In-Plant Process Control
I
G. FRANKLIN TEMPLE Foote Mineral Co., Philadelphia 44, Pa.
The objective of any process control technique is to ensure final products that consistently meet specifications, maximum yield, and production rates maintained at minimum cost. In large continuous processes, automation generally fulfills these requirements most efficiently. Often, automatic instrumentation is the only means of ma inta ining the necessary precision control in highly specialized and critical unit Operations. But in many processes automatic control is not suitable-its cost in many batch or small continuous processes is prohibitively high, or a chemical or physical property capable of being measured by a simple probe may not be available. It is in these last two types of operations that manual control tests are employed most effectively.
tive capacity was reduced 40% because of the necessity of laboratory control in a purification step. Such undesirable delays nated when control tests in the plant by the oper addition to increasing productivity and reducing costs, plant tests give the operator the information he needs when and as often as he needs it, and in less time. This type of in-plant test is the the best substitute for automatic control and in many instances is a necessary development step to good automation. Laboratory Tests Are Not Suitable for Plant Use
Few laboratory procedures can be taken directly into the plant. They are developed for maximum accuracy and require precise weighings of small samples, careful pipetting of solutions, special equipment, and skilled manipulations. They stress accuracy a t the expense of time. A plant control test is usually designed as a “go-no-go)’ test. I t
must be rapid, preferably completed within one or two minutes. Accuracy is of secondary importance, and depending on the particular control ~ 2 0 %is generally sufficient and a realistic figure. The permissible range of accuracy is taken into consideration when limits are established. Plant control tests are usually developed from standard laboratory procedures by modifying the manipulations. Wet solids, of fairly constant moisture content, are measured volumetrically in aluminum or plastic measuring spoons that can be obtained from hardware and novelty stores. Slurries and liquids are measured in graduates instead of pipets, which are taboo for sanitary reasons. Automatic leveling burets are used for titrations and filled before each titration. Gravity and vacuum filtrations are easily performed. Limits are set, or tables prepared, so that the operator makes no calculations whatsoever. Arithmetic is his greatest weakness. A powerful incentive for in-plant
PROCESS
control may be maintained by the analysis of samples in a control laboratory staffed by technicians. T o obtain maximum productivity of the processing equipment, a laboratory must be manned around the clock. More often than not this is a waste of manpower and dollars. I t is impracticable if not impossible to schedule the laboratory work load so that the technicians’ time is used efficiently. I n addition, productive manpower is lost when samples are transported to the laboratory and plant operations are delayed pending results. I n one documented operation, produc-
Preparing raw sample for analysis in the plant control lab at Kings Mountain VOL. 51, NO. 7
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JULY 1959
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Here plant technician at Kings Mountain analyzes by specific gravity a sample of ground ore for its individual components
control is the fact that the average operator or helper is capable of performing any properly designed control test with reasonable accuracy. He can also use such tests to lighten his work load, and even recommend process improvements
that will increase quality and yield. Many technically trained men overlook the tremendous potential for improvement in processing techniques that exists with the operators. The manual control test is an excellent means of developing and
Samples being raised to a very high temperature, one step in the analytical procedure
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
using this potential. It is a real value not gained by automation. Case Histories
Lithium Carbonate ManufacI n the production of lithium carbonate from lithium chloride brines, production per man-hour was increased and raw material costs were reduced by changing a control. This was the test for determining completeness of precipitation. Originally, a fixed weight of soda ash was added to a given volume of brine; a sample was taken and filtered. To a 100-ml. filtrate, 20 ml. of 12% Na2C08 solution were added and the whole was heated. When no precipitate appeared, the batch was finished. This procedure required three to five adjustments and an excessive amount of soda ash. T h e quantity of excess soda ash required for maximum precipitation and the alkalinity of the mother liquor were determined by laboratory tests. The following control test was then developed and given to the operators: ture.
1. Filter a sample of slurry. 2. Measure 25 ml. of clear liquor in
I N - P L A N T PROCESS C O N T R O L a graduate and pour into beaker. 3. Rinse graduate twice with water, adding rinses to beaker. 4. Add 2 to 3 drops of “phenol” indicator. 5. Add 0.5N HzS04 from buret until pink color disappears. 6. Refill buret. 7 . Add 3 to 5 drops of methyl orange indicator and 0.5N HzS04 until color changes from yellow to red. 8. Read buret. The batch is finished when the reading is 11 to 13. This control reduced the soda ash used to within = t l % of the desired amount. After a few days the operators themselves had established a relationship between titers below 13 and the number of bags of soda ash required for adjustment. This information was incorporated into the tests and batches were made regularly with only one adjustment. Lithium Monoxide ,Extraction. Very often an economical direct control test suitable for use in the plant cannot be found. This is often true in lithium processing, because Liz0 analyses are made by flame photometry. Here, the difficulty is circumvented by use of indirect methods. I n the extraction of Liz0 from ores by the lime process, it is necessary to maintain close control of the washing of the residues in order to obtain maximum yields. A control based on the alkalinity of the residual moisture in the filter cake was unsatisfactory because the minimum alkalinity was equivalent to
.
