Table of Contents
The graphic methods and records described in this article have been developed, with satisfactory results, for the use of executives of the Chino Copper Co., in Hurley,. N. Mex. Particular attention is directed to the use of large wall-charts, the quick reference display of large charts, the scale notation of metallurgical charts shown in Fig. 3, and the inclusion of mesh extraction on screen analysis diagrams.
To properly comprehend the operation and metallurgical results from a concentrating plant, a daily record of essential data is kept, usually involving the dry weight of ore milled or treated, dry weight of concentrate or metal produced, assay values of heading, assay values of tailing, assay values of concentrate, and percentage of extraction or recovery. In addition it is often necessary to record moisture, pulp density, or dilution; percentage of weight of oils and reagents employed; screen analyses of grinding conditions, oversize, etc.; assay values of non-valuable or interfering substances; cost data; and such special information as may be required.
Graphic Presentation
Close analysis is necessary of a daily operation involving such a number of factors, the relation and influence of which may be of great importance in the treatment scheme. When the analysis is made from a numerical tabulation, the chance for one or more obscure, but perhaps important, factors to escape notice is considerable, therefore the need for an accurate, readily digestible presentation of metallurgical data is obvious.
Graphic records of milling operations are frequently used, but owing to the wide range of magnitude and values, and the number of factors involved, the customary practice of assigning different values to the same ordinates, as in Fig. 1, results in a complex and intricate record which makes little appeal to a busy operating official. The methods shown in Figs. 2 and 3 are devised to display such data in a clear and legible manner, without confusion.
Semi-logarithmic Method.—This method, Fig. 2, depends on the use of cross-section paper with logarithmic horizontal ruling and arithmetic vertical ruling. Time is laid off on the vertical lines and values are assigned to the horizontal logarithmic scale. Three complete logarithmic scales are required to embrace the range of percentage and tonnage con-
sidered in the average milling operation. Four scales are preferable, but such ruling is not available unless hand-drawn.
The graduation of the scales on the record, shown in Fig. 2, is self-explanatory and any one familiar with the slide rule can grasp the method. By this scheme all factors are shown in their proper relative position and tendencies may be closely observed and compared. The only disadvantage in this method lies in the fact that high points are depressed and low points exaggerated, which sometimes leads to erroneous conclusions.
For this reason it is more applicable to the needs of the engineer than to general use. The great advantage obtained is the presentation of wide variations of magnitude in a small space, a range of from 0.1 to 100 in direct sequence being possible, and without confusion, the range may be increased to 10,000 in the space of a letter-size sheet.
Special Arithmetic Method.—Millimetric ruled cross-section paper, available in 50-yd. (45-m.) continuous rolls 22 in. (55.8 cm.) wide, is used preferably for this method, Fig. 3, which might be described as logarithmic notation on an arithmetic scale. This notation is original and is of marked advantage in plotting metallurgical data. Sheets 4 ft. (1.2 m.) in length, of full width, are recommended. The notation of the scale should
be so arranged as to display the various factors with due prominence. Assay values of heading and tailing which are the minimum and also the governing factors, should be exaggerated. Beyond the maximum range of heading assays of valuable metal, the scale may be reduced, the concentrate assay values, dilutions, extraction, etc., falling principally in this range. The tonnage scale is devised to meet the maximum and minimum variation and may include concentrate production. It is permissible to overlap the tonnage and percentage scales, preferably in multiples of ten, employing lines of different weight or color to distinguish them in conflicting areas. Abnormally high or low points that fall outside the limits of the particular scale should be plotted on the projection of the scale in which they originate, since breaking scale destroys the proportion of the relationship with other lines.
Under date of Mar. 28, Fig. 3, the position of all points except that representing per cent, of iron in heading needs no explanation, as they are referred directly to the notation on the scale. The iron, however, falls below 2 per cent., at which the descending scale breaks from a rate of 2 per cent, per centimeter to 0.1 per cent, per centimeter. If the point is plotted to the scale notation it will be out of proportion to all other similar points, so the 2 per cent, per centimeter scale is projected downward to 1.5 per cent, and the point plotted. On the scale notation it appears at 1.95 per cent., but by referring to the origin of the scale in which all other live points are found, a wrong inference should not occur.
The average line drawn through each curve is the average “per day” for the month, and not the average for the month compounded daily to date, which would appear as a median line on the chart. The three types of metallurgical presentation here shown have been plotted from the same fictitious data, for comparison.
