SCOPE AND A.
ECONOMICS OF ELECTROSTATIC SEPARATION
Ruf&c&
136 Liberty Street, New York. N. Y. of Mines and individual investigators on the apparatus used and conditioning of the feed, improvements in plant 4k conditioning, and improved ehct r i a l eppmtus will aid further development. Attention is u l l e d to the recent development of a separator using nonconducting electrodes to promote safety, unifonnity of field, and more precise regulation of field strength. The costs of four inr*llations and a list of some products now being commercially separated on eIcdror*tic reparotors are given.
Historical development, the theory of electrostatic separation, and the action of the fundamental separator are reviewed. Difference in behavior of conductors and nonconductors, materials of differ. ent dielecbic strength, and materials subject to pyroelectric charge are outlined. Attention i s u l l e d to the possibilities of elecbiul and chemical conditioning of the ked and to the possible impmwment of separation through the selection of pmpe~ supporting surfaces. Dwelopment work by the United Stater Bureau
LECl'EQBTxTIC separation is the of separating E one material from another with charged .el&mdw. They may be a k & with either pimitive or negative eleb
posite charge in the surrounding space. HoweF, the statament that like chargea repel esch other describeswhat appears to happen, and it has bean used in texthooks and in articles on electroetetioseparation for so long that ita continued UBB
art
triaity, whiohever is the more e5cient for the pmblem. The mat&& in the mixture must be phynidly eqmrete and they must carry surface chargea whioh d s e r either in dgn or degree, so that one member may be either attrscted or lapsllen m an opposite manner to the other mat8rialor to a much greater extent than the other material.
&?eQlBadviseble.
The ditierenoe in charges or the variation in strength of chsrge whioh makes electmststio s e w t i o n possible m y
be due to one or more 0 ~ 8 8 8 . Aocordinp to tbeir origin, theae chsgea may be deeoribed an contaot, conductance, p y m e b
The Apparatus Redncad to ita fnudamntal form, an eleotmstatic sepsrator may be pioaYaa an in Figure 1. A feed hopper, A, fitted with a gate, B, is placed over a feed roll, C. Rotation of feed roll C in a counterclooLwise direction feeds the material to be mparated pest the charged electrode, D. Below the feed roll is a two-oompattment receiving bin with a divider, E. This dividermaybe plaoea in suoh a position that, with no charge on electrodeD, sll the m a w will f d into pan F to the right of divider E. If &&ode D is a h g d with negative elee trioity and material is fed from the hopper, any partialee of material in the feed haYing positive surface charges will be deaeoted from their n o d line of fall and will dmp to the left of divider Einto pan a, thua resulting in a separation of the m a w . Separation tskes place ~ccordingto the fundamental law that unlika oharges attraotand like chsrges repel, so that a negatively charged electmde will attrsot the &alee in the mixture whioh carry positive chargea while it will repel the partialee whioh marry negative ohargee. The fundamental law of electrostatic s e w t i o n springs from the primav considerationthat in any system movement of free particles tends to take place in such away BB to reduce the total energy of the a p t e n . Also, the statement that like ohsrges rep4 is not pooording to d n t i 6 c faot, for aceually all movement in an aleotrostatiofield is due to contraoting tubes of force, and the tm partiden with like charges are actuslly neutrsl to each other and sre atgaded in opposite W o r m beasuse of the hbes Of forCe portending frOm their Surface to Surfacee Of O p
tric, and photoelerhric. ConW chargea were probably the b t to be noted in a mix of ~~, an they are sellgenerating. They arise from the frictional electrical efiect whereby two dissimilar sub~tancesacquire opposite and equal electrical ahargee when they are brought into i n t i t e contaot and then separated. For conductsnce chargee to he noted and wed for separating purposes, an outaide source of electrical potential is necessary to condition the material. Thii is heMW conductsnce c b q w arise from the contaot of the mix with the feed o h ~ t e sand r o b of the separator which are charged with an oppodte polarity from that of the attracting dectmde. Nonoonducking OOnStihWnk Of the mix are acted on by induction, and acquire equal and opposite charges so that the resulting total charge is rem. Conduoting particles, on the other hand, permit a flow of electricity which allows the charge differing from the feed ohute or roll to be neutrdized and the perticle to acquireatotal charge which is the same an the feed roll or chute and opposite that of the attraoting eledmde. pymelectrio ohargee are generated in some cryst& from strsins set up hy heating or cooling. The strsins prcduca opposite chsrgee a t the extremities of the crystal, and when one of these chargea is neutralized by the charged surface of the roll or chute, the particle is left with a charge corresponding to that of the roll or obute and is subject to the attractive force of the electrode. Photoelectric charges are produced only on substanca which have the property of Bning off electrons when exposedto radiatiom of the proper frequextcy and are not common in electrostatic separation work In a separation depending on contact chargee, there is movemeat of the constituent materials, not only on them-
