MELTING TECHNOLOGY
`
`Refining Agents, Colourants and
`Decolourisers Used in Container Glass
`
`The refining of glass depends on the gas
`•
`bubbles rising to the surface of the
`glass during the melting process, as well as the
`re-absorption of gases as the glass cools. To assist
`these processes, refining agents are added to the
`batch, with sulphates being the ·most important
`in the soda-lime-silica compositions of container
`glasses. Sulphates also have the added property
`of acting as a wetting agent to help the fluxes
`dissolve silica. Sulphates decompose at high
`temperatures during the melting process,
`resulting in the evolution of sulphur dioxide gas
`bubbles which rise and act as collectors as they
`sweep through the melt, gathering the other
`gases such as carbon dioxide and nitrogen. Any
`sulphur dioxide bubbles remaining after this
`stage are then redissolved into the glass as it is
`cooled down in preparation for forming.
`By employing a reducing agent in the batch,
`the breakdown of the sulphates can be aided
`chemically, with the result that sulphur dioxide
`is evolved earlier.
`This allows lower melting temperatu!:_es (or
`higher throughputs) and the glass is also easier
`to refine to a higher quality. Thus by balancing
`the sulphates ( oxidising) and reducing agents,
`it is possible to optimise the melting process to
`give a refin ing 'system' capable of giving a well
`refined glass at high pull rates economically.
`
`Sulphates
`The choice of sulphate is generally between
`), as
`anhydrite (CaSO 4 ) and sa~tcake (Na2SO 4
`both can be obtained of sufficient chemical purity
`and·consistency and can act with equal effective(cid:173)
`ness in the batch. Their grading and hence their
`ability to mix intimately with the other batch
`ingredients can greatly influence the effective(cid:173)
`ness of a particular material but other factors
`such as batch plant handling and dusting have
`to be taken into account when specifying the
`required grain size. Price, of course, is also of
`major importance and the relative costs of the
`other sources of lime and soda will have a
`signific.:ant bearing on the final choice.
`
`Reducing agents
`Reducing agents fall into two main types. Carbon
`in its various forms such as carbon black or
`processed cokes are examples of where the
`reducing agent is in a highly concentrated form.
`In the other main type, processed blast furnace
`slag (Calumite), the reducing agent is in this case
`sulphide and is present at a level of 1 %- 1. 5 % .
`As a result, the slag has to be added to the batch
`at a rate of approximately fifty times the
`equivalent carbon weight for the same reducing
`power.
`The choice of reducing agent is dependent not
`
`*MC Brew, United Glass Ltd, St Albans, UK.
`
`G L A S S March 1989
`
`by M C Brew •
`
`The refining agents, colourants and
`decolourisers for flint, green and
`amber container glass are reviewed.
`There is considerable interaction
`between the refining system and the
`way in which the various colourants
`and decolourisers act. Where
`appropriate, the factors governing
`the choice of a particular raw
`material from the various types and
`sources available are discussed.
`
`only on the desired chemical effects but also on
`factors such as chemical and physical consis(cid:173)
`tency, handling characteristics, batch weighing
`and storage facilities and, of course, cost. In the
`case of blast furnace slag, the presence of
`alumina, lime and silica already in a glassy
`phase, the presence of impurities such as iron
`and the cost of alumina from other sources all
`have to be taken into account.
`White flint
`Sulphates are added at a rate of approximately
`15kg-20kg SO 4 per 2000kg of sand and this is
`complemented by the use of 40kg-120kg of blast
`furnace slag. In the UK, the slag is often the
`economical choice and so is widely used whereas
`on the Continent and in the USA, carbon is often
`preferred. As the glass is significantly reduced ,
`there is a relatively high ferrous content which
`has to be decolourised. For this selenium is the
`most important material and is usually added in
`the metallic form . As the material is often
`manually handled, there has been a trend to use
`a coarser grade with as few fines present as
`possible. Much of the selenium is lost from the
`melt through volatilisation and there are claims
`that addition in different forms (ie as selenites
`or incorporated within pellets) results in less
`selenium loss. However, the oxidation/reduction
`(redox) balance can have a very significant effect
`on the selenium and the less oxidising conditions
`currently used do result in less colourless
`selenate and selenite formation. In addition to
`the effective pink from the free selenium, some
`yellow/brown selenides are also formed. The
`blue colour generated by a small amount of
`cobalt helps to complement the other colours to
`give the required overall neutral grey.
`
`The amount of decolouriser added depends on
`the amount of iron present in the raw materials,
`the redox conditions and the standard of colour
`required. Addition rates (based on 2000kg sand)
`are usually in the order of 10g-50g of selenium,
`with approximately one tenth to a twentieth of
`that weight of cobalt. Cobalt is usually added in
`the form of a fine cobalt oxide powder.
`Green glass
`The green colour is generated from the iron and
`chromium, which is obtained from iron chromite
`ore. Iron chromite is reducing in nature and so
`lesser amounts of other reducing agents are
`necessary to obtain the required refining. The
`green colour produced is reasonably stable
`through the range of redox conditions normally
`employed, although the iron present can give
`either a yellowish or bluish hue to the colour,
`dependent on the relative amounts of ferrous
`and ferric species present. While the normal
`rules of consistency of composition apply, it is
`the physical sizing of the iron chromite which
`is of paramount importance. This particularly
`refractory mineral must be ground sufficiently
`finely to ensure complete dissolution.
`·
`Amber glass
`Amber is a highly reduced glass, requiring iron
`and sulphide to generate the colour. The
`sulphide is obtained by two main mechanisms:
`• Strong reduction of sulphate;
`• Added to the batch directly as sulphide.
`In the case of the strong reduction of sulphates,
`relatively high quantities of carbon are added,
`with possibly as much as 10kg per 2000kg of
`sand. Where sulphides are added directly, this
`is by the addition of up to 10 % of the total batch
`weight of blast furnace slag. Another sulphide
`source is iron sulphide, which has the additional
`advantage of being an iron source. During
`melting,· sufficient of the sulphide can be
`converted to SO.) by oxidation to give the
`required refining.-Iron oxide is present in the
`sands, where it has to be controlled to ensure
`composition consistency. It is usual, however ,
`to add an additional amount either as the
`sulphide mentioned above iron oxide powder
`or high iron -bearing slag.
`It is common practice, however , to see amber
`batch recipes being made up on the basis of high
`sulphide addition ( either slag or iron sulphide),
`a source of iron to give approximately 0.30 %
`Fe:_P3 in the glass, a small amount of sulphate
`to aid refining and some carbon which can be
`readily altered to control the colour intensity.
`The final choice will again depend on such
`factors as the cost of alumina from various
`sources but consistency of composition of the
`raw materials, as well as the flexibility of the
`batch plant in the number of ingredients that can
`be handled, are important factors. •
`
`79
`
`O-I Glass, Inc.
`Exhibit 1028
`Page 001
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