Laboratory work № 6 Studying of firmphases interactions between СаО – SiO2

Aim of the work- to define interactions between СаО – SiO₂

Theoretical information

In gases and liquid mixtures owing to the large mobility of particles, in presence of favorable thermodynamic conditions, the reactions proceed rather easily. In firm bodies the mobility of particles is rather weak, and basically it’s limited by fluctuations of particles, composing a crystal lattice, concerning their equilibrium position. Therefore processes of chemical interaction between firm bodies proceed hardly and require specific conditions ensuring forward movement of particles and destruction of bonds of a crystal lattice. Firmphases reactions are practically never reached.

At production of most silicate materials, the interaction of components begins with reaction in a firm condition and has great significance for many important industrial technologies. Such processes proceed in cement, glass and ceramic mixtures at their heating up to occurrence of a liquid phase.

The mobility of elements of structure of reacting substances is increased according to raise of temperature and at achievement of its certain size becomes possible reaction in a firm condition. G. Tamman and his followers have found that temperature of reactions beginningin a firm condition practically coincides with temperature of sintering, i.e. with the moment, when grains of a powder mixture under influence of temperature, and sometimes additional pressure is high enough and correspond to 0,8-0,9 of absolute temperature of melting.

At firmphase process baking, polymorphic transformations, crystallization preceed chemical interactions or proceed simultaneously.

The speed of reaction in a firm condition is determined by internal and surface diffusion due to which moving elements of a lattice occur, even one of reagents through a surface line. Internal diffusion or selfdiffussion for oxides according to Hutting and Markus comes at temperature about 0.5 absolute temperatures of melting, and surface or heterodiffusion approximately 0,26 of this temperature. Proceeding from this, for SiO₂, structural diffusion begins at the temperature 870-900 ºС and heterodiffusion at 550 ºС.

Diffusion depends on a structure of crystal lattices of reacting substances, from defects of their structure and structure of intermediate products of the reaction. Polymorphic transformations connected with reorganization and loosening of structure promote to diffusion. Recrysstalization of components causing "treatment" of defects lattices brakes diffusion. Raise of temperature accelerates the diffusion.

Thus, diffusion and temperature are major kinetic factors influencing a course of reaction in firm mixtures.

One more very important kinetic factor is area of direct contact of grains of reacting components, which can be increased at the expense of increase of a specific surface of a powder (its dispersion) and pressing for creation of contact.

But strictly firmphase reactions are rare. In real technological processes frequently reactions between solid substances proceed in the presence of some liquid or gas appearing in the system as a result of dissociation of firm substance (for example, decomposition of carbonate, dehydrotation of clay substance etc.) or formation of small lowtemperature eutectics, caused by impurity in initial products entered by mineralizer. In these cases reactions are considerably accelerated.

The character of reactions in a firm condition makes essential influence on mineralformation at high temperatures and development of sintering being a ultimate goal of many technological processes.

Waterless calcium silicates and its importance

At interaction СаО and SiO₂ four compounds are formed: calcium metasilicate - CaO∙SiO₂(CS), tricalcium disilicate - 3CaO∙2SiO₂(C₃S₂), calcium ortosilicate - 2CaO∙SiO₂(C₂S) and tricalcium silicate - 3CaO∙SiO₂(C₃S) (figure 3).

Figure 3 - Diagram of a condition of twocomponent system СаО – SiO₂

In a nature a mineral wollastonite is most distributed - β-CaO-SiO₂ forming in contacts of abyssal rocks with limestone. Wollastonite rocks are valuable raw material for production of special kinds of ceramics and glasscrysstallic materials. In contact rocks together with monocalcium are found and bredigite - β-2CaO∙SiO₂, larnite - β-2CaO∙SiO₂, shannonite - γ-2CaO∙SiO₂, rankinite - 3CaO∙2SiO₂, however they are rather rare and also have not practical meaning.

At the same time, the minerals of system СаО – SiO₂ are widely distributed in technical products. Among them there is threecalcium silicate, not having analogues in nature. Those or others of the named minerals are contained of in slags of black metallurgy, electrothermophosphorus production is widely used as raw material to receive silicate materials. Exclusive importance of waterless calcium silicates in production of cements, some kinds of ceramics and glasscrysstallic materials.

The basic properties of chemical compounds in system СаО – SiO₂ are given in the table 1.

Performance technique of experiment in research of reaction between silica and calcium oxides

Task of experiment is to study process of dissociation and dehydrotation of initial components in weight losses at heating mixtures and quantitative definition of products of reaction between СаО and SiO₂.

The initial components for work chemical reagents СаСО₃ and Са(ОН)₂ and various natural quartz sand are chosen. The ratio of initial components in mixtures is defined by the teacher. Taking into account that calcium oxide in a mixtures is entered as СаСО₃ and Са(ОН)₂ it is necessary to make the appropriate recalculation. Dissociation of initial components exert essential influence on initial temperature and character of the subsequent chemical interactions. These processes in work are studied in weight losses at continuous heating of mixtures.

General weight M of a mixture in quantity of 0,005 kg (5 g) is carefully rubbed in porcelain motar up to a subtlety powder, weighed on technical balanse, mixed and pressed in a tablet under pressure 150 kgs/sm².

The received tablet is placed in corundum or porcelain crucible on a rod in the centre of the furnace with silicate heaters with balance. After that the furnace is turned on and observation begins.

Changes in weight of a researched material is carefully observed in an interval of temperatures 100-150 °С, 500-600 °С and 800-1200 °С in every 10 ° of rise of temperature. The choice of temperature intervals for observation of losses in weight is caused by temperatures of losses of hydroscopic moisture, dehydratation of Са(ОН)₂ -(530-580 °С) and decarbonization of СаСО₃ - (860-920 °С).

The data of measuring is writen in the form:

Time in minutes., (τ)	Temperature in degrees, (t)	Weight in kg, (m)	Loss in weight in kg, (∆m)

Then the graphic in coordinates are made out t - ∆m and τ - ∆m.

According to the data of Yander and Veier, prepared, confirmed by numerous researches of other authors, the sequence of chemical transformations in the system of СаО – SiO₂ has step character and, regardless of ratio of initial components, a primary product is calcium ortosilicate - 2CaO∙SiO₂. Further 3CaO∙2SiO₂ is formed and only in the subsequent stages metasilicate - CaO∙SiO₂ is vigorously formed (figure 2).

Schematical process can be shown in this way:

СаО + SiO₂ → 2CaO∙SiO₂

2CaO∙SiO₂ + SiO₂ →2 (CaO∙SiO₂)

The process of firmphase interaction proceeds extremely slowly. Taking into account conditions of experiment (final temperature of burning is 1200 °С and time of keeping of a mixture at final temperature is 1 hour). It is expected that calcium ortosilicate is basic product of reaction.

Easily determined phase of the system is free calcium oxide for its quantitative definition ethyl-glycerate method is chosen for which.

If the ratio of initial substances is known, knowing amount of untied calcium oxide, it is easy to calculate amount of formed calcium ortosilicate. The possible insignificant amounts of other products of reactions can be neglected.

On the basis of the given losses of weight and quantitative definition of calcium ortosilicate, the student should make independent conclusions explaining character of course of firmphase interactions in conditions of experiment and their features depending on a kind of calciumcontaining component.

Definition of untied calcium oxide by ethyl-glycerate method

Ethyl-glicerate method of quantitative definition of free lime in cement developed E.I.Nagerova and N.M.Kolendzyan is widely used in the control of production. It is suitable for definition of the total contents of free lime, both as СаО, and as Са(ОН)₂.

The method is based on processing of an analyzed material by a hot mixture of waterless glycerine and absolute alcohol as the result interaction of untied lime with waterless glycerine calcium glycerate is formed. Glycerate is titrated by alcoholic solution of benzoic acid by the reaction:

CH₂OH CH₂O

С аО + CHOH CHOH Ca + H₂O

CH₂OH CH₂O

glycerine calcium glycerate

C H₂O CH₂O

C HOH Ca + 2C₆H₅COOH CHOH + Ca(C₆H₅COO)₂

benzoic acid benzoicsour

CH₂O CH₂O calcium

calcium glycerate glycerine

The calcium glycerate passes into the solution.

Quantitative definition of untied calcium oxide is carried out in the following order: in dry conic retort in capacity of 150 ml, 30 ml of a solution of glycerine in alcohol of barium chloride and phenolphthalein, is poured thinkpounding, weighed on analytical balance, (about 1 g) test. Retort is added is joined with the help of test tube to a return refrigerator above an electrical plate. Periodically stirring up contents of retort is boiled within one minute. The speed of occurrence of pink colouring in test depends on amount of free lime contained in a material. If after half-hour boiling the colouring does not occur, it is possible to consider that free СаО in test is absent.

At occurrence of pink colouring a boiling liquid every 10-15 minute of boiling is titrated by a solution of benzoic acid till wollastonite disappearance of pink colouring. Heating and titration is repeated until occurrence of pink colouring after 20 minutes of boiling stoppes y. Then titration is considered to be completed.

Processing of results

The content of free untied calcium oxide in per cent is calculated in the following formula:

where, V - amount of a solution of benzoic acid used up for titration, ml;

A - dry weight, g;

C - titer of benzoic acid expressed in grams of СаО, i.e. amount of СаО connected in 1 ml of a solution of benzoic acid by reaction (2).

ATTENTION! All contents of retort after titration should be poured out in special vessel.

The table 1 - Properties of chemical compounds in system СаО – SiO₂

Properties and structural parameters	Calcium oxide (lime) CaO	Calcium metasilicate		Rankinite 3CaO∙2SiO2	Calcium ortosilicate Ca2SiO4			Threecalcium silicate
		Monocalcium β-CaOSiO3	psevdovol lastonite		γ= Ca2SiO4	β= Ca2SiO4	α= Ca2SiO4
1	2	3	4	5	6	7	8	9
Chemical composition, (weight, %)		CaO-48.2 SiO2-51.8	CaO-48.2 SiO2-51.8	CaO-58.2 SiO2-41.8	CaO-65 SiO2-35	CaO-65 SiO2-35	CaO-65 SiO2-35	CaO-73.6 SiO2-26.4
Syngony	cubic	monoclinic	monoclinic	rhombic	rhombic	rhombic	monoclinic	monoclinic triclinic
Parameters of a cell (Å)		a=7.94 b=7.32 c=7.07
Parameter of refraction	1.837	p=1.631 p=1.629 p=1.616	p=1.634 p=1.611 p=1.610	p=1.650 p=1.645 p=1.641	p=1.654 p=1.642	p=1.735 p=1.717	p=1.737 p=1.715	p=1.722 p=1.717
Density, kg/m3	3340	2915	2912		3980	3280-3400	3280-3310	3120-3250
Hardness (by Moose)		4.5-5
Temperature of melting (decomposition), ºC	2570±10	β-CaOSiO3 α- Ca2SiO3	Congruently at 1544 ºC	Incongruently 3CaO∙2SiO2 2CaO∙SiO2 melting	γ-Ca2SiO4 Ca2SiO4	β- Ca2SiO4 α - Ca2SiO4	Congruently at 2130 ºC	Incongruently at 2079 ºC
Factor of linear expansion 0	94
Colouring	Colourless	Colourless	Colourless	Colourless	From colourless up to yellowish - brown tone			Colourless
	2.39 (100) 1.69 (63) 2.76 (40) 1.44 (20)	2.697 (100) 3.83 (80) 3.52 (40)	3.23 (100) 2.80 (75) 1.98 (63) 3.42 (15)	3.76 (10) 3.148 (10) 2.675 (10) 4.04 (8)	2.72 (100) 3.00 (80) 2.74 (70) 1.90 (60)	2.78 (10) 2.73 (7.5) 2.60 (4.2) 2.19 (4)	2.74 (10) 3.01 (4.8) 1.90 (4.8) 1.80 (3.2)	2.78 (10) 2.61 (9) 2.75 (7) 3.03 (3)