Impurities in excess of a few duction of high purity manganese
that of lime and bore ries of laboratory tests that this minimum alkalin the sum of the lithium and hydroxides present. control based on the li
as
ssible. An adaptation of the rocedure for determining hardness in water by EDTA titration with Eriochrome black T indicator was eloped. With this test it has n possible to maintain a washing iency of 98y0or better. Spodumene Beneficiation. Bene-
A simple acid-alkali titration, using an indicator with a well defined end point, provides an easy and accurate control test for the determination of liquor strengths in the extraction of lithium from its ore, spodumene (Sunbright, Va.) VOL. 51, NO. 7
JULY 1959
823
ficiation of spodumene by flotation processes presented a challenge to process control. Laboratory analysis by flame photometry required several hours. Process adjustments lagged behind production by as much as 8 hours and it was impossible to maintain product quality at any level. Actual control depended entirely on the empiric judgment of the operators on each shift. A control test accurate to within a few per cent was developed on the basis of heavy-media separations. A 3-gram sample is weighed on a torsion balance, shaken with a liquid having a specific gravity slightly less than spodumene, and centrifuged. T h e float material
technical manpower substantially. Uniformity of product, increase in quality, smoother operations, and significant reductions in costs have been obtained by giving the operators this close control of the process. Lithium Extraction. T h e effective control of the countercurrent leaching and washing process for the extraction of lithium from the spodumene-lime clinker depends primarily on a n accurate determination of liquor strengths in the digest and thickener system. Fortunately, this can be readily obtained by one of the simplest techniques in analysis--s straight acid-alkali titration, using an indicator with a sharp, clear end
Equipment used for indirect test controlling cell feed in the manufacture of electrolytic manganese (Knoxville, Tenn.)
Buret Reading, M1.
10.0
Strength, % LiOH 0.06 0.12 0.18 0.24 0.30 0.60
15 20 25
0.90 1.20 1.50
30
1.80 2.10 2.40
1.0
2.0
3.0 4.0
5.0
35 40
Experience has discouraged the use of simple burets. If the worker does not refill to zero each time, he must subtract the reading at the start from the reading at the end. Failure to observe this procedure has caused no end of difficulties, and discouraged several otherwise very capable operators. Nor is analytical automation the answer. Because of the large volumes handled, the slow rate of change of strengths, and the scaling tendencies of the liquors, automatic titrators have not proved practical. Best results have been obtained with self-zeroing burets. Manufacture of Electrolytic Manganese. Another indirect conrol test has been successfully applied: the plating test for the control of cell feed in the production of electrolytic manganese. Purity of cell feed is of primary importance and impurities are controlled to a few parts per million by chemical tests made by a well trained laboratory technician. T o ensure satisfactory final cell feed, the following performance test has been devised to evaluate it quickly. PROCEDURE FOR PLATING TEST
and the liquid are carefully decanted. The remaining solids are rinsed with benzene, dried, and weighed. T h e spodumene content is then determined from a chart based on the weight of sinks and the LizO analysis of the ore. When the composite on analysis indicates the need, the chart is changed* Because Of the weighings required and the drying of flammable liquids, a small control room has been- constructed in the flotation area. Analyses are made by one of the shift Operators and require approximateb' 15 minutes Per test* These new tests have reduced the cost of
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1. Add SO2 solution to 1 liter of cell feed solution. 2. Prepare a test cathode by washing, buffing, and degreasing. Dry thorLIQUOR oughly. DETERMINATIONOF 3. Heat the solution to 36-8O C. IN DIGESTAND THICKSTRENGTHS 4. Place the cathode in the cell. Fill ENERSYSTEM. the cell with the solution to be tested. 5 . Plate for 30 minutes. Maintain 1. Filter the sample. the current at the red arrow on the 2. Measure 20 ml. of filtered liquor in meter. a 25-ml. graduate. Transfer to a 2506 . When plating is finished, remove ml. beaker. Fill the graduate with the cathode, dip in dichromate solution, water and transfer water"to the beaker. wash thoroughly, and dry. Add 5 drops of phenol indicator. 7. Compare cathode with standard. to the 0 mark with Fill the buret 3. Plate must be bright, show no peeling, 0.5N acid. While stirring constantly, and no rounded corners. add acid from the buret until the pi*& 8. Advise supervisor immediately if color just disappears. Read the buret and find liquor strength from the table. the plate is not standard in any way.
point. The test used by plant operators a t Sunbright is described below.
INDUSTRIAL AND ENGINEERING CHEMISTRY
