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The Importance of Reliability in Automation Applications with Special Reference to Mining, Mineral and Metal Processing
RT2
ROUND TABLe DISCUSSION THE IMPORTANCE OF RELIABILITY IN
AUTO~TION
APPLICATIONS WITH SPECIAL REFSRENCE TO MINING, MINERAL AND METAL PROCESSING
PANELLISTS:
N.T. VAN DER WALT (Chair83n 1 T. HARVEY, R.P. BECK, C. THERON, H.J. STUBLER
M~. Harvey opened the discussion, explaining the problems which arise in the South African gold mining industry, making special mention of existing environmental conditions in these mines, repair facilities, ability and experience of technicians and the determination (or lack of) of management to ensure that instrumentation has a reasonable chance of success. He also mentioned that gold recovery already exceeded 99 percent without sophisticated instrumentation and explained that this was a reason why instrumentation did not enjoy a high priority and why when they were used their reliability should be without question. Mr. Harvey went on to mention several case histories of reliability of instrumentation in the gold mining industry.
9. !-·r~,::;c:!:·o.t::"~15 CA}:r~-:':"·;nc·-.; of instrum17nt dssignsr.s ;;La: ;;Lci~' ins:rum'nts ma~' W02 sup'7rbl; and _flici~fi:l~ ~n c~r:a~~ i~jus tri~s and y t in other~, particularly ~he ;old ~ining indus~ry, they may fail with disappointing regularity. Without apologizing for the gold mines it is desirab~e that the conditions existing there shou~d be understood so that possibly the instrument designer may include that little touch of genius which can make the difference between success and failure.
Mr. Richard P. Beck spoke from the point of view of mineral processing, stressing that the importance of reliability of a specific instrument should be tempered in relation to the specific automation or measurement problem. Financial losses could be high if accounting instruments had but a small error. Alarms, though not required to be accurate, nevertheless had to be reliable. Planned maintenance for a large number of such devices, in view of the fact that they operated only occasionally, was a problem. Mr. Beck went on to speak of unreliability in closed-loop systems, adverse environmental conditions and mentioned the negative effect which the lack of immediate reliability of a system has on operating personnel. Mr. Chris Theron discussed the reliability of instrumentation in the iron and steel industry and stressed the importance of adequate knowledge, documentation facilities and efficient organization as far as maintenance was concerned. Mr. Theron divided system reliability in the steel industry in three categories, namely, data logging and monitoring only with manual back-up, control and data logging with manual back-up, and control with ~o manual back-up. He gave reliability figures obtained in practice from various examples of the above and came to the conclusion that reliability was dominated by interference from outside, equipment malfunction, mean time to repair, and back-up facilities.
Mr. St~bler had presented a paper on Reliability earlier during 'the Symposium.
The reliability of equipment is related to the environment in which it must work, the ease with which it can be repaired if it does go faulty, the ability, the knowledge and experience of the technician in charge, and by no means least of all, the determination of management to ensure that these conditions are effectively provided for.
Of these factors, the attitude of management is really the most important of all. It should be realised that in the gold mines of South Africa in particular, instrumentation, in its more sophisticated sense, is comparatively new and in many cases management has not yet orientated itself to the importance of maintaining instrumentation. In a way this is understandable because a few years ago, before instrumentation was being considered, gold recovery already exceeded 99 percent of the gold originally in the ore and labour was plentiful. The profits were enormous and alt'iOugh instrumentaL.er, could be proved to be economical in th~ sense that the extra 0,1 or 0,2 per c~nt, increase in recovery brougnt about ay the instr~menta tion would more than pay for the complete instr~mentation syste~, the increase in profits of th~ mine were far from being startling. Tne enthusiasm of the few for the use of instr~mentation was consequently not viewed with equal enthusiasm in the board rooms when relative cost advantages were being discuss8d. This is a far different position from, say, the oil industry where success or failure of the whole enterprise depends very much on successful instrumentation. Nowadays, however, the dwindling labour resources are forcing mechanisation and automation - and consequently instrumentation whether people like it or not. The enthusiasts are willing to meet the problem although hampered severely by a lack of real awareness of the plant controllability
T. HARVEY: It is a fairly common and sometimes
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factors, but management is still not really geared up to this new situation.
set it to the new position of the disc. When these units were made as self-contained units with built-in friction-driven wheels running directly off the belt the electrician alone could install them (preferably well away from the tail pulley) and they performed much more reliably.
The result of this is a sad tale of incorrectly installed and poorly maintained equipment with a general background of lack of an effective organisation to ensure a drive towards keeping equipment running correctly.
It is also good practice to employ instruments which do not rely on mechanical activation. When mechanical activation is employed it is surprising how often mechanical shock forces of several magnitudes higher than the designer had ever allowed for are encountered. Mechanical strain gauges on belt weighers fail whe~ oversize rocks pass over them or the electrician decides to use he belt :0 transpor a motor to another part 0: the plan. Til switches used to detect blocked ore chutes tend to vanish completely when inadvertently caught between two pincershaped rocks and even when left intact they operate from ambient vibrations of the structure unless time delays are set so long that the chutes are completely choked and the conveyor belts wrecked before the trip circuit is actuated.
So often reports of poor reliability of instruments prove to be based on trivial faults which even unskilled electricians could have repaired. So often it is easier to leave equipment costing thousands of rands unserviced and to claim that it is not suitable for the job rather than to persevere with maintenance and calibration. Undoubtedly the mining en\iro~~ent is severe in many instances, not the least of the problems being the presence of highly abrasive grit particles in practically all solutions, but the biggest problem facing the instrument designer is the need to produce equipment that ideally never breaks down. If it does break down there is a danger that it will never be made to work again.
In many cases radio-active sources can be used very effectively and reliably, and they are used extensively in level detectors, belt weighers and density gauges. They have, of course, to be applied correctly and in many cases this has meant using far stronger sources than might have been expec ed. This has probably led to greater reliability because strong sources need heavy enclosures for safety against radiation and heavy construction has also resulted in mechanically robust construction.
Coming down to the more specific problems of instrumentation, a very general complaint is the lack of adequate documentation and instructions from the instrument manufacturer. It is noteworthy how many well designed instruments - particularly those supplied from 3uropean sources perform well but cannot be repaired because of lack of back-up serVicing or of documentation. Why are suppliers so reluctant to release this information 0 They might well note the trend for American supplies of equipment to provide very full and complete documentation.
