Causticization of Carbonate Solutions F. E. LITTIIALI" .AND H. J. GASPARI Bustc Reduction Co., Henderson, Aev.
A
LTHOUGH some 500,000 tons of caustic are produced each
droxide. T h e equipment used consisted of 3-liter, 3-necked flasks equipped with a stirrer and thermometer and maintained a t 95' C. in n v a t e r b:tth. It became apparent almost immediately that, a 2-hour run, such a4 used by Olsen and Direnga, did not give equilibrium values a t the higher concentrations. As an illustration, run 107 gave:
year by the lime-soda process in the United States alone, very little fundamental information has been published on the reactions and equilibria involved. Nothing seemed t o be known about this reaction outside the narrow range of concentrations used in the manufacture of caustic soda. T h e region in rrhich the authors became interested during investigations of the leaching of oxidized zinc ores with caustic solutions docs not coincide with t h a t used in producing caustic.
Time, Hours 0
G./Liter
4
T h e causticization of sodium carbonate solutions with lime according to:
+ Ca(OH)2 e CaCO, + 2XaOH
5
.4n appreciable change of compoqition took place between the second and fifth hour. A series of experimrnts mas then run starting with sodium carbonate solutions (Table I), a n d the resulting equilibrium compoqition is plotted on Figure 1.
(1)
has been studied by Lunge ( 3 )and more recently by Goodwin ( 2 ) and Olsen and Direnga ( 4 ) . T h e last-named duplicated Goodwin's work and cheeked his results, which differed considerably from Lunge's. All this work was carried out with solutioiis containing up t o 20y0 sodium carbonate, or about 160 grams pcr liter of equivalent sodium hydroxide. Because most of the leach work with which the authors were engaged mas being done a t total equivalent sodium hydroxide levels of 260 to 350 grams per liter, they were vitally interested in an extension of these data. As a first approximation, Olsen's d a t a were extrapolated to a. total equivalent of 250 grams per liter. These data seemed t o indicate t h a t the sodium hydroxide concentrations could not he raised above 160 grams per liter. An attempt was then made t o duplicate iind extend the data by causticizing sodium carbonate solutions containiiig up t o 400 grams per liter of sodium carbonate, corresponding t o LL maxinium of about 300 grams per liter of total equivalent sodium hy1
Total Equivalent XaOH, G./Liter
SazCOa, G./Liter
1 2 3
EXPERIMENTAL AND DISCUSSION
NaZCOo
NaOH,
Present address, Stanford Research I n s t i t u t e , Menlo P a r k , Calif.
I
I
0
GOODWIN
I
I
I
1
I
1
I
---------
0 AUTHORSILUNGE -'-'-
I I 25 50 75 100 125 150 175
'0
I
I
200
225
250
TOTAL EOUIVAl.ENT NoOH
Figure 1.
I
Comparison of data on composition of causticized solutions
5
IO 20 30 50 100 Na2C0, groms/liter
-
200
500
1000
Figure 2. System calcium hydroxide-sodium hydroxidesodium carbonate in water a t 95" C.
Relation of sodium hydroxide t o sodium carbonate concentration
408
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1956 Table I .
Composition of Causticized Solutions
SaOH, G.jLiter 181.9 169.6 1.56 3 179.0 138.9 167.2 101.7 119.1 153.7 178.3
Run So.
107 110 113 142 144 145 147 218 219 220
Sa*COa, G./Liter 68.0 18.5 31.0 61.1
16.0 39.1 3 .2 11.0 30. 1 5f;.9
Table 11.
Total Equivalent h-aOH, G./Liter 936,2 20G. 2 150.2 217.2 146.0 136.7 105.R 127.4 176.7 220,s
0 /G
Tinie, Hours 5
Conversion 78.3 52.2
4
:e.,
4
7
r8.E 91.,
85.0 96.3 93.5 86 9 80.8
7 94 24 a4
Recalculated Data
Luiige's aiicl Good\\-in's results [as sliorvn by Olven (.$)I m-ere recalculated niid plotted on the same graph (Table 11). The considerable :ind increa.sing differelice betn-een the values obtained by Goodwin and by the present authors can be attributed t o the short running tiriie used by t h e former. \\-henever nii :tttempt \vn5 rilade t o o h t i n points 011 the causticization line : ~ ! ~ o vt ei tot31 r:cpivaleiit of sodium hydroxide of about24Ugrmisper liter, precipitation occurred uiitil the quantity returned to the level oi 240 grams per liter. -4.5 tlie solubility of sodium carbonate iii cawtic solution (as previouJj- determined) is well above 240 yiaiii,. per liter, it \vas cdiicludetl t h a t another phase ~ ~ probulily 1 3 present-in all lilielihood a double carbonate ,sucli as pirssoiiite (CaCOs.SazC0,.2H,0j. T o prove that t h t precipitate was a double carbonate, a batch ( S o . 164) WLLS .>et :I;J a t 210 grains per liter of sodium hydroxide and 12; gr.i:n-. pc'r iiter of sodium carbonate, correzponding very nearlj, t o pciiiit S o n Figure 2 . Lime vas ttdcled, the hatch was :I lit1 a sample was n-ithdr:tn-rl snd titrated. It ' i:i c,8)niposition,Thick1 n-as to be expected, as
409
the starting composition \ m a on the equilibrium line. C:Llciuiii carbonate \vas then added in increnicnts. The total cqiiivalent sodium hj-droxide titration showed a drop of 30 grams per liter in 2 hour,?) Tvhich eontiniied until :t v d u e of 230 grama per liter of total equivalent sodiuni hydroxide was reached. It seems apparent from the ::hove that the preaencc: of calcium cxbonnte is necezsary for the precipit:ition, which siilxtniitiateti the theory that the prccipit:ite i j a doublc carboii:itc. T-ery little inforni:ition on pirsoriitc i:, :ivaila:ile in tiw litwature. The heyt tl:ita :&rcthose bi- I%ur>-and Ttetld ( 1i for :i sy..tern sodium c~ii!ioiinte-c:t!ci~iiii cnrijon::tc-~rate~,cont:ri!:iiig no caustic;. .is &ita over the n-hole r:inge of c::llsiic c;oncentratior~ were of interwt, n &ea of experiments was iet. Bury sild Rcdd's data for 95' C . n-ere rccalculated on a grnrii per liter basis. This gave points (2 anti I? on Figure 2, representing the coilceiitration a t Tvliieh pirs.onite !\-ill appear in a bystem containing no c:~u,-tic,Q, and the mtnr:i.tion of the solutioii n-ith rebpect to aodiiim c;trlion:ite, R. d series of wliitioiis \vas then prepared conttiining not only s d i u n i rurbonate I l i I t illw v:irious amourits of call-tic, tLiliing care t h a t the soluhilifg line of sodium not exceeded G i l ' 'ionate W:LS added carbonate, IZS, 11-a~: in incrrment,s until tile precipit,ation of and the r e d t i i i y valrici were plotted :is ciirvc (ZP. The equilihriuni is nttsiricd s!owiy. 110-t experiments ~ w r erun for 4 to 8 days at 95' C. Tlierc i- nl>o :i dwided contiiig effect, for tlie adtiitioil of fre.jh calcium carbonate is necess'xy t o precipitate ii little more pir5sonite until the filial values are re:tched. Jqriilihrium wvn re:tclied when addition of socliuiii cnuhonate, evcn in the presence of excess calcium carbonate, did not c-harige the composition of the solution-i.e., it was convertrcl t o 1)irsoriite and completely precipitated.
l-
TLYAL EQi! VALENT h o O H - G r o r n s / l i t e r B
.,~. 1,
Figure 4. Relation of sodium hydroxide t o total equivalent caustic concentration S y s t e m calcium hydroside-sodium h?droaide-sodium c a r h o n n t e i n water at 95O C.
0
50
IO0 150 TOTAL EOUIVALENT NoOH-groms/litel
200
250
Figure 3. Relation of sodium carbonate to total equivalent caustic concentration S s s t e m calcium hydroxide-sodium
h:droxide-sodium water a t 95' C.
carbonate in
The intersection of the pirssoiiite line, QP?and tlie causticizrttion line, OS, at point P lies a t 181 grams per liter of sodium hydroxide and 64 grams per liter of sodium carbonate (a total equivalent sodium hydroxide of 2331, n-hich is lower than early results indicated. This discrepanc
