Raw Material Assessment and Control as a Factor in Manufacturing Quality

Topics Covered

Background

Constraints Faced by Australian Organisations

Ceramic Whiteware Raw Materials

Natural Raw Materials

Synthetic Raw Materials

Criteria for the Choice of Materials

Cost and Availability

Key Properties of Raw Materials

Chemical Composition and Loss on Ignition

Particle Size

Mineralogical Composition

Moisture Content

Other Key Properties

The Advantages of Material Characterisation

Materials Characterisation Case Study

Compensating for Raw Material Variations

Summary

Background

Australia is operating in a global market where increased competition is demanding:

         More diverse product ranges

         More complex product shapes

         Higher product qualities

         Automated production methods

         Greater flexibility of production

         Shorter lead times

         Lower manufacturing losses.

These driving forces for change have certain implications for raw materials and supplier/manufacturer interaction. In particular, the raw materials must possess tighter specifications, be consistent in terms of key properties and be competitively priced.

Constraints Faced by Australian Organisations

Particular constraints faced by Australian companies include:

         Lower value of dollar making refined imported raw materials very expensive

         Transport costs both to and within Australia;

         Poor continuity of supply of indigenous raw materials. Local Australian raw material suppliers are very mixed in their attention to a "standard supply". It is probably not viable for multi million dollar mining companies to produce 500 tons per month of clay or feldspar

         Small industry base with little purchasing power.

Ceramic Whiteware Raw Materials

The term raw materials usually refers to all the materials that are physically incorporated into the final product and often comprise auxiliary materials such as binders which affect the products intermediate properties e.g., unfired strength.

In order to appreciate the behaviour of products during manufacture and to develop new products using the most appropriate materials, an understanding of the various raw materials is essential.

Raw materials can either be:

         Natural raw materials e.g., clays, feldspars, quartz or

         Materials that have been highly refined or produced synthetically e.g., frits, oxides, opacifiers, pigments.

Natural Raw Materials

Naturally occurring minerals may either be used "as mined" or in a pre-treated form e.g., calcined, ground, blended. Pre-treatment helps to reduce the natural variability of mineral deposits to give consistent supply when using raw materials and is becoming increasingly commonplace, as processes are automated and require more consistent materials.

Synthetic Raw Materials

The highly refined or synthetically produced materials such as the frits and industrial chemicals which are often used are relatively expensive, but are employed for specific properties which they impart e.g., lower firing temperatures, low contamination.

Criteria for the Choice of Materials

Two basic criteria are used for choosing materials:

         Cost and availability

         Chemical and physical properties

Cost and Availability

Fortunately for ceramics, raw materials are plentiful and relatively cheap. Silicon is the second most abundant element in the earth's crust (after oxygen). Feldspars account for 60% of the earth's crust and clays occur worldwide.

Another element in cost reduction is to look increasingly at the use of indigenous raw materials instead of more expensive, imported raw materials.

Choosing raw materials on cost alone can however, be very dangerous and the following example, emphasises the influence of careful materials choice on the economics of production.

Table 1 compares a raw sanitaryware glaze prepared using different raw materials. Only the cost of he materials, using current United Kingdom (U.K.) tonnage prices, has been considered and any extra processing or production problems which may be associated with lower grade materials has been ignored.

The "accountants" glaze has been prepared using the cheapest raw materials to give overall glaze composition. However, these materials may have high iron and titanium contents which would cause the glaze to be coloured. They may also require more grinding than more expensive materials. This glaze works out three times cheaper than the "technicians" glaze, which uses the cleanest materials and replaces part of the quartz and calcium carbonate with wollastonite. This would give the whitest glaze with the lowest gas evolution.

Clearly the choice of raw materials lies somewhere between the two depending on the quality of fired product required.

Table 1. Glaze cost comparisons.

Raw Glazes

Accountant’s Glaze

Technician’s Glaze

Material

Cost (£/kg/T)

Material

Cost (£/kg/T)

30% Quartz (0.3% Iron)

0.06

19.5% Quartz (<0.1% Iron)

0.2

20% Calcium Carbonate

0.03

25.5% Wollastonite

0.34

10% China Clay (Coarse)

0.11

11% China Clay (Fine)

0.21

40% Feldspar (Coarse)

0.11

44% Feldspar (Fine)

0.21

Glaze Cost

0.07

Glaze Cost

0.22

Key Properties of Raw Materials

From 1993 to 1995, an international project comprising 25 companies (suppliers and manufactures) worldwide led by CERAM, identified key material properties for both raw materials and prepared bodies and recommended a suite of standard, in-house test methods.