111s
INDUSTRIAL AND ENGINEERING CHEMISTRY
11%
selves but also on the supporting and con6ning surfaws, 80 that we have three frictional aourws of charges. For the ideal condition the contact potential of the supporting surfaws should lie between the contact potentials of the ing~&enta of the mix so that the resulting surface potent& of the ingredients will be opposite and at a maximmn. Thia condition iE also a d v a n t ~ g ewhere ~ ~ ~ the separation depends primarily on conduction charges, for then the conducting paaicle will have the same sign an the feed roll or chute, and the production of a maximum charge of opposite sign on the nonconducting constituent will make it stick to the roll or chute and improve the efficiency of the separation. Since it ia possible to reverse the polarity of the charge on the elecW e and frame of the separator,it is always possible to operate the separator with the frame baaring a charge opposite to the contact charge csmed by the nonconductor. ¶tiom are sometimes encountered where the Uerence in contact potential iE slight and where neither constituent can be called a conductor. In such OQBBB it is wmetima possible to b 3 d up a Uerenca in potential by taking advanhze of ditferencea in dielectric value. This is best done by conditioning the feed with a spray electrode juat prior to each separating stage. The charge on the spray electrode iE of the eame sign an that on the attracting electrode, and the lag in the neuWeation of the induced chsrge on the particle of highest dielectric strength aids in the efficiency of the separation.
Historical Development The poaeible w of electmsbtic separation data back to the first recorded electrical experiment of Tbalea around 800 B.C.,when he reported the power of rubbed amber to attmot aeatain partick%. Pmbably he did not use the attractive power of rubbed amber to make separations of materials, but the toolhad beendisoovered and reeorded--s potentialsepa-
rator had been born. Early expehents with amber, glass, SilL, and fur with the pith ball and the elwtroscopeestsblished the fundamenfd hw which hss governed dl d o & and limited all results in dectmstatio eeparation-namely, “Unlike chargea attract, like ebarges red.’’ The potential pmc-
Val 33, Na 9
mental development of efficient electrostatic separators follow~clmly the development of electrid. apparatua for produoing static charges of au5icient strength and uniformity. The development of frictional machinea continued until the 1880’8, at which t i e the Holtz, Voss, and Wimshurst induction machines had been developed to a point where they hecame fairly reliable sources of high-tension electricity and the static charges could be generated outaide of the separator itself. Theseparatingelectrodethentook theformofametal. lic conductor insulated from the body of the machine and charged from an outside s o w . It is interwthg to note that one of the patenta ming this construction wan issued to Edison in 1892 (6)for the separation of orea. While the w of the induction machines 88 a murca of high-potential unidirwtional current WM a great advance over the old frictional methds, the machinea were never of really rugged conshction and their large expased s w f a made ~ them susceptible to conditions of high hnmidity. During 1695 two patents were granted (1,9) for the con&n of albrnating to direot ourrent, and the u8e of the mechanical rectifier and highh i o n transformer w a ~soon adapted to eleotroststic separator work. Improvements in tr8mformers and meohanid rectifiers have contmued until today this class of apparatus iE rugged and dependable. The development of the vacuum tube and the rapid progreas in transformer construction, due to the requirements of the radio, haa produced another d e pendable mure8 of high-tension u n i W i o n a l current which oan be used for efficient eleotrostatic separation. Commercial p r o w wan not only limited by the slow development of reliable m u r of ~ &tic charges, but slso by the fact that efficient eleectroststic eeparation called for many other special considerations. Thua the high valtagea used required eUicient insubtion of the electmdes, not only to save power and maintain the bigh potential, but also to proteat the workmen around the machinse. Efscient separation demanded a uniform and uninterrupted statio field, and this was d i 5 d t to aocomplish before the dap of air conditioning. Good reaulta also depended on elom and nniform 8izing of the feed to the machine, and often the combined costa of BOIBBDB and mparator would render the pro~easeconomicaUy unwia9.
