Showing posts with label Quality control. Show all posts
Showing posts with label Quality control. Show all posts

June 7, 2009

Effects of Grain Size Distribution and Powder Characteristics on Sintering, Densification and some other Properties of Refractory Bricks

We assume that the reader is already aware with the concept of ‘Sintering’, and also the types and effects of sintering on various properties of refractory bricks. In this article we will discuss on the effects of Powder characteristics (Grains size and their distribution) on sintering and densification behaviour of any refractory brick.
Densification is an important objective of sintering. Characteristics of starting powder e.g., particle size, size distribution, particle shape, particle aggregates, degree of agglomeration have a profound effect on the sintering kinetics as well as densification and Microstructural development of a refractory brick. The current understanding of ceramic powder processing has led to the following description of the desired powder and transport processes which led to the high density [R.L. Coble and R.M. Cannon in “Material Science Research Processing of Crystalline Ceramics, Vol.11” Plenum Press, New York, 1978, p.291]:
>> Small, non-agglomerated, monodisperse, spherical powders,
>> Uniform, dense packing of powder,
>> Mass transport during sintering by volume or grain boundary diffusion, no transport by surface diffusion or vaporization and condensation.
How can particle size help in achieving better sintering at a relatively lower temperature and thus, high sintered density in a refractory composition?
It has been found that a higher percentage of smaller particle size or fines in the starting powder lead to a faster densification rate. The theoretical basis of this argument is due to Herring’s scaling law which states that there are simple laws governing the times required top produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical except for a difference in particle dimensions. That is why; those who are interested in high sintered density or reduced sintering temperatures and times strive for fines starting powder (mixture). However, the success in sintering of the fine powder relies on the removal of agglomerates and aggregates. De-agglomeration treatments increase the sinterability of refractory mixtures. Homogeneous mixing of ingredients plays an important role in this regard. There are other limitations too as a very high percent of fines in the initial mixture may cause other problems like -
>> Formation of lamination cracks in green bricks during their pressing or ramming which get exposed after the bricks become dry or has been fired.
>> Less compressive strength, less load bearing capacity and low MOR.
A narrow grain size distribution is imperative for obtaining a high sintered density. H. Kent Bowen stated two postulates for improving the manufacturability of high value added refractory products [H.K. Bowen in “Proceedings of the First China-US Seminar on Inorganic Materials Research”, May 17-21, 1983, Shanghai, Eds. T.S. Yen and J.A. Pask, Science Press, Beijing, 1983, p.55] -
>> Postulate 1: Powders (individual homogeneous or heterogeneous units) with a narrow size distribution are easier to process into uniform microstructures (uniformity of size and distribution of the voids), which results in easier control of the microstructure during densification.
>> Postulate 2: Submicron particles require modification of the interparticle forces by controlling the surface chemistry (usually a liquid phase) such that a small repulsive interaction is achieved by electrostatic, salvation, or steric phenomena.
How can particle size distribution help in controlling the porosity of a refractory composition?
According to Kingery [W.D. Kingery in “Ceramic Fabrication Process, Part IV”, Technology Press, Cambridge and John Willey and Sons, New York], in practice it is found commonly that the porosity is about 40% for a single particle size refractory material, and a combination of cubic and hexagonal packing is observed. In a binary refractory mixture i.e. if two quite different particle sizes are mixed, the apparent volume varies as indicated in the adjacent Figure. with a minimum in apparent volume at about Refractory Technology: Effects of Grain Size Distribution on a Binary Refractory Mixture
Fig.- Particle Packing in a Binary Refractory Mixture
Refractory Technology: Effects of Grain Size Distribution on a Ternary Refractory Mixture
Fig.- Particle Packing in a Ternary Refractory Mixture
70% coarse material. At an infinite size ratio, the lower straight lines are reached, while for identical sizes the top horizontal line is followed. The heavy line is for a diameter ratio of about 10:1. Addition of a third size i.e. in a ternary refractory mixture decreases the pore volume even more as shown in the Figure.
Thus, during processing i.e. before firing of the refractory bricks, the powder characteristics (at first instance can be observed through the green mixture sieve analysis data) can be considered as a set of constant parameters. In conclusion, to achieve a dense fine-grained microstructure it is desirable to have a starting powder with small particle size and a narrow distribution, non-agglomerated particles with equiaxed shapes, and high purity (or controlled dopant / additive content). For all practical purposes and mass productions of refractory bricks and castables of different types like Fire-clay, High Alumina, Basic, Silica, Mag-chrome, Mag-carbon, Slide Gate refractories etc. at industry level there are established standard values for the range of coarse, medium, fine and ultrafine fractions which need to maintained and checked at regular intervals by observing mixture (powder) sieve analysis reports to ensure better control over fired properties of these refractory bricks and castables respectively.

April 27, 2009

Bulk Density of Refractory Samples

Bulk Density

This property is important for both insulating and dense refractories (bricks and castables). Bulk density (B.D.) is the ratio of the mass of the refractory specimen to the bulk volume of the same or in other words it is the weight per unit volume the refractory (including the volume of the pore space present in that refractory sample).

There are two methods for determination of Bulk Density in case of refractories:

1. Direct measurement method

B.D.= (Weight of the specimen in gm.) ÷ (Volume of the specimen in cc)

2. Direct volume determination method

B.D (gm/cc) = (Dry weight) ÷ (Soaked weight - Suspended weight)

Out of these two methods the ‘direct volume determination method’, which is generally used for irregular refractories, gives more accurate results. The nearer the Bulk density approaches the Specific Gravity the lower is the Porosity. Lower B.D indicates higher porosity, lower strength of the refractory material. The BD will also affect other properties of the refractory such as the load bearing capacity and thermal conductivity etc. Unless there is any specific reason, the aim of a refractory manufacturer is to produce products of higher Bulk Density. The ultimate Bulk Density of the product will depend on a number of factors like - type of raw materials used and their processing, process control at every step during green manufacturing and firing etc.

