Relative Solubilities of Quartz, Lead Smelter Slag, Chrome Ore, and Silicate Filter Grits in Hot-Alkaline Waters'" By R. E. UaU,aH.A. Jackson,' and Grant Fit&
ncne*rr OF Milie3 A N D II*C.*N
ConPoa*rron, P m S B U a D H , PA.
I
N THE course of a The for the rate f. solution'ofsand or quartr grit in hot (1) above and ( 2 ) following, 811 the complexities, study on the mechanism maters kinggrcoter than that of materials in which the acid of boiler scale formation has been fully neutralizedby the basic component ham k e n which are so characterand its prevention, made discussed fro,,, the . f equilibrium. sewroi istic of silicate-water sysby the U. s. Bureau of types of mater io^ halie been p,oposed as $Itwing media for tems at higher temperaMines in cwperation with b t and the of three of them, re~atiw tures, have beer1 omittad, the reader being referred the Hagan Corporation of to silica, ham k e n determined. to the papers of G . W. Pittsburgh, data were obMorey.' tained on the relative rates of scilution of some silicates and other materials in slightly alkaThe rate of solution of silica in an alkaline water is more line water at approximately 200"C . These data are of value rapid than in a neutral water, because of neutralization of its in the seloction of inatcrials vjhich are availahle for filtering acidic charat:ter by the hydroxyl ion of the base and the remedia under such conditions and which because of tlicir low sultant ahnost coniplete removal of the hydrogen ion from the rate of solution obviate the neeessit,y of frequently replacing equilibrium. If, in place of an acid anhydride, a substance is the filter grit. The theoretical considerations governing the selected for tlre filtering rncdium in wllich the acidic character rntc of solution and tlie relat.ive rates of a few materials are is alrea.dy neiitralized by a basic radical and in which the solubility in pure water is law, then the rate of solution should be presented in this report. very much leas. Sueli a substance, for instance, is forsterit.e, THEORETICAL COIYSIDZRATIONS .RIg2Si0,, belonging in tlie divine group of minerals. The Sand of appropriate quality or brl,ken quartz isan excel- eqrrilibrium r(?lationsof this Inaterial in an alkaline water may lent filtering medium for use at ordiiiary temperatiires, even be cxPrcssed 11s follows: M~?SiOlT=rlMg,Si0~e2Mgi.+ + (SiOJ ---though i t is an acid anhydride and the watcr is slightly alkalinc. The type of equilibrium which exists under static (solid) (dissolved) 4 N a O H a 4 ( O H - + 4Na+ (2) conditions is as follows: ir it S i O 2 e S i O s f HzOz=tH2SiOaZ=Z2H+ -t(SOa)--(solid) (dissolvcd) ~ N ~ o H ~ ~ ( + o H Z N ) +~ (1)
M
2HaO
IT
N@Oa
The solution of t,lre silica is so slow, and tho ionization of tlre silicic acid so slight, that the loss of the filtering medium hy solubility is of little consequence. In addition, very freqrlently the individual grains bccorne coated wit11 calcium carbonate and are thereby protected. Wien filtration is carried an at higher temperaturns, however, the rate of solution and hydration6 of the silica is so largely increased that loss from solubility is no longer negligible. The solubility of silica is also somewhat larger6 a t the higher temperature, but is ~~~.~ offset by the decrease in ioriisationofsodium hydroxide and the increase in ionization of water with teniporature increase. In the simple equilibria of I Received April 2, 1924. Presented before the Division of Industrial and EoLineering Chemistry at the G7ih Meeting of the A m e r i c a n Chemical Society. Washington, D. C., April 21 t o 28,
1924.
Puhlinhed by perrnirrio~of the Director. U. S. Bureau of Mines.
Pittsburgh Experiment Station, B~~~~~ Mioes. Hagan Corporalion. I J . A m . Chrm. Sac.. 43. 391
'
(1921).
Lenher and hlitcltell. I b i d . . 29, 2630 (1917,
2Mg(OHjl Na.Si0,
If we follow t,llrough the equilibrium from left t.o right, we note that by increasing the amount of hydroxyl ion in the solution we will decrease the amount of magnesium ion by the formation of notiissociabed magnesium hydroxide, and thereby accelerate solotion, and that by increasing the amount of sodium ion w e tend to form more of the sodium ortliosilicate molecules. I3owcver, the ionization of magnesium hydrate, even under these condit,ions, is very large as comparcri to that of water so thiht a relatively large amount 05 magnesium ion will exist io tho solution. Moreover, at equivalent concentrations the ionization of a salt containing a univalent ion, such as sodium, is always markedly higher than that of a salt with a bivalent ion, such as magnesium. Consequently, tho tendency for the magnesium ion and silicate ion, furnished by the orthosilicate of magnesium, to he,removed from their influence in the equilibriurn with the niagnesium silicate, hy being transformed into undissociated m a g ii e s i u m hydroxide a n d sodiiim orthosilicate, is in no wise so large as that for the correspondiog ions in the silica equilibrium. Furthermore, observations on boiler scale formatioil point tMoreyandNigeli. J.Am. Chcm. Sor..35,lOXGlI913);M o i e y , I b i d . . 36, 215 (1914): 39, 1173 (1917).
