible which is then covered by a lead lid with a hole in the middle. A wet piece of black filter paper (available by Schleicher and Schull) is placed over the hole, and a second, standard filter paper (wet and folded) is placed on top to keep the black paper wet. Following warming of the crucible in a water bath for about 10 minutes, silicon flouride escapes which hydrolytically dissociates during deposition of white silicic acid when it comes in contact with the moisture of the black filter paper. Upon completion of testing, the presence of silicic acid in the sample is revealed by a white spot on the black filter paper where it covers the hole in the lid. Very characteristic and highly sensitive. The procedure can also be used to test for flourine by mixing the sample substance with silicic acid and sulphuric acid. Boric acid can be disruptive since it similarly volatilizes.

1.2 Pycnometer

General Ore Mining
Analysis

CONDITIONS OF APPLICATION:

OPERATING PRINCIPLE:

The pycnometer determines the density or specific weight of insoluble mineral fragments or powder. It is a carefully calibrated, very precise volumetric measuring apparatus. Three weighings to determine the density are taken as follows:

- with the dry, empty container (tare)
- with the container filled with dry mineral sample and
- with the container including sample filled with water in the absence of bubbles

The density is determined according to the following formula:

SPECIAL APPLICATIONS:

Determination of mineral density (mineral-identification method) and determination of beneficiation product densities.

REMARKS:

The accuracy of determination is particularly high when the differential quantities to be measured are not too small, such as when the pycnometer is half-full with sample material.

This measuring method, which measures density to an accuracy of ±0.1 g/m³ and therefore meets mining requirements, necessitates only minor equipment expenditures. A mechanical weighing scale accurate to ±0.1 g has been proven sufficient and enables this method to be applied in the field.

SUITABILITY FOR SMALL-SCALE MINING:

Pycnometer assaying is a simple and accurate method for determining density and is therefore well-suited for the evaluation of product quality and for mineral determination.

1.3 Manual magnetic separator by Dr A. Wilke

Metal Mining
General Analysis

CONDITIONS OF APPLICATION:

Bibliography, Source: Manufacturer information

OPERATING PRINCIPLE:

The pocket magnetic separator by A. Wilke is made of a strong, cylindrical permanent magnet with a cylindrical pole gap which can be moved up and down in a brass container by means of a pull rod. The container is covered by a transparent graduated plastic tube with increments in millimeters, and by means of screwing can be adjusted within this plastic tube to change the height of the magnetic surface above the sample material being separated. In this way the separation capability of the magnetic separator is varied, being greatest at the greatest height and smallest at the smallest height (thus biotite is Just barely separable).

To perform the analysis, the sample is thinly spread over a smooth, non-magnetic plate (glass, wood) and magnetically separated over the entire surface by placing the magnetic separator on the plate. The magnetic particles are attracted by and adhere to the magnet. The magnetic separator is then placed on another plate, and the enclosed magnet is lifted by the pull rod, resulting in the release of the magnetic particles. Starting with the minerals exhibiting the highest magnetic susceptibility, the magnetic separator can selectively separate a number of different magnetic fractions. Weighing the entire sample and the products can provide quantitative results when the sample material is completely analyzed.

AREAS OF APPLICATION:

Apparatus for selective separation of magnetic components of mineral sands, ground minerals and ores (beneficiation products).
Generation of monomineralic specimen for microscopic and chemical analysis.
Quantitative determination of composition of mineral mixtures.

Highly magnetic substances which can be separated:
Magnetite, maghemite, franklinite, pyrrhotine;
Moderately or weakly magnetic substances:
arsenopyrite, chromite, hematite, ilmenite, limonite, manganite, wolframite, rhodochrosite, garnet, amphiboles and pyroxenes.

DESIGN INSTRUCTIONS:

In addition to its analytical application, locally-manufactured pocket magnetic separators can be used in beneficiation for the purpose of recleaning concentrates, for example to separate out magnetic heavy-mineral particles from precious metal concentrates. Loud-speaker magnets (strong permanent magnets), placed in a plastic container and calibrated with distance washers made of cardboard, paper, wood, plastic or similar material, are suitable for this purpose.

SUITABILITY FOR SMALL-SCALE MINING:

Pocket magnetic separators are ideally suited for quick quantitative determination of magnetic mineral contents in raw ores and beneficiation products.

The simplest magnetic separators are well suited, depending upon the situation, for recleaning concentrates by removing magnetic components.

