GB2260978A - Glass batch and glass melting process - Google Patents

Abstract

A glass batch composition for production of a coloured glass, including selenium or a source of selenium, characterised in that the composition includes (a) in combination an oxidising agent for increasing selenium retention in the resultant glass and a reducing agent for providing the glass with desired colour characteristics.

Description

GLASS BATCH AND GLASS MELTING PROCESS This invention relates to a glass batch for the production of coloured or decolourised glass, particularly to a batch containing selenium and one or more other colouring agents. The invention further relates to a glass melting process for the production of coloured or decolourised glass containing selenium and one or more other colouring agents.

Selenium is known for use as a colouring agent for glass. The selenium on its own can impart a pink colour to the glass. Selenium can be employed in combination with one or more other colouring agents to provide glasses of different colours. It is possible to produce a decolourised (i.e. white) glass employing selenium and certain other colouring agents. In the following specification the term coloured" glass is intended to encompass such decolourised glass.

GB 1506028 describes a bronze tinted glass for vehicle or architectural use. Selenium and iron oxide along with cobalt oxide, nickel oxide and/or chrome oxide are suggested as colouring agents. The selenium content of the glass is 5-25 parts per million. From the examples it is clear that selenium retention is about 17% at best. Such low retention is undesirable both from the point of view of raw material cost and of environmental considerations. In addition, sodium nitrate is present in the batch. This causes release of NOX emissions when the batch is melted which leads to additional environmental pollution problems.

Various proposals have been made to make tinted selenium-containing glass with a higher selenium retention.

One method is disclosed in US 4886539 where foam is produced in a vacuum refiner and selenium retention levels of 50% are the highest claimed even when known oxidising agents such as sodium nitrate are added to the batch. Apart from the fact that half the selenium is still lost, the addition of nitrate poses a further environmental problem because it adds to the NOX emissions from the melting process.

Recently it has been proposed in EP 0403184 to manufacture float glass from molten glass produced on an electrically heated cold top melter.

The present invention seeks to overcome the problems of selenium retention by proposing the use of a novel batch composition and by providing for a novel process for the production of coloured glasses.

The present invention in one respect provides a glass batch composition for production of a coloured glass, the composition including selenium or a source of selenium, characterised in that the composition includes in combination an oxidising agent for increasing selenium retention in the resultant glass and a reducing agent for providing the glass with desired colour characteristics.

Preferably, the glass is a bronze glass and the composition further includes iron.

The oxidising agent may comprise sodium nitrate, sodium perchlorate, calcium peroxide, sodium persulphate, magnesium peroxide, zinc peroxide or barium peroxide.

The selenium in the composition may be present in the form of one or more of selenium metal, sodium selenite, sodium selenate or barium selenite.

The reducing agent may comprise carbon or ferrous oxalate. The carbon may be in the form of any of the standard glass making materials, such as anthracite, sugar (preferably in the form of sucrose), starch, coke or charcoal.

The present invention also provides a method of producing a coloured glass containing selenium, the method comprising melting the glass batch composition according to the first aspect of the present invention to form a molten glass and forming the molten glass into a desired shape.

The present invention in a second aspect further provides a glass batch composition for production of a coloured glass, the composition including selenium or a source of selenium characterised in that the composition includes a peroxo oxidising agent.

Preferably, the peroxo oxidising agent comprises calcium peroxide, magnesium peroxide, barium peroxide, zinc peroxide or sodium persulphate (disodium peroxydisulphate).

The present invention also provides a method of producing a coloured glass containing selenium, the method comprising melting the batch composition according to the second aspect of the present invention to form a molten glass and forming the molten glass into a desired shape.

The present invention in a third aspect provides a glass batch composition for production of a coloured glass, the composition including selenium, characterised in that the composition includes at least one of a selenite compound and a selenate compound which comprises some or all of the selenium in the composition.

Preferably, the selenite compound comprises sodium selenite or barium selenite. Preferably, the selenate compound comprises sodium selenate.

The present invention also provides a method of producing a coloured glass containing selenium, the method comprising melting the batch composition according to the third aspect of the present invention to form a molten glass and forming the molten glass into a desired shape.

The present invention in a fourth aspect provides a glass batch composition for production of a coloured glass, the composition including selenium, characterised in that the composition includes up to 0.4 wt% of an alkali metal nitrate.

Preferably, the composition includes from 0.055 wt% to 0.4 wt% of alkali metal nitrate, and more preferably from 0.1 wt% to 0.33 wt% of alkali metal nitrate. The alkali metal nitrate preferably comprises sodium nitrate.

The alkali metal nitrate may be in combination with a reducing agent containing carbon.

