US2886051A - Density control - Google Patents

Description

May 12, 1959 A. FQKROLL ETAL' DENSITY CONTROL Filed April 26, 1955 kil i film SLURRY FROM MILL CLASSIFIER D we S L MLMH TLAE mm .M VK Wm WEW N TK mwN W wmm 4A AKF m D N A Y T G NR IE T A W 0 3 2 k 2,886,051 2 Pa,teni;ed May 12, 1:959."

DENSITY CONTROL Alvin F. Kroll, Kellogg, Idaho, and Kingston E. Atwood and Frank M. Mehle, Seattle, Wash., assignors to Bailey Meter Company, a corporation of Delaware Application April 26, 1955, Serial No. 503,846 6 Claims. (Cl. 137-91) The present invention relates to the control of the density of a homogenous mixture of fluid and solid material of relatively small particle size.

The invention, more specifically, relates to the control of the density of a mixture of ore which has been mined and crushed, and suspended in a liquid such as water. The ore is, conventionally, crushed in a ball mill and mixed with water in a classifier. The majority of systems presently arranged to measure the density of this mixture in classifiers employ bubble-tubes as primary elements. The present invention is embodied in a system utilizing this type of primary element.

Some of the systems presently utilizing bubble-tube primary elements, place them. at difierent locations associated with the classifiers inan attempt to get an repre-- sentativemeasurement of the mixture. In some systems the measurement is made in the classifier pool of the mixture. Other schemes establish a sampling cell through Whichonly a small sample of the classifier output flows, and place the bubble-tube primary elements therein. The systems previously employed .have short-comings which include inaccuracy, excessive maintenance and unreliability.

The present invention has as a general object the provision of a system for measuring the density of a mixture of'liquid and crushed solid particles.

Another object of the invention is to provide a control system for the relative amounts of liquid and crushed solid particles making up a mixture, using bubble-pipes arranged at a point in the mixture consistently representative of the percent solids in the liquid.

A more specific object of. the invention is theprovision of a condition of the measured mixture which is consistently representative of the density condition of the mixture.

A still more specific object of the invention is .to provide a structure which will bring the mixture whose density is being measured to bubble-pipes sensing the condition so as to eliminate the efiect'of other factors on the measurement.

Still another object of the invention is to provide an improved system for the measurement and control of density which is accurate, trouble-free and durable.

In the drawing:

The single figure diagrammatically represents a me ferred form and arrangement of elements in which the present invention is embodied.

In the interest of a more comprehensive appreciation of the invention, attention is directed to the fact that the treating of non-ferrous ores to recover the metal from the native ore, has, as an initial step, the dressing of the ore. After the ore leaves the mine, it goes into crushers which reduce the raw ore to a size varying from V2 inch to an inch in diameter. Leaving the crushers, the ore is taken into ball mills where its size is further reduced to a point Where it resembles sand.

The ore is next taken from the ball mills and passed.

into a classifier where thefines are removed and the coarse ore is returned; to the ball mill for additional grinds ing. The fines then go into the flotation circuits wherein the ore is concentrated by removing. a large part of the undesirable earthen deposits. After the concentrate is thickened and dried it is ready for the smelter.

The ball mill is a large cylindrical drum varying in; diameter from 5 to. 10 feet and having about the same length. The drum rotateswith its axis of rotation horizontal, at a constant speed of about 10 to 30 r.p.m. de-

pending upon the mill size. Iron balls Within the drum do the crushing. Raw ore is fed into one end and the crushed. ore is washed out of the opposite end. The process is continuous.

The mixture of crushed ore. and water leaving the ball mills is referred to as slurry which is passed into the classifier. The purpose of the classifier is. to remove. the.

fines from the slurry and return the coarse particles back to the mill for regrinding. Operation is accomplished by floating off the lines which are suspended in the Water. By controlling the amount of Waterptothe classifier, the

density of the slurry in the classifier can be controlled.v

If the slurry becomes too. diluted and its density falls, then only the finest particles will be held in suspension and floated oflf. If the density is increased, the larger.

particles will be held in suspension and also floated ofi.

Hence, it is clear that the particle size desired must be obtained by a control, of density.

At this point it is well to define, more specifically, the variable being controlled. Density is a convenient term of reference for thisvariable condition. Actually, the.

percent solids by Weight in a water mixture is thevariable controlled. It is recognized that should the specific gravity of the constantly supplied ore vary, the density or" the classifier slurry output would vary it the water supply to the slurry remained constant. However, in the present instance, it is presumed that the specific gravity.

of the ore will remain constant and the calibration of the.

system for the measurement of density can be given in terms of percent solids. Therefore, in using the term density it is to be understood that the percent solids is a synonymous term therewith.