1.4 Quick-test-strips merckoquant

General Ore Mining
Analysis

CONDITIONS OF APPLICATION:

Bibliography, Source: Manufacturer information

OPERATING PRINCIPLE:

Merckoquant quick-test-strips consist of plastic strips which have a sealed test-zone on one end impregnated with reagents, buffers and other compounds. These provide a quick preliminary identification in the range 2 1 mg/l (ppm). Application involves dipping the reaction-zone end into the aqueous sample solution for 1 to 2 seconds, and then comparing it to a color scale (included with the strips).

AREAS OF APPLICATION:

For quick determination of metal-contents in water (environmental impact assessments), raw-ore solutions, beneficiation products, etc. Control of reagents during simple cyanide leaching of gold.

REMARKS:

The following can be determined:

Solutions which are too highly concentrated can be diluted with distilled water until the measureable concentration range is reached.

SUITABILITY FOR SMALL-SCALE MINING:

Highly suitable for environmental impact assessment (water) in that it provides fast and location-independent analysis and is very simple to use; unsuitable for raw-material analysis due to substantial difficulties in sample preparation.

1.5 Rifflebox

Metal Mining General
Analysing

CONDITIONS OF APPLICATION:

Bibliography, Source: Schroll, manufacturer information

OPERATING PRINCIPLE:

The sample-divider directs the sample material over riffles which alternately distribute the sample to one side or the other, thereby guiding it into two separate compartments; the sample material of one container is then retained for testing, that of the other is discarded.

AREAS OF APPLICATION:

Sample preparation through a stepwise halving of sample material from individual or composite samples of raw-ores from ore-vein or alluvial deposits, or of beneficiation products.

REMARKS:

Riffleboxes are very simple dividers which are known for their success in producing highly representative sub-samples.

SUITABILITY FOR SMALL-SCALE MINING:

Riffleboxes are highly suitable for small-scale mining application especially since they can be locally manufactured and because they offer an easily-operable method for improving sample preparation, which increases the analytical accuracy associated with small-scale mining.

Fig: Physical Principle of the Rifflebox. Source: Lauer

Fig.: Rifflebox for example preparation. Source: Armstrong

Fig.: Rifflebox with (1) sample divider, (2) feed tray and (3) receiving tray, from Schubert.

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.) B. Underground mining B.1. Definition B.2. Existing situation and problem areas B.3. Organizational measures B.4 Environmental and health aspects

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

B. Underground mining

B.1. Definition

Underground mining includes all aspects of raw mineral extraction by man assisted by the use of technical aids. In addition to the activities involving mining and haulage, it also includes exploration and provision of the necessary infrastructure as well as all measures for the miner's safety. Included among these are:

The frequently-used small-scale mining method in developing countries, characterized as a shallow digging or excavating (cateo), can be regarded as a transitory form of open-pit mining.

Deposit exploration in small-scale mining in Latin America and other developing countries is performed using underground mining development methods (tunnelling), due to the comparatively high cost of core drilling.

B.2. Existing situation and problem areas

Small-scale mining in the developing countries extracts raw ores from extremely varying deposit types. The following ore-deposit types can be considered suitable for small-scale mining methods:

- alluvial or placer deposits,

- oxidation zones,

- magmatic hydrothermal vein ore deposits with defined veins (which, however, frequently contain complex mineralizations as a result of extreme telescoping),

- pegmatite veins,

- low-sulphide gold-quartz-veins,

- veins with high-grade gold-containing sulphides (which can be enriched into a sulphide concentrate by flotation),

- pneumatolytic and metasomatic deposits.

In can generally be stated that in small-scale mining the individual mineralized parts or excavations are of small dimension. The mine buildings are sometimes so small that the use of technical improve meets in the form of standardized, mechaninized mining equipment is impossible. An example of this is the large tungsten deposit Kami in Bolivia, where numerous small veinlets and associated difficulties in mechanizing the operation have led to a predominantly manual extraction of the ore.

Besides the purely technical problems accompanying nonmechanized mining, small-scale mines, particularly the cooperative mining operations (span: Cooperativas Mineras), also encounter a multitude of organizational difficulties, namely;

- inappropriate extracting/ mining methods,
- low degree of division of labor,
- lack of coordinated efforts.

The organizational problems are especially apparent in the structure devided in "cuadrillas", typically four-man mining teams in the cooperatives. At the Cooperativa Minera Progreso in Kami, it could be observed that every cuadrilla in the cooperative was given the right to mine the portion of the deposit extending above or below of 15 meters drift. This results in unplanned, irregular and totally varying mining activities which have limited advance rates due to the lack of ventilation, supports, etc.