E~ioamrrraar b p m m a w m 8TANDABD 8TATIONABY METALLIC E L ~ C T B D OD~
F I O 1. ~
ea lay dormant for some twenty-four hundred years and it iE not until the 1870’sthat we 6nd patent reference to the moat h o l e of eleotmstatic separstors. They 4frictional statio deitricity generated dirkctly on the eiwtmde an the active attrective form, and were designed d y for w on grabs and other light materials. From that time on, the eOrpei-
Uniform multa could be obtained only with material which had been at least surf-, and tbis prohibited the use of the pmca~sin many c u m where modern dryem now make it possible. Uniform f d i of the material and its passsge thmngh the machine in the form of a shallow stream WM a primary considerationin a prooess where every psrticle should
-,
1w1
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1121
contact the feed roll or chute aa it cornea within range of the Figure 1with the new construction shown in Figure 2. The attmtiug electrode, and this was much Mer to aocompliah armngement of the feed hopper, feed roll, divider, and oollecb ingpsn%in~tiauytbeBlllILBinbDthmaohines. The before the day of vibratory feedera. Commeroial propem baa no doubt been further retsrded ohargd metallio conducting electmde D of Fignra 1 in reby the air of mystery which wea allowed to grow up concern- plaoaa by the rotating noncondnoting electmde J of Figure 2. ing ~ ~ t wpmation. i o Moat of the artiolps for and At the surface of the rotating nonconducting eleatrode J and at a point rem& from the frame of the machine, them in a against it were written and praseolted by individualn interconductor, K ,& IYO that when it in ahargedwith 5 highmted in a certain machine or proooess, and only recently was a comprehensive paper (6)presented which analyzed the tension Eummt of eleotrioity, it will induce or set up I) aharge hanic reanone for a di6erentiation of chargm and propwed on the surface of roll J. Since roU J in made of nonconducting mesna forrationalconsidentionor~~separa~nprob~. material, the charge in bound to the surface at the point of term^ such 88 conduotor and nonconductor were used in elec- generation and in married into position opposite tbe strsam of troatatio terminology in I)looeem m , very diflerent from their material to be sepsnted by the rotation of the roll. At that p i n t it attracts partidm carrying cbargea of opposita sign, meaning in electricsl enghedng. From many of the records some of which will adhere to roUJ and be carried around to it would seem that separation problem were not analyzed on an e n g k e h g bssis but that random tests were conducted the point where they will be solsped ofI by wrsper L. This until a suitable sepsration was produced under a lucky set of construction allows the conductom carrying the high-tension current to be &ectively insulated and removea thanfrom the conditions. Although the reason for surface cbargm was d e tarmined when the brst testa were made in ststio electricity, proximity of the frame of the machine. Thin awida sparking and although all the textboob point out that contact chargm and shorting out of the mum of the attractive U d , and it also affords much more &eative protection for the workmen. are dative and that the sign of the cbarge on any particular substance dependa on the material with which it wan in con- Further,aince roll J in of nonconducting material, any piem of stray metal in the feed to the machine which might bridge tact, aa well ea on its own composition, certain materiala the gap between the feed rob and the electrode cannot prohave been regardedan having intrinsic aharges both ea to eign and voltage ea late 88 February, 1938 (8). duce a short circuit whioh will neutralize the entire 6eld of the Modern equipment and engineering knowledge hold out machine, but it will only neutralise the point of roll J on which it reata. The new construction p m n t s another imbright hopes for the future of electmtatic separation. In the portant improvement, in that the amount of cbarge on the modern electron-tube mtilier and in i m p r o d meohanid surface of mUJ can be regulated aa well as ita sign. In the old recti6era and high-voltage transformem, we now have d e pendable ennrca of high-voltage unidmotional current. Air m e W c eledmde construction, since all electmdea were conneoted by conductom, the amount of charge on any electmde conditioning at reasonable coet and drying prounder was limited only by the capacity of the ennrce of power. In sensitive control are now available. The development of the radio baa put excellent insulators on the market at low coat. many asparations on the old machines, particlea bearing an opposite cbarge would be drawn over to the coUecting de+ The preconditioning of the surface of particlea before elm troatatic separation iy baing astinfaotorily worked out (6,7), trode wbere their cbargw would be neutrslired and a charge of like sign induced on their surface. When tbh oocurd, and methods are being developed which promine succ~88in Beparatingmaterialwithaparti~dismeteremallerthanO.04 t h y were no longer attracted by the eleotmde but would be attracted to the n w t point of tbe grounded portion of the mm. (6). Careful drwlopment work by the Bureau of Minea separator. If this n w t grounded portion happened to be is removing the cloik of mystery and in putting the whole qumtiouof eleotmataticseparstion on a rational basis. h m on the other side ofthe divider, portions of the separatsd mai b g results are a h apeeted from the