The slurry in the classifier is continually agitated by rakes of various forms, depending upon the type of classifier. The coarse particles of ore are eventually raked up an incline and thenejected from the classifier whereupon they are returned to the ball mill for the aforementioned recrushing.

As indicated supra, the control over the density in the classifier is through the introduction of diluting water. If a satisfactory system is provided to continually determine th edensity of the classifier overflow slurry, the diluting Water valve can be manipulated in accordance With-1h determination to maintain the density of the slurry in the classifier satisfactorily constant.

The major reasons for controlling classifier overflow u slurry density are seen as:

' urement Where the variable density is satisfactorily represented. The structure at the point of measurement accommodates bubble-tubes as specific primary elements responsive to the variable.

Prior attempts to locate bubble-tubes, as primary eleQ 'ments, in the conduit from the classifier have been unsatisfactorybecause turbulence and frothing of the slurry prevented obtaining a true representation of the density. The bubble-tubes were also positioned directly in the slurry pool within the classifier, but this was not satisfactory because a variation in density, in both a vertical and horizontal plane, was found to exist within the pool, possibly due to stratification and settling.

iBubbletubes have been installed on a float in the classifier slurry pool in order to obviate any changes in depth due to the change in level of the classifier pool. Also,- a weir has been installed on the classifier lip to converge and mix the slurry before it overflowed. These arrangements have not been satisfactory because the variation in density with depth is, itself, a variable over a period of hours or days in the operation of a classifier. tually, the structure in which the present invention is embodied was evolved to provide a point at which the entire classifier overflow will pass and at which point its density can be measured. The structure minimizes turbulence of the slurry and insures a uniform velocity of the slurry at the point of bubble-tube location. The function of this structure is, therefore, to provide a point of density measurement representative of the average, or true,.density condition of the entire body of slurry coming from the classifier.

Reference is now made specifically to the drawing where the classifier overflow slurry is depicted as brought to the point of measurement by conduit 1. The conduit 1 carries the entire slurry product from a classifier 1A arranged to receive raw slurry from a mill (not shown). Coarse particles are separated from the slurry in the classifier and returned to the mill for regrinding as indicated schematically on the drawing. In general, conduit 1 supplies a funnel, or cone 2, which specifically provides the point of measurement. The slurry overflows this funnel-cone and is received by surrounding funnel-cone 3 which passes the slurry on down conduit 4.

Bubble- tubes 5 and 6 have their open ends submerged in the slurry within cone 2. These tubes are supplied a constant amount of air. The farther a bubble-tube is submerged into the slurry, the greater will be the pressure required to bubble air through the tube. If one bubble-tube is immersed farther than the other, a greater pressure will be required to bubble the same amount of air through the deeper one. Using this system of two bubble-tubes, and submerging one farther than the other, two pressures may be obtained. The difference between these two pressures then serves as an index of density. As the density of the slurry increases, the difference between the two pressures will increase and vice versa.

The supply of air for bubble- tubes 5 and 6 is provided by constant differential controllers 7 and 8. These controllers may take the form of simple constant-differential relay-sight gage-control valve combinations, forming the subject matter of catalog 29 of the Fischer & Porter Company, copyrighted in 1952. The sight gages give an indication of the amount of air going to their respective bubble-tubes from their relays. The control valves are adjusted to supply the same amount of air to each bubbletube, and the resulting differential in back pressures between connecting pipes 9 and 10 will vary with the density of the slurry within cone 2.

The back-pressures of pipes 9 and 10 are applied to opposite sides of a diaphragm 11. Diaphragm 11 is mechanically linked to one-half of a fluid pressure couple 12. The fluid pressure couple 12, in turn, controls the output of an amplifying relay 13 whose output loading pressure is, therefore, proportional to the density of the slurry in cone 2. The diaphragm-couple-relay combination is combined in an indicator-controller 14 which establishes the output of relay 13 in pipe 15 for use in control over the slurry density.

' Sensitive to the pressure of pipe 15, a bellows-operated y mechanism 16 is located in a recorder 17. Mechanism 16 may take the form of that structure disclosed in Panich Evenically actuates a fluid pressure couple which controls a relay 20, similar to relay 13. The output of relay 20 is, therefore, controlled by the function of the bellows, and their mechanical linkage with the fluid pressure couple, to produce a pressure in pipe 21 which is a predetermined modification of that in pipe 15.

The modified pressure in pipe 21 is then passed through selector valve 22, which may take the'form of that structure disclosed in Dickey et a1. Serial No. 251,406, filed October 15, 1951, now Patent No. 2,729,222, and applied to air-operated Water control valve 23. The variations in the air pressure passed through selector valve 22, and applied to water control valve 23, vary the opening of the valve and, consequently, the amount of diluting water into the classifier. The variation of the amount of diluting water adjusts the density of the overflow slurry passing through conduit 1 and into cone 2. With bubbletubes 5 and 6 exposed to a true representation of the density of the slurry, control may be exerted over this classifier overflow by the disclosed system of the drawing to maintain its density at a desired value.

The success of the system is primarily due to the function of the sampling funnel, or cone 2, in cooperation with funnel, or cone 3. The'er'nphasis is justified that by weight in water. the funnel, and the shape of the funnel, are such that the slurry remains fairly uniform. At the same time, the

funnel reduces the turbulence of the slurry to the point that its density can be measured readily. The inlet pipe 1 into the funnel 2 makes a right angled turn into the' cone and in so doing creates turbulence with consequent agitation and mixing of the liquid and the solid particles before entering the funnel proper. The turn is purposely not streamlined. The cone then does two things; it tends to reduce turbulence at the point of measurement by decreasing the velocity which in turn minimizes the error resulting from the difference in velocity head being felt by the two bubble tubes. The vertical height of the funnel proper is short to prevent stratification.

In the drawing, pertinent relative dimensions of the structure have been indicated. As in the actual reduction to practice, an eight inch diameter has been indicated for conduit 1, fixing the rate of slurry through the conduit at approximately 200 gallons per minute. Cone 2, actually a geometrical frustum, is 7% inches high, with upper and lower diameters set at 20 inches and 8% inches. Cone 3, arranged about cone 2, is also a geometrical frustum with a height of 7 inches and diameters of 24 and 32 inches. Diameter of discharge conduit 4 is fixed at 8 inches, the same as that of conduit 1.

The result achieved by this structure, and the system in which it is combined, has produced a more uniform operation of classifiers. The sizing of the particle output is more uniform than heretofore attained, and there is an improved flotation control, lower tailings and higher grade concentrates. Uniform particle sizing has leveled operation. Indications have been obtained that the original tonnage fed to the ball mills of the classifier has increased. The value of the automatic control of density in classifiers has been clearly established by the invention embodied in the structure herein disclosed.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. A control system for measuring and calibrating the density of a slurry including, a classifying device, a conduit carrying the entire slurry discharge from the classifying device, a funnel structure vertically arranged and defining a continuous flow line with the conduit, the funnel structure receiving all the slurry discharge in its small lower end from the conduit in such manner that the substance overflows the large end of the funnel without turbulence and at a consistent level, detecting means positioned in the substance within the funnel for producing two pressures whose diflerential is an indication of density of the substance, means differentially responsive to the two pressures for establishing a single fluid pressure of different order representative of the variation in differentials, and valve means responsive to the single fluid pressure for control of an agent to the substance of the conduit which will vary the density thereof.

2. The system of claim 1 in which the detecting means are bubble-tubes vertically inserted into the mixture within the conical frustum at equal distances from the axis thereof and to different depths and supplied equal amounts of air.

3. The system of claim 2 in which the means differentially responsive to the pressures of the bubble-tubes is a movable wall which actuates a fluid pressure relay to establish the single fluid pressure representative of the difierential variation.

4. A system for the measurement and control of the density of a slurry including, a classifier, a conduit for receiving all of the output slurry of the classifier, a conical frustum attached to the end of the conduit by its small end and vertically arranged in order that the slurry will spill over the large end at the top without turbulence 6 and at a uniform rate, the conical frustum and conduit defining a continuous flow line carrying all the discharge slurry of the classifier, a bubble-tube system including two spaced vertical tubes arranged in the conical frustum section to different depths but substantially equidistant from the axis thereof in order to detect the density of the slurry, a differential pressure controller responsive to the bubble-tube system for establishing a fluid control pressure, and a control system responsive to the fluid pressure established by the differential pressure controller for regulating dilution water into the slurry of the classifier.

5. The system of claim 4 in which the conduit is 8 inches in diameter, the slurry passes into the frustum at the rate of substantially 200 gal/min, and the conical frustum is 7% inches high with a lower diameter of 8% inches and an upper diameter of 20 inches.

6. The system of claim 4 in which the said conduit is horizontal and cylindrical and has a flat end closure adjacent the connection to said frustum to provide right angled entry of the slurry into the cone to create turbulence.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,678 Adams June 25, 1940 2,577,548 Vetter Dec. 4, 1951 FOREIGN PATENTS 134,852 Austria Oct. 10, 1933 261,716 Germany June 25, 1913 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0o -2 '886,05l May 12, 1959 Column 5, line 1, for "calibrating" read controlling Signed and sealed this 11th day of April 1961 (SEAL) Attest:

ERNEsT w; SVVIDER ARTHUR W. CROCKER Attestmg Ufiicer A i ommissioner of Patents

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