Salt Lake Mining Review | 1917-08-15 | Page 24 | Answers to Questions on the Flotation of Ores

answers to questions on the flotation of ores BY OLIVER C RALSTON the concentration of sulphide ores particularly ticul arly low grade ores carrying gold s silver R copper lead zinc and other metals seems to be undergoing a revolution except for ores that could be smelted smelter directly or for gold and silver ores that could be treated by stamp milling with or without cyaniding cyan iding or the generally accepted plan of concentrating sulphide ores has been concentration by gravity involving the use of jigs buddies banners van ners or tables within the past fifteen years however metallurgists throughout the world have come to recognize the enormous possibilities ties of concentration methods based on entirely different principles the surface 01 tension of liquids and the adhesion of liquid films to the surfaces of minerals these principles are used in what are known as flotation processes ingalls has aptly characterized flotation as concentration upside down because usually the heavier or ore ini mineral rises while tile lighter gangue mineral sinks for convenience flotation may be defined as tile the process or processes by which the valuable minerals in a mass of finely ground ore are caused to float on a liquid into which the finely ground ore is fed flotation processes can be grouped as of two types film flotation and froth flotation in film flotation the floating mineral particles are sustained on the surface film of the liquid in froth flotation the minerals floated gather in and on the surfaces of bubbles of air or gas driven into or generated within the liquid As the surface of a mass of bubbles overlying a liquid is greater than that of the upper surface of 0 r the liquid itself a froth will carry a greater burden per square toot foot of area of liquid than will the film surface of the liquid in the application of flotation there is s opportunity for endless variety of detail thus in froth flotation the bubbles may be produced by beating air into the pulp or ore carrying liquid by forcing air through gli it by generating a gas through the a action of acids in the liquid on some constitute nt of the ore or by applying reduced pressure to the surface of a pulp saturated with air the great field of flotation has been the preventing of slime losses at ordinary concentrating cent rating mills for ores containing valuable sulphides sulp hides or native metals also flotation is particularly adapted tor for the treatment of an ore carrying valuable minerals in very small grains As regards gold anc and I 1 silver ores ordinarily treated by tion some can be profitably treated by flotation and from some it is possible to united states bureau of mines abines depart ment mp sul of the alper interior 01 1 washington D c technical pape r I 1 recover gold and silver that can not be profitably recovered by the cyanide process however it is hardly probable that flotation will completely displace the cyanide process for gold and silver ore or leaching processes for copper ore because the latter produce metal for shipment whereas flotation ordinarily yields only high grade concentrates cen for shipment to smelters shelters sm elters so rapidly is the art of flotation advancing that any complete treatise on oil the subject is likely to be out of date by the time it is printed however as tile the bureau of mines aunes is continually receiving inquiries regarding a number of more or less practical questions that are seemingly not answered with sufficient clearness in the existing literature era ture attempt is iliade made in this paper to answer such questions as are more pertinent and important answers to some of the questions are to be found in tile the literature of flotation but the majority possibly because they art ar decidedly practical seem to have been overlooked or left unanswered this paper does not attempt to discuss the theories put forth to account for the facts accumulated by tile the many experiments flotation nor does it attempt to describe in detail flotation plants nor the procedure in diff different erent mills rather it seeks to answer questions that are apt to be asked by persons who wish to know the possibility of successfully applying flotation to a given ore or mixture of ores 1 what types of ores can be treated by flotation in general it may be said that only the sulphide ores that is the sulphides sulp hides of copper lead zinc silver iron and other metals can be and are being treated successfully by the flotation methods now in use however free gold and gold telluride can be floated as well as native silver and native copper recently there has been some success in oxidized ores by treating a finely ground pulp with solutions of sodium sulphide or hydrogen sulphide after the particles of an oxidized ore have been converted superficially into sulphides sulp hides they can be floated in the same way as natural sulphides sulp hides no commercial flotation of oxidized minerals without i is known at the time of the writing of tills this paper 2 what are the commercially im methods of flotation nearly all the commercially important plants use froth flotation in this pro process cess tile the finely ground ore suspended in water to form a pulp is isso so treated