chemical of the month Lime
Table 1.
Kenneth W. Watkins
Equilibrium Pressures of CO. in Calcination
Temp ('6)
Colorado State University Fort Collins, CO 80523
Pressure ( a m )
700 600 897 1000
Lime is an extremely versatile substance that has heen used for over two thousand years in construction, agriculture, chemical manufacture, and during recent times, in pollution control and water treatment. Eighteen million tons of lime were produced in the United States during 1981 (1). This places lime fourth in annual tonnage produced behind sulfuric acid, ammonia, and nitrogen. The term lime does not refer to a single substance. Rather, it includes both quicklime and hydrated lime. Quicklime is calcium oxide, CaO, and hydrated lime is calcium hydroxide Ca(0H)a. These substances are sometimes confused with limestone, which is chiefly composed of calcite, CaC03.
2.92 X 1 0 V 0.220 1000 3.87
Table 2.
Preparation of Quicklime
DARRELL H. BEACH The Culver Academies Culver. Indiana 4651 1
Solubilitv of CalOHL
t (OC)
Solubility (gI100 g sat. soln.)
0 20 40 60 80
0.185 0.165 0.140 0.116 0.092
roo
0 071
yielding calcium hydroxide and evolving much heat. heated in a kiln to temperatures of 800-1000°C, it decomposes into calcium oxide and carhon dioxide. CsC031,,rt CaOI,)+ COz(,)AH,ss = 178.1 kJ The reaction is readily reversible and "recarbonation" of quicklime will occur in the presence of C02. The pressure of CO? in eauilibrinm with CaCO? and CaO increases withincreasing temperature. The equilibrium pressures are given in Table 1 ( 5 ) . During calcination in an open kiln, COa is carried away by convection currents. Therefore, its partial pressure is always less than the equilibrium value, and essentially all the CaC03 decomposes. The dissociation temperature of limestone is 897% the temperature at which COa pressure equals 1.0 atm. Lime production is energy intensive, requiring up to 0.33 tons of coal to produce one ton of quicklime. The purity of quicklime depends on the composition of the limestone from which it is derived. Limestone is not 100% a few percent MgO. "High calcium'; quicklikes are those with 95% or more CaO. Physical Properties
Quicklime is a white solid with a specific gravity varying from 3.2-3.4, the value being greater the higher the temperature of calcination. The specific gravity of CaO made by cooling molten CaO is 3.34. CaO melts at 2570°C and boils at 2850°C. It crystallizes to give colorless, transparent, cubic crystals with a hardness between 3 and 4 on Moh's scale (68). One physical property of quicklime has produced a familiar metaphor. The expression "being in the limelight" appears in newsprint quite often. When calcium oxide is heated by an oxvhvdronen flame it nlows with a brilliant incandescence. often than not, took place in that part of the stage cast in the limelight ( 6 , 9 ) . Chemical Properties
Quicklime is the cheapest source of hydroxide ion. CaO reacts vigorously with water in a process called slaking, 60
Journal of Chemical Education
Calcium hydroxide is known as slaked lime and hydrated lime. Hydrated lime is a dry powder made by adding just enough water to quicklime to satisfy its chemical affinity for water. Calcium hydroxide can be decomposed back into calcium oxide and water at temperatures above 400°C. Clear aqueous solutions of calcium hydroxide are known as limewater. The solubility of calcium hydroxide in pure water is low enough for it to be regarded as very slightly soluble. The solubility (Table 2) decreases with increasing temperature (6, 7).
-
Ca(OHh(,)+ H2Ojl~ Ca2+(,,)+ 20H-(,,) AH2sl = 11.7 kJ Like calcium oxide, calcium hydroxide reacts with carbon dioxide. The reaction requires moisture and yields calcium carbonate and water. C~OH)X,,I+ COzi,) + CaCOx,) + HLtc History
Lime is one of the oldest materials known to mankind. Until the rapid growth of the chemical industry in the 20th century, lime was used almost entirely as a building and agricultural material. Many ancient civilizations appear to have discovered lime independently. Lime plaster was used in Crete as early as 1500 B.C. Theophrastos (370-285 B.C.), a pupil of Plato and Aristotle, described lime in his treatise "On Stones" (10, 11). Pliny (23-79 A.D.), wrote how lime was prepared by the Greeks in building the Temples of Apollo and Elis in 450 B.C. The Wall of China was largely laid with lime mortar. Lime was also used by the Incas and Mayans. There is considerable doubt that the Envwtians made swecific use of lime mortar
The slructure. properties, and uses of a variety of chemicals are highlighted inthis feature which isaimed at increasing the use of descriptive Chemistry.
I t was the Romans who made extensive use of lime mortar ( 4 , 6 , 8 , 12). They perfected its production to give aproduct of remarkable hardness. There are many Roman houses, temples, and aqueducts that still stand today as examples of their superior knowledge of mortar. The famous Appian Way which has endured for 2000 years contains lime in three of its four layers. The chemical knowledee " of the Greeks and Romans was not sufficiently advanced to explain what happens during lime burnine. Vitruvius. a Roman eneineer and architect who wrote duringthe reign of Augustus (B.c. 27-A.D. 141, mad&the significant observation t h a t . . ."limestone when taken out of the kiln cannot he as heavy i s when it was thrown in."He reported the loss of about a third of its weight. The theoretical loss of pure calcium carbonate would he 44%. Vitruvius considered the properties of lime to result from the loss of water during the hurning operation (11,121. The first modern explanation of lime calcination was provided by the British chemist Joseph Black who in 1756 pnblished his work "Experiments upon Magnesia Alha, Quicklime, and Other Alcaline Substances" ( 6 , 9 , 11-13). He observed that the gas given up by chalk and limestone on heating was carbon dioxide which at that time was called "fixed air." CaCOs-
CaO
+
COz
chalk quicklime fixed air The hardening of mortar can be explained using two reactions mentioned earlier. Mortar consists of one part lime and ahout three parts sand, to which enough water is added to make a paste. When fresh mortar, which is mostly Ca(OH)2, is exposed to air, it slowly sets or hardens by gaining COz and losing water. Here again, this process was observed by Black who noted that limewater clouded when in contact with air containing fixed air. CaOi,) Ca(OH)zi,,i
+ HnOic + COzi,r
-
-
C~(OH)X.,I CaCOw
+ HzOm
Thus, mortar reverts to something like the original limestone. The sand in mortar is inert. The grains are bound together mechanically by the particles of CaCOs hut do not enter into chemical reaction. The impermeability of mortar to gases is illustrated by modern analysis of Roman mortars, which show in many cases that the interior has not become carbonated in contrast to the exterior. One of the oldest uses of quicklime was in agricultural liming where the addition of lime to soils predates the Christian era. Yields of many diverse crops can he significantly increased bv liminr. This ~ r a c t i c ewas widelv followed in colonial and i9th cenku-y America (12,14),hut since 1914 lime use has declined sienificantlv. In manv sections of the eastern U.S. there was a h&nemade"rock "pot" kiln on almost every farm of any size. In the last sixty years limestone has widely replaced lime use in agriculture because of its lower cost. Modern farmers who refer to "liming" soils are probably using pulverized limestone, not lime. Only in recent times has it become known that the function of liming was to neutralize soil acidity and supply the essential plant nutrients Ca and Mg along with certain trace elements. Lime was also used as a chemical reagent. Xenophon in 350 B.C. records the wreck of a ship near Marseilles that was carrvine a careo of linen and lime "for its bleachine." The Ro&an;~iosc&des, cited the medical use of saturated solutions of limewater in 75 A.D. In the days of wooden sailing ships it was very dangerous to carry quicklime. Occasionally, a vessel sprang .a leak, and the heat evolved uuon the slakine of lime was enough to start a fire. Lime use in construction and agriculture has decreased as lime mortar has been replaced by Portland cement, and farmers have switched to fertilizers and the cheaper limestone to condition soil.
Since 1900, progressively larger quantities of lime have been used in industry as a chemical reagent. In 1973 about 92% of the lime produced was used as a chemical, rather than a building or arricultnral material. Lime is used in metallurgy, .. chemical manufacture, paper and pulp making, water softening, pollution control, and in the food and petroleum industries. Those uses that account for the highest consumption will he discussed. Steelmaking
The greatest use of lime by far is in steelmaking which accounts for almost 45% of lime use. In steelmaking lime acts as a basic flux in the removal of acidic impurities ( 3 , 4 , 6). In most iron ores the chief impurities are various chemical forms of silicon, phosphorus, and sulfur. Quicklime is used extensively in the newer, faster "basic oxygen furnace" which is replacing the older "open hearth" process. In the basic oxygen furnace the impurities are oxidized by a blast of high purity 0 2 . They are then reacted with a flux and the resultant slag is drawn off. To make a strong basic flux, calcium oxide is added in an amount equal to three times the weight of silica, SiOe, present. The calcium oxide then combines with the silica from the slag, plus phosphate and sulfide from the metal phase.
+ SiOz
CaO
3CaO + Fea(PO&
and
+
---
6CaO PnOlo CaO FeS
+
CaSiOs CadPO&
+ 3Fe0
2Caa(P04)z CaS FeO
+
The resulting gangue or slag is a glassy, molten mass of less density than molten iron. Therefore, it collects as a pool on top of the metal. The slag which now contains the impurities, is drawn off and can be used as a component in making cement. The terms basic flux, and basic oxygen furnace come from CaO reacting as aLewis base. The reaction with the Lewis acid SiOz illustrates this.
..-
Ca :O: + SiO,
-
CaSiO,
The hasic oxygen furnace uses from 10tL160 lb of CaO per ton of ingot steel. Magnesium and Magnesia
Hydrated lime is used for the production of magnesium and magnesium hydroxide (magnesia) by the Dow Seawater process ( 4 , 7). Seawater contains 0.13% Mg2+ by weight. Mg(OH)2 is precipitated by adding Ca(OH)2to seawater until a high enough concentration of OH- is reached and magnesium hydroxide precipitates (the soluhility of Mg(OH)2 is 0.009 g/l at 25°C). MgZ+i,,~f CaZ+i,,)
+ 20H-(,,1-
M~(OH)Z(,) + Ca2+i,,)
In this process oyster shells serve as a source of very pure CaCOs from which CaO and then c a ( 0 H ) are ~ produced. Neutralization of magnesium hydroxide with hydrochloric acid yields a solution of magnesium chloride. The water is evaporated off, and the dry MgClz is melted at 708°C. Electrolysis of molten magnesium chloride produces the desired metal. Mg(OHh
+ 2HCl-
MpC12
+ 2H20
-
Calcium
Quicklime is the source of calcium in the preparation of calcium metal. Calcium is prepared by a thermal reduction process that has replaced the electrolysis of calcium chloride Volume 60
Number 1 January 1983
61
( 1 5 ) .In this process calcium is obtained hy the reduction of calcium oxide with aluminum a t high temperature.
6Ca0 + 2A1- 3Ca + Ca3A1206 The quicklime is briquetted with aluminum powder and charged into one end of a long tubular, stainless steel retort. The retorts are evacuated and heated to 1200°C at the end containing the charge, while the other end is cooled. As calcium is formed it then distills and condenses at the cool end. Calcium Carbide Calcium carbide, an important source of acetylene, is formed hy mixing quicklime with coke and heating to a temperature of 2000°C (6, 7).
- + -
Ca(0H)z Ca2++ 20H20H- + 2HCOi 2COg- + 2Hz0 Insoluble calcium carhonate then precipitates Ca2+ COZ-~ CaC08(si The net reaction resulting from these three steps is The net effect of adding calcium hydroxide is to convert Ca(OH)2into CaC03, and HCOl into C0,:-. The carhonate ion then reacts with another mole of Ca2+. The overall reaction for the lime-soda process is Ca(OHI2
+ Ca2++ 2HCO;
-
2CaC03(,)
+ 2Hz0
For each mole of calcium hydroxide added, one mole of Caz+ t d~ uthe ~ l y and two moles of hicarhonate oriainallv mesent in the hard \lulten ,.:~lt.iumctrlridt. l i ~ w ~ t i ~ r ~e ~~n ~~ \ .~t'rom - .. III~I~H e ,C,ind d t e r s d i d i ~ y i nI! ~ i>. m ~ s h ,an11 d grtuld 11, the water are removed. d