702 ..
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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to a decrease in the solubility of the silicates of calcium and magnesium with temperature increase.8 The tendency of calcium silicate to form a scale in boilers has been noted by the H. S. B. W. Cochrane Corporationg in their experiments on filter materials.
Fig. 2 represents a typical time-pressure curve and Fig. 3 shows the relation of loss to time on quartz grit. On 5tccount of the time required by the boiler in heating and cooling, the total time during which the sample was subjected to the action of the alkaline water is somewhat uncertain. Since this uncertainty is the same for all samples, however, it has little effect on the relative rates of solution. The point in Fig. 3 at 14.5 hours represents the summation of the loss of the two individual runs of 4 and 10.5 hours on the same sample. The materials used in the experiments were quartz grit, a substance, A , with the acidic character of the silica neutralized by the basic components, a chrome ore, and a lead smelter slag. The analyses of substance A and of a typical chrome orell are as follows: TYPICAL CHROME ORE
__-_--A---
7
42.24 9.46 3.01 1.72 0.25 38.77 2.6
FIG.2-TIME-PRESSURE
CURVE O F QUARTZ GRITIN DILUTEAQUEOUS SODIUM CARBONATE SOLUTION
AVAILABLE MATERIALS We would expect to find, therefore, in those minerals which in pure water have a very low solubility, that if the acidic character of the negative components is entirely neutralized by the basic component, the solubility in the presence of a few hundred parts per million of hydroxyl ion would not be greatly increased, even though the material is in contact with it a t the temperature of boiler water. Such materials are found in the olivine group of minerals in which the basic component may consist of iron, magnesium, and calcium in various proportions; also in certain of the pyroxene and amphibole group of minerals. Dibase and basalt, selected with reference to their small content of uncombined free quartz, might serve the purpose. It is reasonable to suppose, also, that we need not limit ourselves to silicates; that materials such as ilmenite, magnetite, chromite, spinel, etc., would exhibit the same properties.lO
Vol. 16, Yo. 7
CrzOa FeO AlrOa MgO Si02 CaO
New Caledonia 54.5 19.5
11.0 8.0 3.1 1.5
California 52.7 14.2 12.5
15.5 4.0 0.5
There is little or no uncombined silica in either A or the chrome ores. The lead slag from its nature was considered presumably nearly free from uncombined silica, and no analysis was made. TABLE I-Loss
MATF~RIAL Quartz grit Lead smelter slag Chrome ore A (re-run)
::
WEIGHTPER HOURPER EQUIVALENT SURFACE Loss in Weight per Hour per Gage Pressure Equivalent SurSpecific Gravity of Boiler face X 106 250 2.64 2460 250 3.76 250 250 3.92 110 250 80 3.40 3.40 120 40 120 100
IN
The writers were more interested in relative rates of solubility of materials than in the total amounts dissolved. For strictly comparable figures, they would want to know how much solution occurs on unit surface of the material per unit 11
McDowell and Robertson, .I. A m . Ceram. SOC., 6 , 865 (1922).
EXPERIMEKTAL Experimental work to test the validity of these conclusions was carried out in a small laboratory boiler of the type indicated in Fig. 1. It was sufficiently rugged to work under 109 kg. (250 pounds) pressure, and was fitted so that a sample could be rotated in it as shown by the mechanism attached to the right-hand side of the boiler. The desired material, sized to 10-14 mesh, was enclosed in a container made of monel metal gauze. In this form it was attached to the rottting spindle, which revolved a t about 17 r. p. m. The boiler was filled with ordinary tap water made up to an alkalinity of approximately 20 milliequivalents of sodium carbonate. The boiler was heated until a pressure of approximately 109 kg. (250 pounds) 'was attained, and was then maintained at this pressure for periods varying from 4 to 10 hours. It was then allowed to cool, and the sample was removed and weighed. The loss in weight showed how much of the material had dissolved. 8 Hall, Smith, and Jackson, Proceedings Prime Movers Committee, Natl. Elec. Light Assoc., 1924. 9 Manufacturers' Statement, Chemical Treatment; Statement by H . S. B. W. Cochrane, 1923 Report of Prime Movers Committee, Natl. Elec. Light Assoc., Technical National Section, Part B , p. 157. 10 The authors are indebted to George Otis Smith, dgrector of the U. S. Geological Survey, for suggesting a number of minerals.
" FIG. 3-TIME-LOSS
TIME, HOURS CURVE OF QUARTZ GRITI I D I L U T IAQUEOUS ~ SODIUM CARBONATE SOLUTION
INDUSTRIAL A h T D ENGINEERING CHE*WISTRY
July, 1924
time a t a definite temperature and concentration of dissolving agent. What they actually obtained was the amount of material which dissolved in several hours’ time, while temperature and conrentration remained approximately constant, and while the surface of the material was changing slightly due to the quantity being dissolved. The amount of surface exposed per unit volume of material may be made very approximately the same by using material of the same mesh-in this case through 10 on 14. If, for comparison, a volume of material and therefore a surface corresponding to that a t the middle of the determination is used, the results will not be much in error because of change of surface during the experiment. Choosing as the unit of volume the volume occupied by 1 gram of quartz, then in terms of this surface the figures in Table I are obtained, showing loss in weight pw hour per equivalent surface.