that a froth will form on its surface and the proper conditions are arranged so that the commercially merci ally valuable sulphides sulp hides will adhere selectively to the thin films of t the ie a p bubbles that make up the froth usually these bubbles are of air which has been introduced trod by beating it in with mechanical beaters or impellers or by injecting corn com pressed air into the ore pulp by subjecting such a pulp to a vacuum the dissolved air in the water is liberated in small bubbles which likewise rise and form a froth bubbles of gas may be made by adding acid to the pulp in the presence of carbonate minerals like calcite and siderite jv with th tu the result that carbon dioxide bubbles form in the body of tile the pulp and rise to form a f roth froth thus froth notation flotation is recognized under four heads mechanical 11 pneumatic vacuum and acid frothing methods essentially the same underlying principles are involved in them all ir in present practice froth notation flotation alwa always ys depends on air or gas bubbles for the flotation of the mineral particles and the oils added are not used for their buoyant effect as they were formerly in bulk oil flotation usually a frothing agent is added in order to aid the formation of a froth and to increase the tendency of the liquid films of gas bubbles to attach themselves selectively to only the sulphide particles of the ore frothing agents are usually oils or substances of oily consistence and hence the use of the misnomer oil flotation skin flotation or film flotation is at present used in concentrating a very small tonnage of ore practically troth froth flotation may be regarded as the only type used 3 what are the fields of skin iskin and of froth flotation the application of processes for concentrating ores is limited by the types of ore they are capable of treating most alost of the film flotation methods are expensive because of tile the small weight of mineral that can be floated at any one time on the surface film of water they are not applicable to the treatment of finely divided slimes froth fa flotation and as the present popularity of depends on its success in the treatment of ore slimes the reader can easily see that skin methods have not Is as wide avide a field as frothing methods skin flotation by tons method is at present in m use at tile ali for the treatment of table middling s morning mill at lU mullen tillen idaho idabo barite ai alid siderite are there present as gangue enill minerals zinc and lead ie containing in a middling minerals elnera s sulphides sulp hides As these he heavy avy gangue methods mauo can not be separated by gravity alternative 0 of f grind gr of concentration the or 0 flotation in ing them fine enough for froth has ha 10 flotation of treating them by skin to sur method date enabled the latter he at ai the competition of froth flotation ore testing plant of H E wood of denver is also a skin notation flotation machine removing from a low grade ore probably froth flotation could be used for every separation of minerals that Is now being accomplished by skin flotation provided tile the material were ground somewhat finer although 20 mesh material can be successfully floated by skin flotation 50 mesh material is usually about the limit of coarseness for froth flotation flotation of some coarser particles than those mentioned has been observed but usually there were large losses of the valuable minerals miner alK in tile the coarser sizes 4 what Is the difference between mechanical and pneumatic flotation froths frothy As s the froth produced by mechanical methods of beating air into a pulp is more or less tough and permanent whereas tile the froth produced with the same frothing agents by releasing small bubbles of compressed air in the pulp is rather evanescent many people have wondered whether one froth was not different in kind from the other air bubbles produce both froths frothy and often the same quantities of oil are used in pulps culps made up with the same quantities of water yet the pneumatic froth dies as soon as the supply of air is shut off and tile the mechanical froth does not break down for a considerable length of time occasionally weeks the froths frothy usually look much alike while the machines are running except thai the froth on the pneumatic machines appears whiter and does not seem to contain as much mineral As far as can be learned the differ difference eDee between the two froths frothy lies largely in the different percentages of air and moisture as compared to the amount of solids the continual supply of air by the pneumatic method produces a condition under which each rising bubble carries only a few grains of mineral whereas in the mechanical methods the bubbles come up fairly heavily laden when the bubbles burst the minerals carried by them are caught by the bubbles just below A mechanical machine is provided with a box in which the pulp is relatively quiet and consequently there is opportunity port unity for the collection of froth which is burdened to the limit the close packing of the mineral particles seems to give strength to the froth bubbles so that such a mechanical troth froth will stand for a considerable length of time before it breaks down much of the water has drained out of it then so that often it may be called rightly a froth of mineral mud if a pneumatic machine is allowed to run for some time with no removal of froth the layer of froth on the top finally becomes similarly loaded although the continual rush of large volumes of air through this layer keeps it more or less fluffy and expanded and wet ter than a mechanical froth on letting such a froth subside in a quiet place a small amount of permanent froth will remain consequently one may say that a pneumatic froth probably contains more water and more air for a given weight of supported mineral 5 what are the metallurgical and eco differences between mechanical ch anical and pneumatic froth flotation the two methods of flotation give nearly near ay the same metallurgical results however comparisons made in a number of competitive tests indicate that the mechanical machines give a somewhat better extraction of the coarse particles of mineral and that tile the pneumatic machines make a better separation of the finer slimes provided the oil has been well mixed with the pulp with the same pulp the pneumatic machines have given a very slightly better total extraction than the mechanical machines in practice the machine that gives the highest total extraction is usually considered the best this is especially true in copper mills in zinc mills the highest possible grade of concentrate is of importance and should be considered operation of tile the pneumatic system requires slightly less power than most of the mechanical machines especially if the oil or other frothing agent has been added to the pulp before the latter passes to the tube mills in either system the grinding machinery requires the same amount of power and it might as well mix the oil with the ore with such mixing the saving of power by pneumatic machines is from a half to one kilowatt hour per ton of ore if the experience of the larger copper mills is to be considered as determining the point the pneumatic machines also require less labor 6 what Is the effect of diluting the pulp in froth flotation if a given ore be crushed to a definite size and treated by flotation tile the more dilute the pulp the cleaner is the concentrate this is because less gangue is in trained in ill the froth made from the more dilute pulp it is also noticed that the extraction also falls off mainly because the coarser particles are not floated it would seem that in more concentrated pulps culps the smaller particles of mineral assist the floating of tile the larger particles when flotation is attempted in dilute pulp these larger particles are not as close to their smaller neighbors and are not carried into the froth with them if tile the ore is already finely ground tile the thicker pulps culps often give such a poor concentrate that dilution greatly increases the grade or of concentrate in fact some slimes can not be treated in any other than a dilute pulp five or more tons of water these slimes are usually to one ton of ore from ores with a gangue containing much colloidal matter another effect noticed on treating pulps culps of different dilution is that dilute dains pulps re rc quire more oil for a good froth than do thicker pulps culps of course there is a greater weight of water to be oiled and like likewise wisp there is more water to be agitated or otherwise treated in order to form a froth this means a greater expenditure of power per ton of ore treated 7 what difference Is there between flotation of granular matter and of slimes from the foregoing discussion it is evident that granular matter can be treated in a very thick pulp whereas the successful treatment of slimes usually requires a more dilute pulp of course the surface presented by a ton of slime is immensely larger than the surface of the particles in hn a ton of sand so that more oil is needed in treating slimes furthermore when the material is very finely divided it usually tends to gather into flocks in some of which a number of gangue particles may surround each sulphide particle such flocks must be broken up in one way or another in order that the sulphides sulp hides can be oiled and floated one way of accomplishing this is by extreme agitation such as is possible in the janney flotation machines another way is to the gangue slimes by the addition of a such ass as caustic soda sodium silicate or similar agent because of the necessity of floating the slimes in a more dilute pulp the greater amount of oil power water etc involved makes the treatment more expensive the use of a dilute pulp cuts down the capacity of a flotation machine as far as the tonnage of ore is concerned and sometimes it even cuts down the total volume of water and ore together put through the machine thus one can say in general that a given machine will have a greater capacity for granular matter than for slimes A true colloidal slime such as has been called a primary slime resulting from pulverization of the constituents ov of the ore or gangue often proves extremely difficult to treat by flotation in this COU coo lection tion the work of the metallurgists at tile the mill enill of the inspiration consolidated copper company has been most in interesting terest the primary slime produced at that mill could not be successfully treated by itself addition of iron filings made possible the formation of a good mineral froth and later it was found