The key properties of raw materials are defined as:

         Chemical composition and loss on ignition

         Particle size

         Mineralogical composition

         Soluble salts

         Moisture content

Bulk chemistry and particle size are particularly important as without these two characteristics vitrification and densification at acceptable firing temperatures in acceptable times would not be possible. This means that these two properties can only vary between narrow limits otherwise it will lead to major differences in the way in which formulations interact with water and deflocculating chemicals.

It is often not practical to carry out chemical and mineralogical analysis of the materials at the factory site. However, it is feasible to measure moisture content and particle size and simple observations can be made on the state of the incoming raw materials e.g., specks.

Chemical Composition and Loss on Ignition

The materials primary functions are to supply the requisite oxides to produce the right product when fired and thus should be consistent chemically otherwise they can create problems particularly where automated systems are used or colour matching is required. Both organic and inorganic constituents are important.

The material supplier should provide up-to-date information concerning the materials and provide assurance of consistency of supply.

Particle Size

Along with the composition of the incoming materials, the particle size should be well known and controlled if any grinding operation takes place. Under or over ground materials may both have an adverse effect on the quality of the finished article.

Control of particle size in plastic materials such as clays can influence casting rates, deflocculation demand, packing density, viscosity on standing, cast firmness, strength etc.

Control of particle size distribution for non plastics e.g., flux and silica is crucial in ensuring that vitrification takes place at acceptable temperatures. Too fine (90% <10µm) a flux results in bloating in the body, crawling in a glaze or less intense, pale fired colours.

Mineralogical Composition

Identifies the crystalline materials present e.g., the level of flux and filler. Different clay mineral compositions can severely affect processing properties such as drying shrinkage, cracking and slip deflocculation characteristics.

Soluble Salts

Soluble salts e.g., sulphates and chlorides are also important as these can influence deflocculation demand and hence unfired properties such as strength, firmness, and plasticity.

Their presence may also promote glaze crawling.

Moisture Content

A regular moisture content is important and should be measured not only for slip calculations but also to ensure that water is not being purchased in place of the material.

Other Key Properties

Other factors combine to determine the final choice of material such as the shrinkage on drying and firing, suspension and binding properties, dry strength, and plasticity.

Vitrification characteristics, decomposition on firing, presence of colouring oxides, particle size and mineralogy also influence the choice of raw materials.

Often the requirement of one part of the process requires a material with a property that conflicts with that required for another. For example, the demands of a slip casting process are influenced by the plastic (clay) materials whilst the vitrification process relies on the fineness and nature of the non-plastics. Compromise once again needs to be made and accepted.

The Advantages of Material Characterisation

Material characterisation plays a vital role in trouble shooting and ensures manufacturers become pro-active rather than reactive to loss reduction.

Specking faults for example are a constant problem in the Whiteware industry and can account for up to 10% of total faults. They may be caused by contaminants, which can be introduced by raw materials. Identification prior to manufacture can reduce production losses.

Materials Characterisation Case Study

Particle size variations also account for many production losses. For example, during the course of one year the ball clay used by one sanitaryware organisation gradually become coarser. To compensate for the resultant increase in casting rates, greater levels of organic deflocculant were used such that the viscosity on standing continually increased. The casts that formed had a hard face at the mould surface with a soft centre. Cracks developed from the inside and the casts had a tendency to tear. The slip was also difficult to drain. Production losses gradually increased. Once the particle size increase had been identified as the problem, alternative deflocculation practices were recommended to compensate for the change. Production yields increased by 14%.

Compensating for Raw Material Variations

Although suppliers are taking increasing care in the refining of materials it is inevitable that the clays, in particular, will have variable properties. It is therefore important to understand the effect that variations in the properties of the raw materials may have before they are used and to adjust the composition accordingly (± 2-3%).

The advantages of this approach are that problem batches are detected early and manufacturers have a flexible attitude to product reformulation although it does require investment in acceptance test equipment and procedures.

Summary

By appreciating the requirements placed on each raw material and the properties and oxides they impart, an insight into the performance of the production process and subsequent products can be gained.

Close control of raw material properties is important for automated production, loss reduction and higher quality as the products will posses certain features such as good dimensional stability and colour.

For more details on the recommended suite of test methods to measure the most important properties of raw materials contact CERAM Research.

 

Primary author: Sally Alsop

Source: Abstracted from Journal of the Australasian Ceramic Society, Vol. 34, no. 2, pp. 179-84 (1998)

 

For more information on this source please visit The Australasian Ceramic Society.

 

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit