Chapter 5

Chemicals from Limestone

The next major raw material for which we discuss the derived chemicalsis calcium carbonate, common limestone. It is the source of some carbondioxide, but, more importantly, it is used to make lime (calcium oxide) andslaked lime (calcium hydroxide). Limestone, together with salt andammonia, are the ingredients for the Solvay manufacture of sodiumcarbonate, soda ash. Soda ash is also mined directly from trona ore. TheSolvay process manufactures calcium chloride as an important by-product.Soda ash in turn is combined with sand to produce sodium silicates tocomplete the chemicals in the top 50 that are derived from limestone. Sincelime is the highest-ranking derivative of limestone in terms of total amountproduced, we discuss it first. Refer to Fig. 2.1, Chapter 2, Section 1, for adiagram of limestone derivatives.

1. LIME

CaO

Before going further, let us clarify the various common names oflimestone and lime. The following is a summary of the nomenclature andthe chemicals referred to. Industrial chemists quite often use the commonnames for these substances rather than the chemically descriptive names.

CaCOs: Limestone, calcite, calcium carbonate, marble chips, chalkCaO: Lime, quicklime, unslaked lime, calcium oxide

Ca(OH)2: Slaked lime, hydrated lime, calcium hydroxide. A saturatedsolution in water is called limewater. A suspension in wateris called milk of lime.

1.1 Manufacture

Lime is one of the oldest materials known to humankind. It was used byRomans, Greeks, and Egyptians for the production of cement and wasemployed in agriculture as well. One of the first things done by Americansettlers was to set up a lime kiln for the "calcining" or heating of limestone.

1.1.1 Reaction

CaCO3 ^ CaO + CO2

CaO + H2O ^ Ca(OH)2

Common temperatures used in converting limestone into lime are 1200-130O0C. For this reason lime is a very energy-intensive product. It takes theenergy from a third of a ton of coal to produce 1 ton of lime.

1.1.2 Description

Fig 5.1 outlines lime production. The limestone is crushed and screenedto a size of approximately 4-8 in. There are different heating techniques andkiln styles. The one diagrammed is a vertical Dorrco Fluo Solids system.The limestone enters the top. Air entering the bottom "fluidizes" the solidsto get better circulation and reaction. Approximately 98% decarbonation istypical. When a kiln is used in conjunction with the Solvay process and themanufacture of soda ash, coke can be fired in the kiln along with limestoneto give the larger percentages of carbon dioxide needed for efficient soda ashproduction by the reaction of the carbon with oxygen to give carbon dioxide.If a purer lime product is desired, the fine lime can be taken from area 4. Aless pure product is obtained from the bottom kiln section. Another kind ofkiln is the rotating, nearly horizontal type. These kilns can be as much as 12ft. in diameter and 450 ft. long. Limestone enters one end. It is heated,rotated, and slowly moves at a slight decline to the other end of the kiln,where lime is obtained.

CaCO3

LimestoneCrusher Dryer Screening

Ca(OH)2

Slakedlime

Slaker

Finelime

CaOLime

CoolerCooler

Kiln

Fluidizing air

Figure 5.1 Lime manufacture.

1.2 Uses

For most applications slaked lime is sold. The hydration of lime is veryexothermic and could ignite paper or wood containers of the unslakedmaterial. Slaked lime is slightly soluble in water to give a weakly basicsolution.

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux inthe manufacture of steel. Silicon dioxide is a common impurity in iron orethat cannot be melted unless it combines with another substance first toconvert it to a more fluid lava called slag. Silicon dioxide is a Lewis acidand therefore it reacts with the Lewis base lime. The molten silicate slag isless dense than the molten iron and collects at the top of the reactor, where itcan be drawn off. Over 100 Ib of lime must be used to manufacture a ton ofsteel.

CaO + SiO2 ^ CaSiO3

Table 5.1 Uses of Lime

Metallurgy 40%Pollution control 15Water treatment 10Chemical manufacture 10Pulp paper 5Construction 5Miscellaneous 15

Source: Key Chemicals

The uses of lime in chemical manufacture are too numerous to discusssince over 150 important chemicals are made with this basic material. Infact, only five other raw materials are used more frequently than lime forchemical manufacture: salt, coal, sulfur, air, and water. The most importantchemical derivative of lime is soda ash, although the synthetic product hasbeen a small percentage of all soda ash in recent years.

A growing use of lime is in pollution control, where lime scrubbersplaced in combustion stacks remove sulfur dioxide present in combustiongases from the burning of high sulfur coal.

SO2 + H2O ^ H2SO3

Ca(OH)2 + H2SO3 *• CaSO3 + 2H2O

Lime is used in water treatment to remove calcium and bicarbonate ions.

Ca(OH)2 + Ca+2 + 2HCO3" +> 2CaCO3 + 2H2O

Lime is employed in the kraft pulping process to be discussed in detail inChapter 22, Section 3.1. Most of it is recycled. Without this recycling thepulp and paper industry would be the largest lime user. The main reaction oflime in the kraft process is for the purpose of regenerating caustic soda(sodium hydroxide).

Na2CO3 + CaO + H2O ^ CaCO3 + 2NaOH

The caustic soda is then used in the digestion of wood. The lime isregenerated from the limestone by heating in a lime kiln.

A large part of Portland cement is lime-based. Sand, alumina, and ironore are mixed and heated with limestone to 150O0C. Average percentages ofthe final materials in the cement and their structures are given here.

21% 2CaO^SiO2 Dicalcium silicate52 3CaO*SiO2 Tricalcium silicate11 3CaO^Al2O3 Tricalcium aluminate9 4CaO • Al2O3 • Fe2O3 Tetracalcium aluminoferrite3 MgO Magnesium oxide

The percentage of dicalcium silicate, sometimes abbreviated as C2S in theindustry, determines the final strength of the cement. The amount oftricalcium silicate, C3S, is related to the early strength (7-8 days) required ofthe cement. Tricalcium aluminate, C3A, relates to the set in the cement.

Tetracalcium aluminoferrite, C4AF, reduces the heat necessary inmanufacture.

1.3 Economics

The production history of lime is given with other chemicals in Fig. 2.2,Chapter 2, Section 1. Production dropped more for lime than most otherchemicals in the 1980s, 2.4% per year. Lime production is very dependenton the steel industry, which in turn fluctuates directly with automobile andhousing demand. But production rose again in the 1990s. Lime, being anenergy intensive chemical because of the high temperatures required to makeit from limestone, fluctuates more with energy prices than most otherinorganic chemicals. From 1970-1975 the price rose from $12/ton to$28/ton, mainly because the oil embargo increased energy costs. Presently itsells for $57/ton or about 2.90/lb and the commercial value of its 45.2 billionIb is $1.3 billion.

2. SODA ASH (SODIUM CARBONATE)

Na2CO3

2.1 Manufacture

The LeBlanc process for the manufacture of soda ash was discovered in1773 and was used universally for many years in Europe. Salt cake (sodiumsulfate) reacts with limestone to give soda ash and a troublesome sideproduct gypsum (calcium sulfate). The process is no longer used.

Na2SO4 + CaCO3 —-—»»Na2CO3 + CaSO4

In 1864 Ernest Solvay, a Belgian chemist, invented his ammonia-sodaprocess. A few years later the soda ash price was reduced one third. TheSolvay process had completely replaced the LeBlanc method by 1915. TheSolvay method is still very popular worldwide. However, in this countrylarge deposits of natural trona ore were found in the 1940s in Green River,Wyoming. In the last few years there has been a tremendous conversionfrom synthetic to natural soda ash. The first and last Solvay plant in the U.S.closed in 1986 (a large Allied Chemical plant in Solvay, NY). Trona ore isfound about 50Om below the surface. It is called sodium sesquicarbonate

and is mostly 2Na2CO3*NaHCO3*2H2O (45% Na2CO3, 36% NaHCO3, 15%water + impurities). Heating this ore gives soda ash. The conversion fromthe Solvay process to natural soda ash has been called one of the mostsuccessful chemical industry transformations of the late 1970s and early1980s. The ratio of production for selected years certainly proves this point.

Year 1948 1974 1981 1985 1986

Solvay 94 46 9 6 ONatural 6 54 91 94 100

Despite the fact that no new Solvay plants have been started since 1934in this country, it is still an important method worldwide. There is somefascinating chemistry in this involved process and we will discuss it.

2.1.1 Solvay Reactions

FCaCO3 +> CaO + CO2 ~~|source of CO2

[CaO + H2O *> Ca(OH)2J

2NH3 + 2H2O ^ 2NH4OH source OfNH4OH

"2NH4OH + 2CO2 *• 2NH4HCO3

2NH4HCO3 + 2NaCl *• 2NaHCO3 + 2NH4Cl mam S°lvay

reactions_2NaHCO3 *• Na2CO3 + CO2 + H2O

2NH4Cl + Ca(OH)2 ^ 2NH3 + CaCl2 + 2H2O recycle of NH3

CaCO3 + 2NaCl ** Na2CO3 + CaCl2 overall reaction

2.1.2 Description

A detailed description of salt mining will be postponed until the nextchapter, but it is important to note that soda ash is made from both limestoneand salt, the two major raw materials. As outlined in Fig. 5.2, the brine (saltsolution) is mixed with ammonia in a large ammonia absorber. A lime kiln,using technology similar to that discussed earlier, serves as the source ofcarbon dioxide, which is mixed with the salt and ammonia in carbonationtowers to form ammonium bicarbonate and finally sodium bicarbonate andammonium chloride. Filtration separates the less soluble sodium bicar-bonate from the ammonium chloride in solution.

NaHCO3NH4Cl

CO2, H2O (recycle)

CaCO3 _Limestone

Figure 5.2 Manufacture of soda ash.

The sodium bicarbonate is heated to 1750C in rotary dryers to give lightsoda ash. The carbon dioxide is recycled. Light soda ash is less dense thanthe natural material because holes are left in the crystals of sodiumbicarbonate as the carbon dioxide is liberated. Dense soda ash, used by theglass industry, is manufactured from light ash by adding water and drying.

The ammonium chloride solution goes to an ammonia still where theammonia is recovered and recycled. The remaining calcium chloridesolution is an important by-product of this process, although in largeamounts it is difficult to sell and causes a disposal problem.

2.2 Uses

Table 5.2 outlines the uses of soda ash. Glass is the biggest industryusing soda ash. The 49% used by this industry is divided into 44% bottlesand containers, 38% flat glass, 9% fiberglass, and 9% other. The glassindustry is very complex and would take some time to discuss at length.There are about 500 different kinds of glass. However, 90% of all glassmade is soda-lime-silica glass, which incorporates ingredients to be heated togive an approximate weight ratio of 70-74% SiO2, 10-13% CaO, and 13-16% Na2O. These glasses can be used for windows, containers, and manytransparent fixtures. The sand must be nearly pure quartz, a crystalline formof silicon dioxide. These deposits often determine the location of glass

NH3 (recycle) CO2 (recycle) CO2 (recycle)

NaClBrine

Am

mon

iaab

sorb

er

Car

bona

ting

tow

er

Car

bona

ting

tow

er

Lim

e ki

ln+

sla

ker

Am

mon

iast

ill

Vacuumfilter

NH4Clsolution

NaHCO3

solidRotarydryer

LightNa2CO3

DenseNa2CO3

Densifler Mill

Table 5.2 Uses of Soda Ash

Glass 49%

Chemical manufacture 27

Soaps/detergents 11

Flue gas desulfurization 3

Pulp and paper 2

Water treatment 2

Miscellaneous 6

Source: Chemical Profiles

factories. Sodium oxide is principally supplied from dense soda ash, butother sources of the oxide include sodium bicarbonate, sodium sulfate, andsodium nitrate. Some nitrate is generally used because it will oxidize ironimpurities and avoid coloration of the glass. Limestone is the source of lime.When these substances are heated the following reactions occur.

Na2CO3 + 0SiO2 ^ Na2O - aSiO2 + CO2

CaCO3 + ^SiO2 ^ CaO • 6SiO2 + CO2

For common window glass the mole ratio may be 2 mol Na2O, 1 molCaO, and 5 mol SiO2. Glass is essentially an amorphous, multicomponentsolid mixture. Specific CaO-SiO2 or Na2O-SiO2 compounds do not exist.The addition of borax increases the glass resistance to acids and thermalshock. This is called Pyrex® glass.

In many other uses soda ash competes directly with caustic soda as analkali. The chemical of choice is then dependent on price and availability ofthe two.

3. CALCIUM CHLORIDE

CaCl2

Calcium chloride is no longer in the top 50, but it is very high in thesecond 50 chemicals (see Chapter 13). Because it is an important by-productof the Solvay process, we will mention it here. Besides being a Solvay by-product it is also obtained from natural brines (especially in Michigan). Atypical brine contains 14% NaCl, 9% CaCl2, and 3% MgCl2. Evaporation

Table 5.3 Uses of Calcium Chloride

Road Deicing 40%

Road dust control 20

Industrial processing 20

Oil and gas wells 10Concrete 5

Miscellaneous 5

Source: Chemical Profiles

precipitates the sodium chloride. The magnesium chloride is removed byadding slaked lime to precipitate magnesium hydroxide.

MgCl2 + Ca(OH)2 ** Mg(OH)2 + CaCl2

The uses of calcium chloride are given in Table 5.3. A large amount ofcalcium chloride is used on roads for dust control in the summer and deicingin the winter. The dust control use percentage includes some for roadwaybase stabilization. It is less corrosive to concrete than is sodium chloride. Adebate on which is worse environmentally on local plant life because of highsalt concentrations remains to be resolved. The home ice-melt market forcalcium chloride has grown recently. Local governments are also usingmore calcium chloride. Another recent competitor in the market is calciummagnesium acetate, made by reaction of high-magnesium content lime withacetic acid. This salt could prove to be a noncorrosive alternative to thechlorides. Calcium chloride is used for some industrial refrigerationapplications. Saturated calcium chloride does not freeze until - 5O0C,whereas saturated sodium chloride has a freezing point of- 2O0C.

4. SODIUM SILICATE (SILICA GEL)

Soda ash is heated with sand at 1200-140O0C to form various sodiumsilicates (over 40 of them), which collectively are produced at levelssufficient to rank in the top 50. Some common ones are listed here.

Na2CO3 + ^SiO2 *» Na2O • ̂ SiO2 + CO2

Table 5.4 Uses of Sodium Silicate

Soaps and detergents 38%

Silica gel and catalysts 15

Pulp and paper 12

Rubber and elastomers 7

Food and health care 5

Agriculture 3

Paints and coatings 3

Miscellaneous 17

Source: Chemical Profiles

Ratio ofSiO2TNa2O

Sodium tetrasilicate Na2Si^ 4

Sodium metasilicate Na2SiOs 1

Sodium sesquisilicate Na3HSiO4 • 5H2O 0.67

Sodium orthosilicate Na4SiO4 0.50

Table 5.4 gives the uses of sodium silicate. It is a partial replacement forphosphates in detergents as a builder (see Chapter 24, Section 6) that doesnot pollute rivers and lakes. This is a growing use of sodium silicate. As afine silica gel with a large surface area it is used for catalysis and columnchromatography. In the pulp and paper industry it is used as a hydrogenperoxide stabilizer. Hydrogen peroxide is a pulp bleaching agent that isgrowing in use, replacing chlorine. In rubber tires it is replacing some of thecarbon as a reinforcing agent, yielding the so-called "green tire."

Suggested Readings

Austin, Shreve 's Chemical Process Industries, pp. 181-185.Chemical Profiles in Chemical Marketing Reporter, 1-18-99, 2-1-99, and 6-

5-00.Kent, Riegel's Handbook of Industrial Chemistry, pp. 409-414.Thompson, Industrial Inorganic Chemicals: Production and Uses, pp. 123-

148.