A record of departmental milling costs and general statistics is kept on separate similar sheets, changing the scale from logarithmic notation to direct arithmetic, covering the necessary range of magnitudes.
Mining Control
Upon sheets of the same size, a monthly graphic record of tonnage of ore mined and yardage of waste stripped is kept with incidental costs of various operations. Corresponding deep-mining data can be added or substituted with facility. This form is not illustrated as it follows conventional fines and is simple in construction.
Wall-Charts
Charts of the type shown in Fig. 3 are particularly adapted to large-scale wall-charts for permanent record. Very neat “month by days” or “3-year by months” records may be kept: time is laid off on the width in days or months, one unit to the centimeter, leaving ample space for marginal notation. Such records, reinforced at the top and bottom with thin clamping strips of wood, screwed together, are easily handled.
For current daily record they are also well adapted to ordinary pin-posting: mounted on a cord and strawboard backing, in which the colored pins may be easily inserted, they are convenient to work upon and may be accurate to a fine degree. Colored connecting cord, to match the glass-headed pins used to represent the different values, adds to the legibility of the chart. When the period covered by the record is ended, the pin-heads are encircled by a pencil mark before the chart is removed from the pin-board. The connecting lines are inked in colors and the record transferred to a cabinet, in which it may be shifted, making it possible to display a number of charts at the same time for comparison.
Display of Large Charts
The principal reason for the use of graphic records, after their value in analysis and comparison, lies in the rapidity with which the information displayed may be assimilated. This makes them exceptionally useful for reference. Therefore, when an elaborate and comprehensive graphic record is rolled up and filed away, safe from harm and service, a large share of the value is lost. After preparation, such charts must be accessible to yield full usefulness. A visible record cabinet now in use illustrates one way of achieving this result.
This wall cabinet is of wood. Its height is determined by the length of the record sheets, its length by the number of records it is proposed to display simultaneously, and its depth by the number of records it will eventually contain. A pin-board with cork composition backing is built in at one end of the cabinet for the posting of current sheets. An overhead monorail system of the requisite number of tracks is built in at the top of the cabinet, the rails extending its full length. This system is improvised from sliding-door equipment, the track and ball-bearing roller trucks being used. Two trucks in tandem are divided by a spacing bar to the width of the record sheets. The wooden clamps reinforcing the top of the sheets are attached to the spacing bar by a simple lug and thumb latch of brass. The latches on the spacing bars and the lugs on the clamps are laid out to jigs, so all sheets are interchangeable on any track. One sheet only is mounted on each rail. A curtain suspended from a spring roller hangs in front of the cabinet, concealing the contents when desirable. The large sheets are well adapted to photostatic reproduction, and may be reduced to letter size without losing legibility,
Periodic Reports
A useful chart for periodic reports is the especially designed form shown in Fig. 4. The cross-section is engraved on thin letter-size tracing paper, on the lower half of the sheet, leaving the upper part for typewritten tabulation. The tracing of the ordinates corresponds to type-
writer spacing, and also to that of the multigraph, so standard forms may be skeletonized in large lots at small expense.
Essential data from the tabulation are plotted on the cross-section. This form is of special use in the presentation of progressive reports, in which summarized periodic figures are carried from month to month for the year, etc. Data for the current month appear in the tabulation, while essential material for past months is plotted. Twelve or twenty-four months may be carried on this form. Many special reports are adequately presented in this manner.
Screen Analysis Diagrams
These diagrams, shown in Fig. 5, are furnished on letter-size tracing paper, by the W. S. Tyler Co., who devised the ratio plotting scale. Comment upon their use is unnecessary, particularly in the analysis of crushing and grinding problems. In the study of performance of various stages and methods of concentration, the value of these diagrams has been increased by adding the mesh extraction. The percentages by weight of material and copper on each mesh in heading and tailing are delineated. The mesh extraction is computed from the actual weight of copper in the material retained on each mesh in heading and tailing, from the form:
Heading—Tailing/Heading
The work done by the process on various sized material is thus clearly and quickly shown. If desired, the individual mesh extraction may be plotted, either with or without the cumulative total extraction.
Monthly records of stage concentration have been kept in this manner over a considerable time and are a valuable index to operating conditions. In the comparison of qualities of the many new types of concentrating equipment frequently tested in a large plant, this scheme is especially useful.
In conclusion, it may be said that the graphic records outlined above, modified or amplified to suit special conditions, have facilitated both reference to past performance and analytic study of current operations at this plant, well justifying their adoption.