new dielectric electmde terial would paw back into and contaminate the other inseparator (4)which provides a completely insulated machine gmdient of the mixture. With the diel&c construction, the with a uniform field whiob in not subject to ahort circuit amount of cbarge can be adjusted en that attraction taLea plsce without complete neutralhation or recharging. Under tbrougb the presence of metallic particles. Thin new dielectria electmda separstor wan developed by this condition the separated material continued to stick to the the author to avoid mme of the hublea eaperienced with the roll and be ~ m e mund d to a point where soraper L reold separatom of conventional deaign. In the machines availmoves it and drops it into collectmg pan G. Thin allows much more efficient separation. able in the past, the eleotrodm consisted of mds, tubes, or wirm of conducting material, with or without an insulating covering. Most of the machinea were constrncted with bare Scope of ElectrostaticSeparation eleatmdea, and where inenluted electrodes were used, it was nBo8881vs.to increase the d t a g e h t to the NPture point On the basis of the fundamental law tbat unlike charges of the insulation if an eleotrostatio 6eld of any &&ve attract and like chargea repal, electrostatic separation will be strength was to be maintained. This conatruction oalled for found to apply to almost any mixture of dry materiala, where the we of direot cnrmnt vol@w ranging from 14,000 to 86,- they have teenground h e enough so that the possible aurface attraction becomea great enough to influenae motion tuking 000 volts. AU of the eleotrodea on a separator were joinea d plsce under tbe action of gravity or inertia. Electrostatic topther in a continuous eleatriast cimuit en that any I separation will take place beoauw there in a difference in fluohtion was tranemitted to all p o d of the separating polarity of charge or a difference in intensity of charge on the areaa. Thelargeerpoaedaressofllighlyahargedme~were subject to heavy losaee of *tic ahmrge dnring damp weather, conntituenta of the mix or beosw one m e m h in a better mschineopesatorswere erposedtohighwlbgm, and a tingle conduotor than the other. When any two materials haviug ground at any point of the machine rendoed theentlesepa- dry surfam are p d or rubhed together, they q n i r e rating 6eld inoperative. In the new co&&n, rotating qnd and opposite charge8 (8) and they C B n b arrangedin a A m , such ea in Table I, where if any two materiala are eleotmdeamadeof a highlydieleatricmaterialareused. A surface charge of the dmired polarity in gsnersted on thenuchoaen and rnbhed together, the one stamkg 6mt becoma face of the nonconduating e k o t d e , and the mtation of the positivelycbarge€landthe~becOmesnegative(S). These e b t m i e bringa tbh muface chsge into the field of Mtion. contact charges pmduca a condition in many mixturea of The essential dflerenca from the old oonstruotionare a k l y materiala wbich courta separation, if the mixtam in exposed seen by eomparinp the conventional co&ction &own in to suitaldy charged elechudea. Where the contact o h a m
INDUSTRIAL AND ENGINEERING CHEMISTRY
1122
are feeblebecause of the nature of the materjals or because the eftect of the contact chsrges 8from the supporting mlaceedecressesthepotentialgradient; thenatureandstrength of the chsrgas can often be changed favorably by selecting n proper support material or the 5mhg material may be favorably conditioned by means of indued c b a r p from a spray electrode. Where oontect chakgea are ~ t u r a l l ydand feeble, but where there is a considerable dilTempx in conduotivity, it is often possible to regulate the speed of the material tbrougb the machine 80 that, in conjunction with proper $ea trical conditioning,the nonconductor will p ~ s athe attracting electrode canping the same sign, while the conducting ingredient will carry the opposite charge and will be attrsatad out of the normal line of fall. If one ingredient is a @ conductor, electrical conditioning may be dispensed with and it is neoessav only to seleot the proper sign for the attracting electrode, which sbould compond to the natural sig0 of the nonconducting particles. If one member of the mix is subject to pyroelectric cbarging, the chsrge on the feed roll or chute can be selected to be the opposite to that of the nonpyroelectric member 80 that attraction to the feed roll will occur, in the cas8 of the pyroelectric ingredient, the opposite charge will be neutralized by the feed roU or chute and the charge remaining on the particle will cause it to be drawn over by the attracting electrode. If the possible applications of eleotmstatic separation are so universal, the question naturally arises m to whether it can be regarded aa a cure-all in the separation field. The answer is, “Decidedly, no”. Electrostatic separationis n sensitive method of separation. The strongest forces which can be generated in an electrostatio separator are actually very small. The macbine must be mmrately adjusted and kept in good condition if uniform multa are to be obtained. The feed to the maabine should be uniformly dried and it is nesary either to size the material witbin rather close Uta or to make elaborate mangemats for controlling the rate of feed nnd the distribution of the feed aaross the face of the macbine. Some separations require electrical or ohemical conditioning of the materials. EBch separation problem should be studied by iW and the deciaion made on an economic basis.
1. Fur 2. FI.nnel
3. Ivon 4.
O h
6. Cotton
8. 10. 11. 12.
Ebnita The hand
Metals 8ulfur 13. Celluloid
-
Paper 7. 14. Rubbertubing -The list i s m arranged that if any two materisla are placed in rubbing ODntllot and mpusted. the h t on the list will boome tively ohm& while the other will be ne.ativs. 6.
m
Economia In taking up a particular problem, the first question should be: Is the mixture suaceptible to eleotmststic separation? This can be determined by sending a Bample to nome d d e r of electrostatioequipment, or records at the Bureau of Mines may cover the c88e. From plant w r d a it is usually possible to determine the cost of grinding and &hg the material. Grinding must be oarried to a point where the ingredients are physically separate and, to ensure scient use of the nonetoostrong foroea in the static field, a -6 mesh grind is required in most separations. The grinding should be done with the production of a minimum amount of dust and, if a very dean aepsrstion is desired, it would be well to figure the Oost of
VoL 33, No. 9
grading the feed to the separator into portions of, my, -6 to +20 mesh, -20 to +@Itmesh, -60 to +120 mesh, and -120 mesh. The cost of surfwe drying should also be cons i d e d at this point.
Consideration of the cost of separator equipment can be divided into two headinga: the cost of the electrical apparatus and the cost of tbe separator proper. The electrical equipment should be of first grade quality and should be fittad with the proper safeguards and controls. The ohst of the electrical equipment will vary but little, re@= of whether the separator is a mudl single unit or a large multiple hatallation, for in m y aase we are dealing witb the same higb voltages and therefore wfththe same exding insulation requhmenta and with controls and Bsreguards that are easentislly identical. whatever small dilTer8nce in cost there may be wiU bedue primarily to an increaeed copper requirement in the transformer w i n d i i for the higher current output and in the higher cost of connectingup the larger m a a h . However, in the case of the trsnafonner cost, S k h I d .U d wire size and mechanical and structural detgils are more likely to determine the &e of the conductors tbm an inm?aee in Calrrent-aarryinp capacitywhiah is in the order of milliamperes rather than ampem. Where electron tube reati6era am used, there will be an inmew in Oost for the larger,higher capacity tubes. It is safe to figure at least $750 for eleatrid equipment for the smallest commercial separator and $1500 for large multiple unite. The cost of the separator proper will depend on the method netswry to feed the material, the mate% of connh’uetion required, the number of separating stsgee wbiah must be used, and the tonnage to be separsted.
sePtedm, 1941
INDUSTRIAL A N D BNQINBBRINQ CHEMISTRY
1123
4oQpmmperhour.
kfoot of faoe per hour. Perhaps the beat idea of equipment costa wiU he given by listing enme a d d installation costa which have come within the writer's experienca. A double-baals eight-atage S-foo& face toboggm-chute machine fittedwith revolving eleetmdea wan furnished complete meobanical rectifies and oontml bosrd for $8850. This machinehad aaspaoity of 2 tone per hour. Ita in%tsll.tionradnoed lomm of a hesic msterialfrom 5 to leas than 1 per cant,and it furni$led such a clean feed to other equipment thst operatiom could he carried on at a much higher rate of produotion and the pumhase of fmm )6oooto m a o r t h o f addltkmal epuipment WaB avoided. This unit wan inatslled in 1931 and has been in d d y o p r a tion sinca that time. A duplex toboggan-type eepsrstor, having a ainglebacl six-stage 4-foOtfsce unit feeding a doubleback nix-ntage 4 fwbface unit, both unita fitted with '/Finoh stationary b r w electrodes, waa furnished complete with drive motor, mechanioal rectifier, and control board for about W. This nachineeJlowed oertakrraclaimed mataridstobeeaparated en eompldy thst they both moved into a higher quality dd6oation and commanded higher prim. The aspaoity a ~ 1 8 0 0 p o u n d s p e r h o u r .Tbismachinewssinst8ued in 1032 and m running to aspaoity today. During1932 alargeshipment of mustard need wan received in the port of New York and found to he in a filthy condition due to animal excreta. The shipment waa consigned to one of the hrgest manufactuma of mustsrd and his hveatigatiom convinced him that the d could he cleaned only by eleotmntatic methods. A doubbbsck 4-fooLfm toboggantype eepuator, having twelve stsges and fitted with stationargbrass electmdes,wea imtslld in his plant at a total cost of around $2850. This msohine made e b l e the sal+ of ovw S16,WJworth of mustard 4, and the report in that every pound of mustard seed received in that plant since that time has pMBed through thisseparator. hthernIa&ine WBB m e imtslled in the food industries in 1934 for removing ohati from M)OB nib. The m a o h i a a w a s d o u ~ s l r d B s d a n M o o t f a a a There . were twelve & a p of separation,ea& with ita own d e r fed: eight rotating eleotmdss and foor &ationary electrodas m used on ench face. The maahine had a oapoity of ao00 p o d per hour and &)B9M) oopnpleta withsiriacreen, air separator for dust, drive motor, rmchamd . red&r,and psnelborrd. Thismschinedesndthecwosmbwi~t the loss of good material previously expeienced with an air separator, and the recovered materisl dto 800 pounds per Show ut. As the material wan valued at 6 mta per pound at that point, them was a Bsving of $48 per ehift.
lb?&m a list of of t&e prodnots on which eleotrostatic eeparatora are now in sueawful operation will give a clue an to whether this method of separation may have appliastion in any particular indmtry. Ckmaccial separstors are now separating silicon carbide abrasive from Alundum and oleaniog foreign material from both the silicon wbide and the Nundun. Spiw sue being cleaned not only of chaff and Bimilar msteNb W rPa0 of animal and bug excreta. Chati is beingrempoved ban gmmd masted coffeewith no loas of &ea. Machines which were introduced in the abraavemanutacturingindustryyears ago am still wed to eeparate impuritka from &e mwky made egjns of abrasive (7). It m @eo reported @3)W miosis baing eeperated fromsand,. ilmenite and rutile from zircon, feldspar from quarts, and f m i l i c o n from alumina abrasiva. From the costa given, it wiU be 8 e ~ that, n in general, the wt of electmtati~mparatora in higher than that of screens, jiga, and sir eaparatm of equal tonnage. Also, in a great many rather elaborate conditioning equipment ia required in the way of screens, ek., to prepare the feed properly for the saperstor. If the desired multa c8n be obtained with m-ww, jiga, or air ~epsrstors,it in usually more emnomid to lme them. on the other hand, where valuable materiala can he recovered which would otherwise be lost, where s u b quent operations can be made more dcient by the more wmplete cleaning of a pxdurh, or where bettm gradii will allow a materisl to oommsnd a higher price, it is recommended that full consideration be given to the modern electamtatic mparator. Likntun cited
~.~~~ .
(a) I&.., P. 7.
~~~~
H.L., U. 8. P.trmt.%~.ooB (Dso.17,1040). (6) Edi.on. T.A...XM.. 476.991 (June 14,1SSa). (6) Fm~:Fc8ter,and Iiahtnn, 0. C.. IUD. EUQ.CknM.. s2, 000-4 (4) Bull&
(1040).
0 Joheon, F. E.,0 . 8 . p.tamt 1,090,418 Uuc. 17,1887). (8) JohmDs E.B..&u. M MiniugMet. !bsm.. T& P a p Sn (1988).
(9) 8W. W.J..
-
U.8.P W t 647.043 (Oat. 1,1895).