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DISCCSSION The effect of using a material in which the acid component is fully neutralized is marked. It may be noted in addition that the rate of solution of A when the boiler was operated at 43.6 kg. (120 pounds) pressure was greater than a t 109 kg. (250 pounds) pressure. This is in accord with the idea that the solubility of the silicates decreases with increase of temperature. Also, a re-run of A , even a t the lower pressure, gave a slower rate of solution than the primary experiment at the higher pressure. Probably incidental impurities were present in the initial run, which dissolved and affected the rate of solution. Another advantage of the minerals discussed over quartz is their greater specific gravity. This permits more rapid back-washing of the filter without loss of the filtering medium.
The Volumetric Determination of Fluorine’ By Wilfred W. Scott COLORADO SCHOOL OF MINRS,GOLDEN,Cor,o.
The Of LUORINE is comA brief reoiew is gioen of the more generally practiced methods though necessary in a gravimonly d e t e r m i n e d for the determination of fluorine. The method described by the metric method, is not necesgravimetrically by writer is the result of his experiences with the methods cited and is saryin a volumetric determibased on the principle of precipitation of fluorine as the calcium salt, nation of fluorine. Precipitation as lead chlorofluoride or as Calcium fluordepending upon its insolubility in acetic acid. Experiments were The Of phosphoric acid by silver nitrate, as recide. I n the second method conducted using fluorspar of tested purity, with prepared calcium ommended, with a subsethe Calcium is generally fluoride, and with alkali fluorides. Three procedures are dequent precipitation of the excess of silver as silver chlotreated with acetic acid to scribed: ( A ) determination of calcium and cquivalentfluorine, (B) remove ilnpurities, the Caldefermination of fluorine by the acetate method, and (C) r i d e ~requires e a e care, since silver phosphate cium fluoride being Practidetermination of fluorine in alkali fluorides. Experimental data may be quantitatively concally insoluble in this reare gioen showing the degree of accuracy that may be expected of verted i o silver chloride and agent. Two v o l u m e t r i c these procedures, sodium phosphate by addition of sodium chloride, as recmethod?, are generally ommended, thus defeating the k n o w n - - t h e method of purpose of the silver reagent. Ag,PO* 3KaC1 = 3AgCl f h’aaPO4 Greef, which depends upon the principle that a neutral aqueous solution of ferric chloride forms a white, crystalline precipitate with neutral solutions of alkali fluorides, making ~ ~ $ ~ ~ ~ ih $~ ~ ~ ~ ~~ e~ ~~ possible the titration of fluorine with ferric chloride; and the of silver for phosphate has to be relied upon for the removal of method 01’ Offerman. which evolves. bv sulfuric acid treatment. DhosDhoric acid. the fluorine as silicon tetrafluoride, with subsequent absorpIn the determination of fluorine by the first procedure tion of this fluoride in standard alkali, determination of the given, the writer obtained lower results than by the second excess of alkali giving the necessary data for obtaining the procedure. This low result was attributed to a partial defluorine absorbed. of calcium fluoride by sodium oxalate, as shown composition In a recent edition of Low’s work on ore analysis appears by the reaction a volumetric method for fluorine worked out by W. V. Norris, CaF2 NagC204 F? CaCsOc f 2NaF which depends upon the precipitation of fluorine from an The method outlined in this paper is a result of the writer’s acetic acid solution by addition of a measured amount of a standard solution of calcium acetate, the excess of which is a experience with the methods cited. Like the Korris method, measure of that required by fluorine: Norris removes the it is based on the principle of the second gravimetric method silica present in the solution by the method outlined in the given-the precipitation of fluorine as the calcium salt, deauthor’s work,2 and follows this by removal of phosphoric pending upon its insolubility in acetic acid. Details of the acid by means of silver nitrate. Two options are now given: procedure differ considerably from the other methods. Experiments were conducted with fluorspar of tested the first precipitates the excess of calcium, in presence of the calcium fluoride, by means of sodium oxalate, and titrates purity, with prepared calcium fluoride, and with alkali the calcium oxalate; the second method calls for a separa- fluorides. tion of solution containing the excess of calcium from the Experimental fluoride before attempting the calcium determination. FluorTwo procedures are suggested-a rapid method depending ine is calculated from the calcium removed from the solution, applying the formula CaFz to the compound formed. The upon the estimation of fluorine from the percentage of calcium writer’s experience with the Norris method led to the fol- present with fluorine, the calcium combined with commonly occurring substances being extracted by glacial acetic acid; lowing co~iclusions: and a procedure that depends upon separation of fluorine 1 Received December 13, 1923. from its combination by converting it to soluble alkali salt 2 “Standard &lethods of Chemical Analysis,” 3rd ed , p . 216.
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