These technical and organizational deficits are responsible for the especially low specific performance rates characteristic of small-scale mining in developing countries. As a result, many small mines, although rich in ore, must be classified as economically marginal operations.

These economical inefficiencies lead to further problems specific to small-scale mining:

- Insufficient safety measures. Deficient cash availability has caused mine operators to save on expenses, particularly in the areas of ventilation and support, as well as in supplying safety equipment for miners.

- The economic problems of miners' families force the women and children to work in the mines (see photo). While women, due to tradition and religious beliefs, are only allowed to work above ground, i.e. in the beneficiation processing activities, children as young as 10 - 12 years old are already working underground mining the ore. These children frequently work in extremely small holes which are inaccessible to adult miners.

- The high exploitation costs and related costs incurred in poorly-organized, manual small-scale underground mining force the operators to more selectively mine only the high-grade zones in the vein. This screening-out method of mining (= highgrading) which follows only the rich portions of ore veins is a form of destructive exploitation which can lead to substantial macro-economic damages. In areas where poorer deposits become inaccessible or are abandoned due to destructive exploitation of rich ores, a later mining becomes technically difficult or if not impossible. Even under changing economic conditions (as for example higher world market prices for mineral resources), the deposits that have been destroyed through exploitation mining practices still may not be mineable. This situation applies only to unorganized small-scale mining of large deposits; the unique macro-economic value of small-scale mining lies in its ability to adapt to small deposits which could not be mined by any other organinized form of mining.

A further goal of the handbook "Tools for Mining" is to help solve the problems associated with small-scale underground mining. Recommended work-organization improvements are presented below which can benefit small-scale mining operators without requiring additional investment costs:

- lowering the cut-off grade,
- extending the life-span of the deposits,
- improving work conditions (increased safety, elimination of child labor practices),
- improving mine productivity,
- increasing incomes, and
- stimulating the economy through job security.

B.3. Organizational measures

Small-scale mining frequently employs inefficient loading and transport methods. Loose material is rehandled a number of times through reloading, redumping, relocating. Particularly primitive and unproductive are the mining methods practiced in the small-scale mining cooperatives where the miners are organized into small mining teams (cuadrilla) which work from the haulage level downward. In these mines, hoists are the standard form of ore transport (see photo, Technical Outline 9.1), sometimes being found every 15 - 20 meters. This is a situation in which changes in mining procedures in order to increase production, listed below as a three-fold concept, are not only logical but also necessary:

1. A reorganization which incorporates division of work duties (job specialization) should be established.

2. A mining procedure should be chosen which employs gravity to increase the efficiency of loading and transport activities.

3. Shaft haulage should be centralized where possible; this involves planning and driving of haulage drifts for the transport of raw ore to the haulage shaft or blind shaft.

A planned loading procedure through the use of loading platforms, raise chutes and bunkers can significantly increase mine productivity and reduce loading and transportation costs. Furthermore, a centralization of shaft haulage can simplify a mechanization of the hoisting equipment.

B.3.2 BACKFILL WITH HAND-PICKED ROCKS

Another method for reducing haulage costs is the hand-sorting of waste rocks underground for further use as packing or backfill material in the excavations. In deposits where portions of unmineralized hanging or foot wall also need to be mined, hand sorting can significantly decrease the volume of raw-ore to be transported. Although hand-sorting in small-scale underground mines in developing countries is a frequent occurrence (see photo, bottom), the sorted-out waste material is not always used in the excavations for backfill, but rather hauled separately out of the mine and deposited on the surface. A change in this practice could contribute significantly to lowering transport costs, improving safety at the mining face and, especially in the small manually-operated mines, alleviating drift and shaft haulage activities. Aside from these, a systematic back-filling can also contribute towards improving the ventilation in the mine, for example by filling in old man (abandoned) workings and thereby preventing short circuits in the ventilation flow.

B.3.3 DIVISION OF LABOR IN UNDERGROUND MINING

One basic organizational deficiency in small-scale mining is the frequent lack of work specialization. Especially the cooperatives' "cuadrilla" work procedures repeatedly lead to difficulties due to the parallellism or duplication of work performed by these small mining teams. As a result, a continuous working operation is not possible, and due to economic and organizational necessities, the work activities are limited to a few critical areas. Mining, haulage and beneficiation are performed sequentially, and other essential tasks are negelected for the present time; as a result, work activities such as development of deposits (even where this is possible internally inside the ore-body structure), timbering and maintenance of galleries (see photo) are not performed.

This has the following effects:

- lack of safety in the mining operations

- a steadily worsening mineral-reserves situation which further lowers the ability of the operations to receive credits and further impedes potential investment through exploration funds (e.g. from the Fondo Nacional de Exploracion Minera, FONEM, Bolivia).

These problems can be countered by a systematic division of labor in the mining operations. This normally requires, however, that the existing distrust first be eliminated. This lack of confidence has been the primary cause of failure so far for numerous projects which attempted to promote a cooperative work system, despite the fact that a concept incorporating rotating job responsibilities not only contains components for specialized training, but also most ideally encompasses the cooperative idea.

Furthermore, a system of work specialization could also facilitate essential planning and coordination activities such as ventilation, supply of energy, mine planning and mine safety.

The introduction of work specialization should include the negociation of personnel salaries based on performance or productivity.

B.3.4 COST REDUCTION IN DRILLING, BLASTING, MAULING, CRUSHING

Depending upon deposit geology and existing mechanization and equipment both on the surface and underground, possibilities exist for reducing costs for drilling, blasting, hauling and crushing. The following relationships regulate potential savings within these cost categories:

fewer drill holes per drilling round (lower drilling costs) produces coarser material (higher crushing costs), stronger explosives (higher blasting costs) results in fewer and/or smaller drill holes (lower drilling costs), electrical milk-second detonator (higher detonator costs) yields finer-grained material (lower loading and crushing costs).

In mines with defined veins without impregnation zones, coarser mined materials can make the hand-sorting or backfilling work easier. Optimization possibilities are dependent on the specific mineralization conditions and the technical capabilities of the mine operation.

B.3.5 SELECTING AN APPROPRIATE STOPING METHOD

The primary deficiency in underground mining operations is the lack of mine planning. As a rule, a type of exploitation mining in the form of irregular excavating or room-and-pillar mining is practiced without any prior planning. This results in lower recovery, lack of safety, and adverse macroeconomic effects due to a partial destruction of the deposits. The different mining methods can be classified according to the type of mine development and support and roof-control measures as follows:

Table: Main Classification of Mining Methods

In the following sections, mining methods are presented (according to Stoces) which, under the special conditions of small-scale mining in the Andean region, contribute to lowering costs, increasing productivity, improving the use of resources (through higher recovery) and decreasing the effort required to extract the ore (consequently increasing mine safety).

Pillar mining

Fig.: Development of pillar mining in an inclined deposit. Source: Stoces

Pillar mining is characterized by irregular forms and arrangements of the excavation chambers, determined by the characteristics of the deposit, the chambers being separated by pillars of varying shapes to support the roof.

It can be applied in deposits with competent mineral and host rock.

Room-and-pillar method

Fig.: Room-and-pillar method. Source: Stoces

This mining method is characterized by the development of parallel headings which resemble long drifts in their form and dimensions. The width of the headings depends upon the competence of the host rock and can reach 10 meters; the height can total up to 3 meters.

The individual headings are laid out either parallel to each other, or either perpendicularly or diagonally crossing each other. Support pillars are left between the headings to support the roof. The roof and floor of the headings usually correlate with the hanging and foot walls of the vein being mined; in some cases, however, the mineralization may extend beyond the upper and lower heading boundaries.

This mining method can be applied in flat or slightly-inclined deposits with competent ore and country-rock.

Panel mining

Fig.: Panel mining. Source: Stoces

Within the deposits, only the narrow, long panels are mined, the valuable mineral contained in the support pillars between panels is left unmined. The deposit is normally developed by a main gallery from which other drifts branch off. These drifts are then widened into panels, leaving a stretch of unwidened drift between the main gallery and the panel for support reasons.

This method of mining is characterized by the construction of panels of regular, mostly rectangular shape. These panels are usually larger than headings, being developed according to preplanned, defined measurements.

Support pillars are left between the panels, consisting of either a solid wall (without cross-cuts), or a row of singular pillars (separated by cross-cuts connecting adjacent panels), depending on the method of ecavation employed.

In gently-dipping deposits, either the hanging wall and foot wall, or portions of the mineralized ore itself, form the roof and floor of the panels. In steeply-dipping or massive deposits, the mineralization can extend beyond the chamber boundaries in all directions. The panels can be mined by various methods, for example, a full advance to the final dimension, or with overhand or bench stoping, with or without backfilling or roof caving.

Panel mining can be applied in thick and massive deposits with competent mineral and host rock regardless of dip.

Shrinkage stoping

The blasting is performed from small chambers in the roof of the stope itself which are sunk from the overlying drifts.

Fig.: Shrinkage stope. Source: Stoces

With this procedure, the extracted ores are stored in the excavation chamber for the duration of mining of the individual slopes. The advantages of shrinkage stoping lie in the fact that no support measures are required and recovery is very high. Shrinkage stoping in small-scale mining in the Andean region is particularly suitable where local conditions permit only seasonal execution of certain processing steps; for example where there is a lack of processing water during the dry season, so that raw-ore beneficiation can only be performed in those months with sufficient rainfall.

Sub-level stoping

Sub-level stoping is an irregular form of panel mining.

This method is characterized by the blasting of large chambers, varying in size depending upon the structure and stability of the deposit and the host rock, and therefore, contrary to panel mining, not precisely defined prior to mining. The excavation chamber must be designed so that gravitational forces alone enable the blasted ore to slide down out of the chamber. Only in rare exceptions (for example, in particularly competent host rock) in only slightly inclined deposits, can a scraper be installed to assist in removing the blasted ore from the chamber.

The stoping, contrary to that in the panel stoping method, does not occur within the chamber itself, but rather at the chamber perimeter, either from horizontal sublevels or through long drillholes, since for safety reasons the chamber may not be entered.

The sublevel stoping can be performed either with roof-caving or with backfilling. It is applicable in steeply-diping deposits of lesser or greater thickness, and in flatter, more massive deposits where a required minimum stope height of around 15 meters can be realized.

A sufficient host-rock stability is important since the stopes can only be worked as long as they remain open. Due to the specified minimum sizes of the chambers and the corresponding greater degree of mechanization, sublevel stoping cannot be considered suitable for small-scale mining.

Fig.: Sublevel stoping. Source: Stoces.

Sublevel stoping (sublevel widening and sublevel caving). When competent ore is being mined from sublevel drifts, then mining from the lower sublevels can proceed.

Cut-and-fill stoping

Fig.: One-Sided cut-and-fill stoping of overhand faces with brace support. Source: Stoces.

This type of stoping is defined primarily according to the type of advance and not the shape of the excavated chamber. The overhand stope, which aside from bench stoping is the oldest form of mining, is characterized by the arrangement of the overhand-stope faces in a step-like pattern of advance whereby each stope cuts into the roof of the preceding stope. The floor of the stope generally is constructed with backfill' although in rare cases square sets are employed for chamber support.

Bench stoping (Underhand stoping)

Figure

Bench stoping is sometimes employed for mining smaller regions of deposits which lie below the haulage level where it would be uneconomical to develop additional levels.

Fig.: Bench (or underhand) stoping. Source: Stoces

Fig.: underground bench stoping or glory-hole mining in a steeply-dipplig coal mine in Checua Region, Cundinamarca, Colombia.

This mining method is the graphic opposite of overhand stoping. Here also, the type of development rather than cavity shape characterizes this mining method. The step-like stoping advances in such a manner that each face mines the floor of the preceding one.

In more massive deposits, the bench stoping develops into an underground glory-hole mining without backfill. It is applicable in deposits of smaller thickness and steep dip, and also as underground glory-hole mining in more massive deposits.

Inclined cut-and-fill mining is differentiated from the regular cut-and-fill method only by the inclined position of the face, which occurs as a result of applying this stoping method in steeply dipping deposits.

This method is only applied in steeply-dipping seamlike deposits of smaller thickness.

Inclined cut-and-fill mining

Fig.: Example of double-sided inclined cut-and-fill mining. Source: Stoces

Sub-level caving

Fig.: Sub-level caving. Source: Stoces

This form of stoping is characterized by the drifting of underground sub-levels, aligned underneath each other, separated by vertical distances of two to three times the height of the roadway. Mining progresses, as the name implies, from the top downwards, followed by caving which automatically also advances downward from sublevel to sub-level.

At each sublevel, the mineral is mined by a two-step form of "small panel mining" as follows:

advance mining involving the driving of individual parallel headings (similar to drifts in height and width), which is then immediately followed by retreat mining whereby the in-situ mineral above the sub-level is mined and the pillars between the drifts simultaneously weakened to the furthest extent possible. The stoping can be performed either sequentially, one sublevel after the next, or simultaneously with several staggered sublevels in deposits of sufficient thickness.

The sub-level caving method is predominantly applied in steeply inclined deposits, of smaller or greater thickness, or in rare cases in thick flat deposits.

A comparison of the various mining methods with regard to their technical and economic characteristics is presented below:

In any case, the application of a systematic mining method leads to a reduction in costs and improved mine saftey compared to the visual mining methods currently being used. The selection of one of the above-mentioned mining methods must give serious consideration according to the deposit characteristics.

Table: Compasion of the essential mining parameters of the major mining methods