Standard Methods for Testing of Refractories [Read]

Types of Testing of Refractories [Read]

April 22, 2009

Types of Testing of Refractories

The durability of refractories is a matter which vitally concerns all manufacturers and users since it is intimately connected with economics and efficiency of the process in they are employed. Refractories are characteristically anisotropic in nature which makes it all the more difficult to judge exactly the durability while it is in use. The furnace designer has to choose the refractories according to the data from different tests available to him. That means in order to fulfill the users or various application requirements and to achieve long service lives, refractory products must be assessed by testing.

The main functions of refractories testing may be classified into three types:
1. Evaluation of new materials before use.
2. Quality control by the manufacturer or user.
3. Post-mortem examination of refractory bricks that behaved differently than the normal.

The testing of refractories can be categorized as:
  1. Non-destructive type.
  2. Destructive type.

A list including both these types of testing is given below:

=> Particle size analysis
=> Refractoriness Under Load (R.U.L)
=> Modulus of Rupture (M.O.R)
=> Modulus of Elasticity (M.O.E)
=> Permanent Linear Change (P.L.C)
=> Reversible Thermal Expansion (R.T.E)
=> Thermal Conductivity
=> Differential Thermal Analysis (DTA)
=> Thermo-gravimetric Analysis (TGA)
=> Spalling Resistance (water quenching and air spalling)
=> Hydration Resistance Test
=> Creep in Compression Test
=> Abrasion Resistance Test
=> Microstructural analysis under Optical Microscope
=> Mineral Phase identification by X-ray Diffractometer (XRD pattern)
=> Chemical Analysis

There are several Standard Methods to carry out the above testing which have been accepted and performed globally. Here one must remember that the sampling of the refractories specimen for testing become very important because of several reasons. Methods of carrying out all the tests given in the above list will be discussed individually in separate posts at this site. To know more about Standard Methods and Sampling read the following -

Standard Methods for Testing of Refractories [Read]
Sampling of the Refractory Specimen for Inspection [Read]

October 14, 2008

How to Prevent Formation of Iron Spots (Crater) in Refractory Bricks as a measure of Quality Control

What makes a refractory product sell? The factors determining the sales success are many and varied. They include market conditions, the nature of the product, and the image of the manufacturing organization in the market as well as of the product created by advertising, the socio-cultural background of customers, credit facilities, Customer Delight and so on.

But one major factor that appears certainly in all conditions (excluding monopoly or extreme scarcity) is product quality as perceived by the customer. The Production Department must accept its fundamental responsibility for the manufactured quality of its refractory products. The workforce with effective training and adequate equipment must be capable of -

· consistently producing to specification and

· recognizing and reacting when an operation goes out of control.

Adequate systems must be established to ensure that corrective action is taken. The quality control for Refractory Bricks refers to the following aspects:

· Manufacturing defects.

· Dimensional tolerances.

· Physical & Chemical material properties.

· Quantities, Marking, Labelling and Packing.

While Sampling for visual and dimensional inspection, iron spot (crater) is always an important criteria of inspection. It is not an uncommon site or if one had a chance of visiting brick yards of a few Refractory manufacturing units in this country then he must have noticed that bricks worth crores (millions) of rupees or thousands of tons lying rejected for not satisfying the customer specified criteria of iron spots (craters). Before going into the steps as how to control the formation of these iron spots in Refractory Bricks let us discuss some more about Craters.

Craters are defined as melt phenomena, caused by mainly iron oxide or lime or some other low temperature melting elements like, alkalies. An Iron spot on the surface of a refractory brick can be tested with a hammer (I assume Refractory persons would be acquainted with this tool :-) ) to determine the possible presence of crater which can be dangerous for the brick and so, for the furnace while in operation. The various reasons/sources of these iron spots and actions required to be taken to prevent their formation in a refractory brick are outlined below:

Refractory Raw Material

1. Preventive Measure: Checking the Raw Material & removing free iron from it before grinding / using. Sorting of Slag / Iron patch especially in calcined fire clay and other calcined raw materials at the kiln yard itself.

2. Responsibility: Mill house, Calcination Kiln, Checking & RM yard.

Own Rejection (Refractory Grog)

1. Preventive Measure: Iron containing (contaminated) grog to be kept separately.

2. Responsibility: Checking & RM yard.

Outside Grog

1. Preventive Measure: Visual inspection in the Truck by breaking up the lumps into pieces before unloading. Briquette test in the Laboratory.

2. Responsibility: Checking & RM yard. Laboratory (Quality Assurance).

Maintenance Waste

1. Preventive Measure: Cleaning of waste & scraps after Maintenance e.g. iron chips, welding tips etc. Handover to Production Deptt after maintenance of any Machine (Handover – Takeover system). Cleaning before & after maintenance of each machine (Mill House, Mixer, Press etc).

2. Responsibility: Concerned person of the Maintenance Deptt. Shift in charge of the concerned area (Mill House, Press, and Production).

Free iron in Mill House Silo Material

1. Actions Required: Due to Beater => regular cleaning of Magnetic drum, Plate & Roller Magnets. Due to Rusted plates => Silo cleaning once in a fortnight.

2. Responsibility: Mill House. Maintenance. Production departments.

Free iron from Press Hoppers

1. Actions Required: Due to Rusted plates => Hopper cleaning once in a fortnight.

2. Responsibility: Production.

These are the certain necessary steps which, if taken properly, can arrest the formation of craters and iron spots which become visible on the surface of the refractory bricks only when the firing is over that means everything is over!