Tt would be thought that an instrument which could surely be designed for reliable operation would be one for indicating the sump level of slime solutions. The fact of the matter is that every conceivable principle has been tried, ranging from floats through to differential pressure transducers, bubble tubes, capacity electrodes and ultrasonic detectors.
Another thorny problem, this time in the domain of the application engineer, is the problem of calibration. Comparing operating instruments with such procedures as belt cuts done rather haphazardly or the use of tonnage boxes of small capacity is just not good enough. There should be calibrating facilities either easily accessible nearby or, preferably, built in to the plant, and this calibration ~ust be accurate and reliable and must not introduce excessive lag times. There is nothing more frustrating than to test an instrument against another instrQ~ent and in the end not know which reading is reliable. The type of ir.strQ~ent which can be and mainta~ned by one tradesman should preferably always be used. If two or more tradesmen are involved it can always be assumed that the instrQ~ent will never work properly. An example of this has been belt-slip detectors. One type of unit originally used operated from electromagnetic pulses picked up from a disc with holes drilled into it and driven from the tail pulley. This hardly ever worked because of buckling of the disc caused by spillage, by over-enthusiastic operators using the disc as a fulcr~m point for levering out jammed rocks, or the result of the fitter having changed a tail ~nlley. If the fi ter had ensured that the disc was running true then the pic~-up ur.it did not work because the electrician had not i~stalled
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0: these the ultrasonic detectors have proved in the end to be the most reliable. Initially they, too, had problems caused mainly by echoes returning through the mounting steelwork and ghosting echoes from the sides of the sumps. Mounting the units on resilient rubber diaphragms, choosing appropria e beam widths and prevailing on the manufacturers to incorporate range-tracking gates have overco~e most of these problems. However, maintenance is still high as the heads must be kept clean and have to be replaced frequently, and the connection cables between the heads and the electronic unit are very sensitive to ~oisture and capacitance effects. Another very vexing problel~ has been to obtain reliable operation of proportional control valves. A common feature of the cany valves tried has generally been the~r high failure rate which seems to bear a direct relationship to their cost. 1he most successful valves to date have been pneU"-atic types with a separate con~on air supply system. ,This combinatior. has, Of, course, violatea the prlnclple elaboratea earlier of the Q~desirable practice of mixing trades, and there have been many
~ ooint that ~ust be considered whp.n discussing the r01iability of auto~~tcd equipment is the environment in which the equipment operates. In the mineral processing industry the sensors are often subjected, for example, to quantities of large falling rocks, of pulp s reams containing very abrasive particles or of streams of only partially-dissolved chemicals or froths.
control centre and thus the control processing units at the centre of the plant, where he can watch the instrumentation and the machines at the same time. This is an ideal that is not normally practical if reliability is to be ensured. It will be interesting to hear how this problem has been overcome in operating plants.
The environments of these ins ruments tend to be wet and/or dusty and the instruments are often subjected to vibration, all of which must reduce reliability and which must be allowed for in the design of the instrument.
A major concern when instrumentation and automatic systems are being added to plants is the immediate reliability of the items. It is difficult to get operator acceptance of a new installation if it works for only part of the time. This becomes of great importance in the experimental stage of any installation and can be overcome only if the operators are kept fully informed at all stages of the work.
The siting of the automation processing control units is a function of reliability. These units normally require vibra ionless and dus -free surroundings, and are sited away from the main plant operations. Unless remote s ations are installed 0 relay the data to the centre of the plants, which would be costly, reliability is achieved at the expense of the operator. The operator would prefer to have the (conttl. next column).
I believe that this is enough in he way of a general introduction to this subject, and I look forward to the discussion on the importance of reliability in automation applications, a matter which is very near the heart of every operator of an automated plant.
0000000
Plant
Type of System
Date Inst.
Memory
Avail. for AUB· '76 %
Core Tinning Line
Data Log and Alarms
Feb. '68
12K 14 Bit
B.O.F.
Control with Jan. '74 manual back-up
16 Bit
Hot Strip Mill
48K 32K 24K 24K
No manual back June '74 up on S.P.C. 3 computers
24 Bit Batch Annealers
Limited manual Jan. '75 back-up
Tandem cold Mill
No manual back Feb. '75 up on 320's (mini comp) 4 computers
Control with manual B.D.
Dec. '74
Bulk
H/W
S/W
0
100
100
100
98,98
100
98,98
99,85 99,98 99,98
100 100 100
99,85 99,98 99,98
1,5M Disc 0 512K
256K
Drum
Overall
24K
128K Disc
95,50
98,77
94,32
16K
512K 0 0 0 Drum
98,43 100 100 99,85
98,4 98,84 100 100
98,12 98,84 100 99,85
0
100
99,5
99,5
16 Bi
52K 56K 52K 24 Bit 16 Bit
Temper Mill
:
32K 16 Bit
C. THERON:
doing automatic control and monitoring with no manual back-up.
The reliability of a computer system may be defined as the probability that a system will operate at an agreed level of performance for a specified period, subject to specified environmental conditions. It can be expressed as the ratio of the mean time between failures to the sum of the mean time between failures and the mean time to rectify.
Availability figures obtained in practice for various applications in the above categories are in the Table shown above. Apart from failure of the equipment itself, other factors may also influence the reliability of a system. These are interference from the operation or frvm nutside the system, factors which influence repair time, and back-up facilities for various functions.
Computer systems in the steel industry can be divided into three categories, namely, systems doing monitoring and data logging only with manual back-up, systems performing automatic control and datagathering with manual back-up and systems (con~d. next cQl~n below Table)
In order to prevent interference from outside the system there should be a stable power supply and good air conditioning
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surpirsing moments when air lines have been opened and have been found to be full of water. It has at least been established that the air supply system to instruments should be completely separate from the mine air system.
be very accurate but as they are required to function only under abnormal conditions and therefore non-continuously, it is important that they be reliable. Failure of an instrument which normally works continuously is easily detected. This, however, will not happen, say, when the high-level alarm on a tank fails; such a failure is likely to be noticed only when the tank overflows, and is likely to occur more often than that of a temperature alarm on a grinding mill bearing, the overheating of which would have drastic results. Although planned maintenance will ensure that all such alarm devices are working at all times, the very large numbers used in modern automated plants makes this an almost impossible task and I should be interested to hear comments and discussion on this point.
Some success has been obtained by using hydraulic actuators coupled to Saunders valves and this combination is low in cos. It is too early to say whether these will replace the pneumatic valves. As a last word on the whole problem of instrumentation in the gold mines, while what has been said might be construed as being hyper-critical and even pessimistic, the enthusiastic efforts of a small core of people are rapidly improving all aspects of reliability ranging fro~ correct selection and application of instruments right through to the education and organisation of technicians and the provision of adequate testing and calibration facilities.
R.D.
The problems outlined so far are compounded when the instruments are included in automatic loops or systems. The instrument then becomes a link in a chain that includes controllers, processors and actuators. The failure of any of these will cause failure of the automation system. It is the reliability and the possibility of failure of the system that must be planned for in any installation. It must be determined whether an operator can take over control and whether control can be performed manually. But it must be borne in mind that on an automated plant the operators will tend not to have the experience of good plant control through lack of experience of manual operations. If this is the case, standby control units might be required and the remarks regarding the reliability of instruments which are not often used apply here also.
B~CK:
There is no such thing in mineral processing as a reliable instrQ"ent. This, no doubt, is a controversial enough statement for the maker of instrQments and some amplification is obviously necessary. I would thus like to talk briefly about the effects of this lack of reliability and throw out some points for discussion. To start with I want to suggest the ideal control circuit or instrument. This is equipment tha is installed and calibrated very simply, which has a high degree of accuracy and which then produces the required control action or information without further need for adjust~ent, maintenance or attention.
I should be interested to hear comments as to how this balance between instrument failure, operators' process control knowledge and the provision of standby equipment has been handled in practice.
The major requirement of such an installation is that it should be reliable. Firstly, I want to talk about instruas individual items for the logging of operating information and as alarms and then go on to discuss the reliability of control leaps and systems.
~ents
Although at all times a high degree of reliability is desir.d of any instrument, this degree of reliabili~y, or the ability th~ ~utput
0:
instru~~n:
to continue to giVE an
of the desired degree of accuracy, ~ay ce temper~d in relation to the specific automation or ~casure~ent application. The highest accuracy is usually reqUired of those instr~ents used for accounting. The most obvious is the weightometer. Since the tonnage processed in modern plants is very high, a small inaccuracy or unreliable result can give a jiscrepancy of thousands of tons. =f unreliable and inaccurate instruments are used to mcnitor or control the addition 0: reagents to a process, financial losses will be incurred because of either the addition of inadequate quantities of reagent with consequent low recovery, or the addition of excessive and therefore, wasteful, quantities of reagent. Alarms are not normally required to 752
The reliability of instruments and control systems can be maintained by good and regular servicing and calibration checks. The amount of work involved with each one and the extent of ~raining or qualifications of the man perfor~ing the work must be kept to a minimum. The need for specialist service engineers increases in proportion to the increasing complexity of the systems. The plug-in module system of electrical circuits has certainly helped to overcome this need, but the Doint must be brought strongly to the attention of manufacturers of equipment for this industry, since mining operations are often in remote parts of the world and it is difficult to attract personnel to maintain sophisticated electronic instruments at such places. Adequate means for the calibration of instruments ffiUSt be provided, but these are often omitted. The accuracy of an instrument is only as good as its" calibration and if the means for sampling and varying operating conditions are left out, the accuracy of the system will be poor. Off-line calibration is often a useful method but facilities for this are rarely seen on installations.
and the computer should be free of dust and vermin. It should also be ensured that no interference is caused during electrical isolation of the plant, and when switching relays. Electrical noise should also be avoided in switching large blocks of power. Power to the hardware should be disconnected only when this is absolutely necessary.
The advantages of this organization are that the technical staff specialise in hardware systems and peripherals and that no problem arises in interfacing between hardware and software staff. With regard to reliability of software, all software changes and the reasons for such changes should be documented. An up to date program should be available for reloading with minimum loss of time. Program listings and flow charts should be up to date and powerful reassembly facilities should be provided. Software errors can be prevented by bulk memory protection.
A good common earth for the computer should be provided isolated from the cabinet and any other earthing system. Down-time will be minimized by preventive maintenance. Repair time is influenced by a large number of factors. Spares should preferably be available in the form of electronic cards and as complete units for equipment such as analogue-to-digital converters, selsyns and ho metal detectors. They should be stored in the computer room in order to ensure rapid location of faults. Special test instruments such as storage scopes and paper and tape recorders are a grea relp in tracing intermittent faults.
Reliability can be improved by providing back-up facilities. It is not necessary, for example, 0 go off-line for all malfunc ions. In any system function failures can be divided in to three categories: (1)
Failures that are alarmed to notify only the operations personnel.
All documentation should be correct and up to date. Test descriptions and logic fault tracing methods are of grea help in reducing repair time. Descrip ions should be written in comprehensible English.
(2)
Failures which require that the operation be sopped and operator action be requested, as when the operator can override the decision of the compu er.
Communication panels which give access to various registers and step-by-step operating sequencing to check timing signals and alarm conditions are of great assistance in reducing repair time. Mimic panels which display sequencing are very necessary in some of the systems.
(3)
Failures which necessitate taking the system off-line, or, for example, in the case of a power failure or loss of position calibration.
Operating according to such categories will increase system up-time.
In the area of computer/plant diagnostics, the computer itself can be very useful in reporting on the malfunctioning of certain equipment, for example failure of hot-metal detectors or load ' cells.
An expensive, but perhaps worthwhile back-up is the provision of an extra main frame unit. Other extra items of equipment which can be provided as back-up facilities are primary data input card readers, typewriters and sensors.
As regards program loading, adequate facilities should exist to read-in programs rap~dly and correctly, particularly on systems which have no bulk memory.
F.W. YOLK: If we include Mr. Stubler in the panellists we hear voices from four different directions, which I wou~d like to put into two groups.
Motivated and skilled maintenance personnel are a very important factor in minimising down-time. The maintenance organization at ISCOR at Vanderbijlpark is as follows:
The first group is related to mineral processing with special regard to gold reduction plants. Instrumentation and automation are, as mentioned by the speakers, s ill in an early stage. The second group relates to highly sophisticated computercontrolled steelworks. I expected from all speakers at least a hint concerning the human inadequacy or failure within the total automatic system, but this fact was mentioned only by the first group who referred to artisan and operator faults and to lack of knowledge in general.
Mills area
For the last ten years I have observed advanced, partially automated, ore-dressing plants and have noted that their labour complements were low. A low labour force would, therefore, appear to be a prerequisite for further development of a high degree of automation.
SI W maintenance and development work Comp. H IW peripherals
753
For this reason the question is, is it right that the issue, manual operation versus automatic control is not the point of discussion, but rather the application of automation versus more reliable application of automation?
should be considered the most important instrument and should be made as reliable as possible. The interpretation of mean time to repair a failure should be done very carefully, taking into consideration the total number of failures which have occurred. A poor preventative maintenance routine will cause a high nQ~ber of minor failQres which, together with the norrr.al rate of serious failures, which normally take a long time to repair, will yield a relatively low mean time to repair. A high mean time to repair should thus not categorically be interpreted as bad craftsmanship in repair, since it could also be the result of a well funcioning preventative maintenance program.
H.W. SMITH: Too many systems are designed to include sensors which provide interesting but unnecessary information. When these fail, they are not maintained, thus giving a completely wrong impression of the system as a whole. Design simplicity is a very important precondition for system reliability. Many instruments, even when new, do not meet their specifications. Such evaluation services as SIRA (Great Britain) can be of great help in choosing the right instrument in the first place.
G.
I believe that a cost-benefit analysis of instrumentation and automation should be conducted to convince management of the importance of the equipme~t and staff employed in instrumentation.
D.e. FARQUHARSON: In our experience at ISeOR with a microprocessor connected to a load cell weigher, with regular computer-initiated electrical range checks, we have several times observed progressive failure of the load cell - test failed once a week, to once a day, to several. times a shift, to finally continuous failure.
Documentation is generally provided in sufficient amounts, but is lost or not understood by maintenance personnel. Staff turnover is obviously also an influence on good maintenance. I also believe that no facilities are available to calibrate beltweighers or magnetic flowmeters, but that such a facility should be available.
Perhaps with some statistical 'near failure' on-line monitoring, many sensor failures could be predicted in good time for repair or replacement of the sensor.
We achieve almost 99 percent extraction of gold and therefore the motivation for instrumentation is not as great as In the metal industry. Safety is generally built into the equipment in mineral processing and therefore the need for ins rumen ation to improve safety is not as dominant as in metal processing.
2. With small systems, particularly microprocessors, the need for airconditioning, dust filtering and rat-proofing is beginning to fall away. At least one process control installation in he ISeOR Vanderbijlpark Works has shown this over some three system-years. H.~.
eOHEN:
W. BOONZAISR:
The problems of costs underlie many of the questions of reliab~lity. If one examines the costs of processing equipment and the contained values of the materials being processed, one finds a large difference between mining/mineral processing and steel production. The high cost of rolling mills and the high value of steel ensure reliability through expensive instrumentation and control. In mineral production, the contained value of the ore is usually low and the processing equipment is far less costly than, say, rolling mills. Hence, relatively low-priced instrumentation is sought. The total costs of developing such equipment are inevitably out or proportion to the expected marKet value. Thus the problem arises of someone having to bear the development costs. In terms of reliability, it should be noted that these costs comprise not only the initial invention, design and construction of control instrumentation, but also on-stream development which is vital for bridging the gap between laboratory expectation and plant performance. R.R.W. ~ore
SO~lER:
In dealing with the question of multiple computer systems, the reliability of multip:e process systems on hot standby operation requires vigorous hardware and sof ware checks to ascertain, for instance, which computer of a two-computer hot standby system has developed a fault. ~xperience has shown that it is necessary to have a third electronics unit to carry out this monitoring and control fur.ction so that a clear and unambiguous decision can be made, especially when the hardware or software monitoring ~~its in the computers are causing the fault. R.
GR~~NWOOD-SMITH:
Reliability is obtainable at a cost. tends to stress low capital cost and the cheapest instrument is purchased. The difference in cost is often more than used in trying to get the cheap instruments to work.
~anagement
It is often the case where the user overspecifies the accuracy needed, for example, grade accuracy reqwrements .. The cost and complexity of the resultlng lnstruments leads to the purchase, or construction of, an unreliable instrument.
W~H&~ANN:
The safety monitoring instrument much so than the accounting instrument
754
The use of redundant instruments in a control system allows alternative calculation of the measurement to be performed from the results of other measurements. This can retain the integri ty of a control system after failure of an instrument.
W. WEIDEMANN:
In optimising the overall p~formance of a plant (and for overall safety or lack of disruptive disasters), would it not be advantageous to draw a 'consensus vs. expected frequency' curve?
If a control system can be seen on 'manual' for any length of time without 'management' noticing a reduced performance from the plant, then the control system is hardly worth having.
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A.R. ATKINS:
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DESIRED ENVELOPE
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A reliable system utilising automatic control depends upon the reliability of both the equipment as well as the operators and maintenance staff.
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INCREASING COST OF DISORDER OR OUTAGE (LOG SCALE)
I do not think sufficient attention, particularly in the mining industry, is given to human factors concerning these two groups of people.
One could plot various 'disasters' vs. their expected frequency from faultfree analysis, making use of s a istical data for the components making up a system. This will focus attention on hose eventtrees where remedial action or re-design, or more frequen maintenance will prove most beneficial. Also, it will identify common reasons for outages (e.g. common power supply).
There is a lack of understanding of what is expected from the controls by the operator. Because of this the operatur is unaware of failure of an instrument. There is a lack of communication between management, operator and maintenance staff. The reliability of maintenance staff could be improved if they were given better training and enjoyed a better status. At the moment, most technicians rank with electricians in the mining industry. A.H. MOKKEN: I would like to correct a misconception with regard to the percentage of gold recovered in the gold mining industry's reduction works. Two previous speakers have mentioned a figure of 99 percent and have indicated that there is therefore little financial incentive to spend money on instrumentation and systems aimed at improving recovery. In the Union Corporation Group extraction averages about 96 percent from an overall grade of about 7 g/t. It is conceivable that an individual mine in the industry, treating a high grade nonrefractory ore, could reach 98 percent. If the ore grade was, say, 20 g/t, the amount discarded would be of the order of 0,4 g/t. This is still about 50 percent more gold per ton than that discarded on average by the mines of Union Corporation. During the period when the price of gold soared to $200 an ounce increasing attention was given to the magnitude of the gold loss. This is understandable but not condoned. Now is the time to look closely at the factors influencing the efficiency of gold extraction and install those facilities and controls that will promote recovery in order that we may be prepared to take full advantage of any future rise in the price of gold.
755
ROUND TABLe DISCUSSION THE IMPORTANCE OF RELIABILITY IN
AUTO~TION
APPLICATIONS WITH SPECIAL REFSRENCE TO MINING, MINERAL AND METAL PROCESSING
PANELLISTS:
N.T. VAN DER WALT (Chair83n 1 T. HARVEY, R.P. BECK, C. THERON, H.J. STUBLER
M~. Harvey opened the discussion, explaining the problems which arise in the South African gold mining industry, making special mention of existing environmental conditions in these mines, repair facilities, ability and experience of technicians and the determination (or lack of) of management to ensure that instrumentation has a reasonable chance of success. He also mentioned that gold recovery already exceeded 99 percent without sophisticated instrumentation and explained that this was a reason why instrumentation did not enjoy a high priority and why when they were used their reliability should be without question. Mr. Harvey went on to mention several case histories of reliability of instrumentation in the gold mining industry.
9. !-·r~,::;c:!:·o.t::"~15 CA}:r~-:':"·;nc·-.; of instrum17nt dssignsr.s ;;La: ;;Lci~' ins:rum'nts ma~' W02 sup'7rbl; and _flici~fi:l~ ~n c~r:a~~ i~jus tri~s and y t in other~, particularly ~he ;old ~ining indus~ry, they may fail with disappointing regularity. Without apologizing for the gold mines it is desirab~e that the conditions existing there shou~d be understood so that possibly the instrument designer may include that little touch of genius which can make the difference between success and failure.
Mr. Richard P. Beck spoke from the point of view of mineral processing, stressing that the importance of reliability of a specific instrument should be tempered in relation to the specific automation or measurement problem. Financial losses could be high if accounting instruments had but a small error. Alarms, though not required to be accurate, nevertheless had to be reliable. Planned maintenance for a large number of such devices, in view of the fact that they operated only occasionally, was a problem. Mr. Beck went on to speak of unreliability in closed-loop systems, adverse environmental conditions and mentioned the negative effect which the lack of immediate reliability of a system has on operating personnel. Mr. Chris Theron discussed the reliability of instrumentation in the iron and steel industry and stressed the importance of adequate knowledge, documentation facilities and efficient organization as far as maintenance was concerned. Mr. Theron divided system reliability in the steel industry in three categories, namely, data logging and monitoring only with manual back-up, control and data logging with manual back-up, and control with ~o manual back-up. He gave reliability figures obtained in practice from various examples of the above and came to the conclusion that reliability was dominated by interference from outside, equipment malfunction, mean time to repair, and back-up facilities.
Mr. St~bler had presented a paper on Reliability earlier during 'the Symposium.
The reliability of equipment is related to the environment in which it must work, the ease with which it can be repaired if it does go faulty, the ability, the knowledge and experience of the technician in charge, and by no means least of all, the determination of management to ensure that these conditions are effectively provided for.
Of these factors, the attitude of management is really the most important of all. It should be realised that in the gold mines of South Africa in particular, instrumentation, in its more sophisticated sense, is comparatively new and in many cases management has not yet orientated itself to the importance of maintaining instrumentation. In a way this is understandable because a few years ago, before instrumentation was being considered, gold recovery already exceeded 99 percent of the gold originally in the ore and labour was plentiful. The profits were enormous and alt'iOugh instrumentaL.er, could be proved to be economical in th~ sense that the extra 0,1 or 0,2 per c~nt, increase in recovery brougnt about ay the instr~menta tion would more than pay for the complete instr~mentation syste~, the increase in profits of th~ mine were far from being startling. Tne enthusiasm of the few for the use of instr~mentation was consequently not viewed with equal enthusiasm in the board rooms when relative cost advantages were being discuss8d. This is a far different position from, say, the oil industry where success or failure of the whole enterprise depends very much on successful instrumentation. Nowadays, however, the dwindling labour resources are forcing mechanisation and automation - and consequently instrumentation whether people like it or not. The enthusiasts are willing to meet the problem although hampered severely by a lack of real awareness of the plant controllability
T. HARVEY: It is a fairly common and sometimes
749
factors, but management is still not really geared up to this new situation.
set it to the new position of the disc. When these units were made as self-contained units with built-in friction-driven wheels running directly off the belt the electrician alone could install them (preferably well away from the tail pulley) and they performed much more reliably.
The result of this is a sad tale of incorrectly installed and poorly maintained equipment with a general background of lack of an effective organisation to ensure a drive towards keeping equipment running correctly.
It is also good practice to employ instruments which do not rely on mechanical activation. When mechanical activation is employed it is surprising how often mechanical shock forces of several magnitudes higher than the designer had ever allowed for are encountered. Mechanical strain gauges on belt weighers fail whe~ oversize rocks pass over them or the electrician decides to use he belt :0 transpor a motor to another part 0: the plan. Til switches used to detect blocked ore chutes tend to vanish completely when inadvertently caught between two pincershaped rocks and even when left intact they operate from ambient vibrations of the structure unless time delays are set so long that the chutes are completely choked and the conveyor belts wrecked before the trip circuit is actuated.
So often reports of poor reliability of instruments prove to be based on trivial faults which even unskilled electricians could have repaired. So often it is easier to leave equipment costing thousands of rands unserviced and to claim that it is not suitable for the job rather than to persevere with maintenance and calibration. Undoubtedly the mining en\iro~~ent is severe in many instances, not the least of the problems being the presence of highly abrasive grit particles in practically all solutions, but the biggest problem facing the instrument designer is the need to produce equipment that ideally never breaks down. If it does break down there is a danger that it will never be made to work again.
In many cases radio-active sources can be used very effectively and reliably, and they are used extensively in level detectors, belt weighers and density gauges. They have, of course, to be applied correctly and in many cases this has meant using far stronger sources than might have been expec ed. This has probably led to greater reliability because strong sources need heavy enclosures for safety against radiation and heavy construction has also resulted in mechanically robust construction.
Coming down to the more specific problems of instrumentation, a very general complaint is the lack of adequate documentation and instructions from the instrument manufacturer. It is noteworthy how many well designed instruments - particularly those supplied from 3uropean sources perform well but cannot be repaired because of lack of back-up serVicing or of documentation. Why are suppliers so reluctant to release this information 0 They might well note the trend for American supplies of equipment to provide very full and complete documentation.
Tt would be thought that an instrument which could surely be designed for reliable operation would be one for indicating the sump level of slime solutions. The fact of the matter is that every conceivable principle has been tried, ranging from floats through to differential pressure transducers, bubble tubes, capacity electrodes and ultrasonic detectors.
Another thorny problem, this time in the domain of the application engineer, is the problem of calibration. Comparing operating instruments with such procedures as belt cuts done rather haphazardly or the use of tonnage boxes of small capacity is just not good enough. There should be calibrating facilities either easily accessible nearby or, preferably, built in to the plant, and this calibration ~ust be accurate and reliable and must not introduce excessive lag times. There is nothing more frustrating than to test an instrument against another instrQ~ent and in the end not know which reading is reliable. The type of ir.strQ~ent which can be and mainta~ned by one tradesman should preferably always be used. If two or more tradesmen are involved it can always be assumed that the instrQ~ent will never work properly. An example of this has been belt-slip detectors. One type of unit originally used operated from electromagnetic pulses picked up from a disc with holes drilled into it and driven from the tail pulley. This hardly ever worked because of buckling of the disc caused by spillage, by over-enthusiastic operators using the disc as a fulcr~m point for levering out jammed rocks, or the result of the fitter having changed a tail ~nlley. If the fi ter had ensured that the disc was running true then the pic~-up ur.it did not work because the electrician had not i~stalled
750
0: these the ultrasonic detectors have proved in the end to be the most reliable. Initially they, too, had problems caused mainly by echoes returning through the mounting steelwork and ghosting echoes from the sides of the sumps. Mounting the units on resilient rubber diaphragms, choosing appropria e beam widths and prevailing on the manufacturers to incorporate range-tracking gates have overco~e most of these problems. However, maintenance is still high as the heads must be kept clean and have to be replaced frequently, and the connection cables between the heads and the electronic unit are very sensitive to ~oisture and capacitance effects. Another very vexing problel~ has been to obtain reliable operation of proportional control valves. A common feature of the cany valves tried has generally been the~r high failure rate which seems to bear a direct relationship to their cost. 1he most successful valves to date have been pneU"-atic types with a separate con~on air supply system. ,This combinatior. has, Of, course, violatea the prlnclple elaboratea earlier of the Q~desirable practice of mixing trades, and there have been many
~ ooint that ~ust be considered whp.n discussing the r01iability of auto~~tcd equipment is the environment in which the equipment operates. In the mineral processing industry the sensors are often subjected, for example, to quantities of large falling rocks, of pulp s reams containing very abrasive particles or of streams of only partially-dissolved chemicals or froths.
control centre and thus the control processing units at the centre of the plant, where he can watch the instrumentation and the machines at the same time. This is an ideal that is not normally practical if reliability is to be ensured. It will be interesting to hear how this problem has been overcome in operating plants.
The environments of these ins ruments tend to be wet and/or dusty and the instruments are often subjected to vibration, all of which must reduce reliability and which must be allowed for in the design of the instrument.
A major concern when instrumentation and automatic systems are being added to plants is the immediate reliability of the items. It is difficult to get operator acceptance of a new installation if it works for only part of the time. This becomes of great importance in the experimental stage of any installation and can be overcome only if the operators are kept fully informed at all stages of the work.
The siting of the automation processing control units is a function of reliability. These units normally require vibra ionless and dus -free surroundings, and are sited away from the main plant operations. Unless remote s ations are installed 0 relay the data to the centre of the plants, which would be costly, reliability is achieved at the expense of the operator. The operator would prefer to have the (conttl. next column).
I believe that this is enough in he way of a general introduction to this subject, and I look forward to the discussion on the importance of reliability in automation applications, a matter which is very near the heart of every operator of an automated plant.
0000000
Plant
Type of System
Date Inst.
Memory
Avail. for AUB· '76 %
Core Tinning Line
Data Log and Alarms
Feb. '68
12K 14 Bit
B.O.F.
Control with Jan. '74 manual back-up
16 Bit
Hot Strip Mill
48K 32K 24K 24K
No manual back June '74 up on S.P.C. 3 computers
24 Bit Batch Annealers
Limited manual Jan. '75 back-up
Tandem cold Mill
No manual back Feb. '75 up on 320's (mini comp) 4 computers
Control with manual B.D.
Dec. '74
Bulk
H/W
S/W
0
100
100
100
98,98
100
98,98
99,85 99,98 99,98
100 100 100
99,85 99,98 99,98
1,5M Disc 0 512K
256K
Drum
Overall
24K
128K Disc
95,50
98,77
94,32
16K
512K 0 0 0 Drum
98,43 100 100 99,85
98,4 98,84 100 100
98,12 98,84 100 99,85
0
100
99,5
99,5
16 Bi
52K 56K 52K 24 Bit 16 Bit
Temper Mill
:
32K 16 Bit
C. THERON:
doing automatic control and monitoring with no manual back-up.
The reliability of a computer system may be defined as the probability that a system will operate at an agreed level of performance for a specified period, subject to specified environmental conditions. It can be expressed as the ratio of the mean time between failures to the sum of the mean time between failures and the mean time to rectify.
Availability figures obtained in practice for various applications in the above categories are in the Table shown above. Apart from failure of the equipment itself, other factors may also influence the reliability of a system. These are interference from the operation or frvm nutside the system, factors which influence repair time, and back-up facilities for various functions.
Computer systems in the steel industry can be divided into three categories, namely, systems doing monitoring and data logging only with manual back-up, systems performing automatic control and datagathering with manual back-up and systems (con~d. next cQl~n below Table)
In order to prevent interference from outside the system there should be a stable power supply and good air conditioning
751
surpirsing moments when air lines have been opened and have been found to be full of water. It has at least been established that the air supply system to instruments should be completely separate from the mine air system.
be very accurate but as they are required to function only under abnormal conditions and therefore non-continuously, it is important that they be reliable. Failure of an instrument which normally works continuously is easily detected. This, however, will not happen, say, when the high-level alarm on a tank fails; such a failure is likely to be noticed only when the tank overflows, and is likely to occur more often than that of a temperature alarm on a grinding mill bearing, the overheating of which would have drastic results. Although planned maintenance will ensure that all such alarm devices are working at all times, the very large numbers used in modern automated plants makes this an almost impossible task and I should be interested to hear comments and discussion on this point.
Some success has been obtained by using hydraulic actuators coupled to Saunders valves and this combination is low in cos. It is too early to say whether these will replace the pneumatic valves. As a last word on the whole problem of instrumentation in the gold mines, while what has been said might be construed as being hyper-critical and even pessimistic, the enthusiastic efforts of a small core of people are rapidly improving all aspects of reliability ranging fro~ correct selection and application of instruments right through to the education and organisation of technicians and the provision of adequate testing and calibration facilities.
R.D.
The problems outlined so far are compounded when the instruments are included in automatic loops or systems. The instrument then becomes a link in a chain that includes controllers, processors and actuators. The failure of any of these will cause failure of the automation system. It is the reliability and the possibility of failure of the system that must be planned for in any installation. It must be determined whether an operator can take over control and whether control can be performed manually. But it must be borne in mind that on an automated plant the operators will tend not to have the experience of good plant control through lack of experience of manual operations. If this is the case, standby control units might be required and the remarks regarding the reliability of instruments which are not often used apply here also.
B~CK:
There is no such thing in mineral processing as a reliable instrQ"ent. This, no doubt, is a controversial enough statement for the maker of instrQments and some amplification is obviously necessary. I would thus like to talk briefly about the effects of this lack of reliability and throw out some points for discussion. To start with I want to suggest the ideal control circuit or instrument. This is equipment tha is installed and calibrated very simply, which has a high degree of accuracy and which then produces the required control action or information without further need for adjust~ent, maintenance or attention.
I should be interested to hear comments as to how this balance between instrument failure, operators' process control knowledge and the provision of standby equipment has been handled in practice.
The major requirement of such an installation is that it should be reliable. Firstly, I want to talk about instruas individual items for the logging of operating information and as alarms and then go on to discuss the reliability of control leaps and systems.
~ents
Although at all times a high degree of reliability is desir.d of any instrument, this degree of reliabili~y, or the ability th~ ~utput
0:
instru~~n:
to continue to giVE an
of the desired degree of accuracy, ~ay ce temper~d in relation to the specific automation or ~casure~ent application. The highest accuracy is usually reqUired of those instr~ents used for accounting. The most obvious is the weightometer. Since the tonnage processed in modern plants is very high, a small inaccuracy or unreliable result can give a jiscrepancy of thousands of tons. =f unreliable and inaccurate instruments are used to mcnitor or control the addition 0: reagents to a process, financial losses will be incurred because of either the addition of inadequate quantities of reagent with consequent low recovery, or the addition of excessive and therefore, wasteful, quantities of reagent. Alarms are not normally required to 752
The reliability of instruments and control systems can be maintained by good and regular servicing and calibration checks. The amount of work involved with each one and the extent of ~raining or qualifications of the man perfor~ing the work must be kept to a minimum. The need for specialist service engineers increases in proportion to the increasing complexity of the systems. The plug-in module system of electrical circuits has certainly helped to overcome this need, but the Doint must be brought strongly to the attention of manufacturers of equipment for this industry, since mining operations are often in remote parts of the world and it is difficult to attract personnel to maintain sophisticated electronic instruments at such places. Adequate means for the calibration of instruments ffiUSt be provided, but these are often omitted. The accuracy of an instrument is only as good as its" calibration and if the means for sampling and varying operating conditions are left out, the accuracy of the system will be poor. Off-line calibration is often a useful method but facilities for this are rarely seen on installations.
and the computer should be free of dust and vermin. It should also be ensured that no interference is caused during electrical isolation of the plant, and when switching relays. Electrical noise should also be avoided in switching large blocks of power. Power to the hardware should be disconnected only when this is absolutely necessary.
The advantages of this organization are that the technical staff specialise in hardware systems and peripherals and that no problem arises in interfacing between hardware and software staff. With regard to reliability of software, all software changes and the reasons for such changes should be documented. An up to date program should be available for reloading with minimum loss of time. Program listings and flow charts should be up to date and powerful reassembly facilities should be provided. Software errors can be prevented by bulk memory protection.
A good common earth for the computer should be provided isolated from the cabinet and any other earthing system. Down-time will be minimized by preventive maintenance. Repair time is influenced by a large number of factors. Spares should preferably be available in the form of electronic cards and as complete units for equipment such as analogue-to-digital converters, selsyns and ho metal detectors. They should be stored in the computer room in order to ensure rapid location of faults. Special test instruments such as storage scopes and paper and tape recorders are a grea relp in tracing intermittent faults.
Reliability can be improved by providing back-up facilities. It is not necessary, for example, 0 go off-line for all malfunc ions. In any system function failures can be divided in to three categories: (1)
Failures that are alarmed to notify only the operations personnel.
All documentation should be correct and up to date. Test descriptions and logic fault tracing methods are of grea help in reducing repair time. Descrip ions should be written in comprehensible English.
(2)
Failures which require that the operation be sopped and operator action be requested, as when the operator can override the decision of the compu er.
Communication panels which give access to various registers and step-by-step operating sequencing to check timing signals and alarm conditions are of great assistance in reducing repair time. Mimic panels which display sequencing are very necessary in some of the systems.
(3)
Failures which necessitate taking the system off-line, or, for example, in the case of a power failure or loss of position calibration.
Operating according to such categories will increase system up-time.
In the area of computer/plant diagnostics, the computer itself can be very useful in reporting on the malfunctioning of certain equipment, for example failure of hot-metal detectors or load ' cells.
An expensive, but perhaps worthwhile back-up is the provision of an extra main frame unit. Other extra items of equipment which can be provided as back-up facilities are primary data input card readers, typewriters and sensors.
As regards program loading, adequate facilities should exist to read-in programs rap~dly and correctly, particularly on systems which have no bulk memory.
F.W. YOLK: If we include Mr. Stubler in the panellists we hear voices from four different directions, which I wou~d like to put into two groups.
Motivated and skilled maintenance personnel are a very important factor in minimising down-time. The maintenance organization at ISCOR at Vanderbijlpark is as follows:
The first group is related to mineral processing with special regard to gold reduction plants. Instrumentation and automation are, as mentioned by the speakers, s ill in an early stage. The second group relates to highly sophisticated computercontrolled steelworks. I expected from all speakers at least a hint concerning the human inadequacy or failure within the total automatic system, but this fact was mentioned only by the first group who referred to artisan and operator faults and to lack of knowledge in general.
Mills area
For the last ten years I have observed advanced, partially automated, ore-dressing plants and have noted that their labour complements were low. A low labour force would, therefore, appear to be a prerequisite for further development of a high degree of automation.
SI W maintenance and development work Comp. H IW peripherals
753
For this reason the question is, is it right that the issue, manual operation versus automatic control is not the point of discussion, but rather the application of automation versus more reliable application of automation?
should be considered the most important instrument and should be made as reliable as possible. The interpretation of mean time to repair a failure should be done very carefully, taking into consideration the total number of failures which have occurred. A poor preventative maintenance routine will cause a high nQ~ber of minor failQres which, together with the norrr.al rate of serious failures, which normally take a long time to repair, will yield a relatively low mean time to repair. A high mean time to repair should thus not categorically be interpreted as bad craftsmanship in repair, since it could also be the result of a well funcioning preventative maintenance program.
H.W. SMITH: Too many systems are designed to include sensors which provide interesting but unnecessary information. When these fail, they are not maintained, thus giving a completely wrong impression of the system as a whole. Design simplicity is a very important precondition for system reliability. Many instruments, even when new, do not meet their specifications. Such evaluation services as SIRA (Great Britain) can be of great help in choosing the right instrument in the first place.
G.
I believe that a cost-benefit analysis of instrumentation and automation should be conducted to convince management of the importance of the equipme~t and staff employed in instrumentation.
D.e. FARQUHARSON: In our experience at ISeOR with a microprocessor connected to a load cell weigher, with regular computer-initiated electrical range checks, we have several times observed progressive failure of the load cell - test failed once a week, to once a day, to several. times a shift, to finally continuous failure.
Documentation is generally provided in sufficient amounts, but is lost or not understood by maintenance personnel. Staff turnover is obviously also an influence on good maintenance. I also believe that no facilities are available to calibrate beltweighers or magnetic flowmeters, but that such a facility should be available.
Perhaps with some statistical 'near failure' on-line monitoring, many sensor failures could be predicted in good time for repair or replacement of the sensor.
We achieve almost 99 percent extraction of gold and therefore the motivation for instrumentation is not as great as In the metal industry. Safety is generally built into the equipment in mineral processing and therefore the need for ins rumen ation to improve safety is not as dominant as in metal processing.
2. With small systems, particularly microprocessors, the need for airconditioning, dust filtering and rat-proofing is beginning to fall away. At least one process control installation in he ISeOR Vanderbijlpark Works has shown this over some three system-years. H.~.
eOHEN:
W. BOONZAISR:
The problems of costs underlie many of the questions of reliab~lity. If one examines the costs of processing equipment and the contained values of the materials being processed, one finds a large difference between mining/mineral processing and steel production. The high cost of rolling mills and the high value of steel ensure reliability through expensive instrumentation and control. In mineral production, the contained value of the ore is usually low and the processing equipment is far less costly than, say, rolling mills. Hence, relatively low-priced instrumentation is sought. The total costs of developing such equipment are inevitably out or proportion to the expected marKet value. Thus the problem arises of someone having to bear the development costs. In terms of reliability, it should be noted that these costs comprise not only the initial invention, design and construction of control instrumentation, but also on-stream development which is vital for bridging the gap between laboratory expectation and plant performance. R.R.W. ~ore
SO~lER:
In dealing with the question of multiple computer systems, the reliability of multip:e process systems on hot standby operation requires vigorous hardware and sof ware checks to ascertain, for instance, which computer of a two-computer hot standby system has developed a fault. ~xperience has shown that it is necessary to have a third electronics unit to carry out this monitoring and control fur.ction so that a clear and unambiguous decision can be made, especially when the hardware or software monitoring ~~its in the computers are causing the fault. R.
GR~~NWOOD-SMITH:
Reliability is obtainable at a cost. tends to stress low capital cost and the cheapest instrument is purchased. The difference in cost is often more than used in trying to get the cheap instruments to work.
~anagement
It is often the case where the user overspecifies the accuracy needed, for example, grade accuracy reqwrements .. The cost and complexity of the resultlng lnstruments leads to the purchase, or construction of, an unreliable instrument.
W~H&~ANN:
The safety monitoring instrument much so than the accounting instrument
754
The use of redundant instruments in a control system allows alternative calculation of the measurement to be performed from the results of other measurements. This can retain the integri ty of a control system after failure of an instrument.
W. WEIDEMANN:
In optimising the overall p~formance of a plant (and for overall safety or lack of disruptive disasters), would it not be advantageous to draw a 'consensus vs. expected frequency' curve?
If a control system can be seen on 'manual' for any length of time without 'management' noticing a reduced performance from the plant, then the control system is hardly worth having.
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A.R. ATKINS:
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u
DESIRED ENVELOPE
=>Q..
O(f)
A reliable system utilising automatic control depends upon the reliability of both the equipment as well as the operators and maintenance staff.
w~
a::: u.. zw > w
INCREASING COST OF DISORDER OR OUTAGE (LOG SCALE)
I do not think sufficient attention, particularly in the mining industry, is given to human factors concerning these two groups of people.
One could plot various 'disasters' vs. their expected frequency from faultfree analysis, making use of s a istical data for the components making up a system. This will focus attention on hose eventtrees where remedial action or re-design, or more frequen maintenance will prove most beneficial. Also, it will identify common reasons for outages (e.g. common power supply).
There is a lack of understanding of what is expected from the controls by the operator. Because of this the operatur is unaware of failure of an instrument. There is a lack of communication between management, operator and maintenance staff. The reliability of maintenance staff could be improved if they were given better training and enjoyed a better status. At the moment, most technicians rank with electricians in the mining industry. A.H. MOKKEN: I would like to correct a misconception with regard to the percentage of gold recovered in the gold mining industry's reduction works. Two previous speakers have mentioned a figure of 99 percent and have indicated that there is therefore little financial incentive to spend money on instrumentation and systems aimed at improving recovery. In the Union Corporation Group extraction averages about 96 percent from an overall grade of about 7 g/t. It is conceivable that an individual mine in the industry, treating a high grade nonrefractory ore, could reach 98 percent. If the ore grade was, say, 20 g/t, the amount discarded would be of the order of 0,4 g/t. This is still about 50 percent more gold per ton than that discarded on average by the mines of Union Corporation. During the period when the price of gold soared to $200 an ounce increasing attention was given to the magnitude of the gold loss. This is understandable but not condoned. Now is the time to look closely at the factors influencing the efficiency of gold extraction and install those facilities and controls that will promote recovery in order that we may be prepared to take full advantage of any future rise in the price of gold.
755