The present invention further provides a method of producing a coloured glass containing selenium, the method comprising melting a glass batch composition in a cold top melter to form a molten glass, the glass batch composition including up to 0.4 wt% of an alkali metal nitrate, and forming the molten glass into a desired shape.

In any of the glass producing methods of the present invention, the melting may be carried out in a cold top melter.

The present invention will now be described in greater detail by way of example only with reference to the accompanying drawings, in which: Figures 1 to 11 are graphs showing for a number of examples the relationship between composition parameters and chemical and optical results for antisun bronze glasses.

The present invention has particular application in the production of antisun bronze glasses. However, the present invention can also be employed for the production of other coloured glasses which contain selenium and various colouring agents.

Antisun bronze glasses are known for use as architectural and vehicle heat absorbing glasses. Such glasses incorporate selenium and iron. An antisun bronze glass must have the desired standard antisun bronze colour.

The target for the bronze colour composition relates to a specific chemical composition in the glass which is total Fe2O3 equals 0.4 weight percent; total ferrous equals 18 weight percent; Co3O4 equals 39 ppm; and Se equals 27 ppm. It is possible to carry out a spectrophotometric analysis to provide an indication of the proportion of the correct bronze colour and presented as a percentage of the target specified above. The wavelengths which are measured in the spectrophotometric analysis in order to determine the optical contribution for each component are Se 475nm:, Cho 304 590nm and ferrous 1000nm.

A typical glass composition and batch composition for an anti sun bronze glass is shown in Table 1 below.

TABLE 1

Glass Composition Batch Composition Oxide Contribution Raw Material g to make 100 g z of glass SiO 72.49 Chelford Sand 74.56 Al2O3 1.07 Ben Bennet Limestone 5.07 Fe2O3 0.282 Rouge 0.165 CaO 8.51 Varmsuorth Dolomite 18.46 MgO 3.91 Soda Ash 20.68 Na2O 12.81 Saltcake 0.90 Co3O4 0.0037 Co3O4 0.0037 Se 4 0.0026 Se metal - 0.0135 SO3 0.2 Sodium nitrate 10.4 K2@ 0.589 Batch S03 = 5 In production the glass composition for an antisun bronze is generally fixed apart from small variations which are usually made to obtain the correct colour in the glass.

It is believed that the bronze colour resulting from the selenium results from an interaction between iron and selenide ions. Thus the bronze colour depends not only on the amount of selenium in the glass composition but also the proportion of the selenium which is present as selenide ions. In addition, for the antisun bronze colour the total weight percent ferrous (Fe++) content of the glass is important. The proportion of the ferrous as a percentage of the total iron content depends on the reducing or oxidizing effects in the furnace and the glass. If the glass is very reduced (i.e. has a high percentage ferrous state) only a small amount of total iron is required to produce the target total ferrous. Ferrous oxalate instead of rouge may be employed to incorporate iron into the glass.

In order to study experimentally the batch compositions suitable for producing bronze glasses, a simulator for an electrically heated cold top melter was employed. This simulator was adapted to melt small amounts of glass batch material so as to have a body of molten glass covered by a desired thickness of unmelted batch. Molten glass is drawn off from the bottom of the melter and as batch at the top of the melter is progressively melted into glass, additional batch is added to the top of the melter. The simulator had a glass production rate of from about 1.7 to 2 kilograms per hour and a batch crust thickness of around 12 to 15 centimetres. The temperature of the melter was around 1400 C.

In the experiments, a series of tests were conducted employing various batch compositions. The primary results obtained were the ferrous and selenium optical percentages, these being determined by the spectrophotometric analysis identified above. The experiments also assessed both the initial batch selenium content in parts per million and the final retained selenium content in parts per million. The melting tests were carried out for a period of approximately 16 hours although some tests were terminated earlier than this period in which case the results were taken at the end of the test. A variety of parameters was altered in order to assess the changes of the parameters on the ferrous and selenium optical percentages and the selenium retention.As was explained above, the ferrous and selenium optical percentages give an indication of the degree to which the actual colour of the resultant glass corresponds to the desired antisun bronze colour.

Before describing the experiments in detail, the broad technical results are described hereinbelow.

In the composition shown in Table 1 the selenium is introduced into the glass composition in the form of selenium metal. We have found it is possible to introduce selenium into the glass composition in the form of barium selenite or sodium selenite as well as in the form of selenium metal.

During large scale production of antisun bronze glasses, typically 135 ppm of selenium metal is introduced into the batch of which approximately 27 ppm is retained i the glass product. We have found that if selenium is introduced into the glass composition by including barium selenite in the batch, this improves the selenium retention. Sodium selenite gives about the same selenium retention as selenium metal.

It is believed that the improved selenium retention with barium selenite results from the greater thermal stability of this substance when compared to selenium metal or sodium selenite. It is also possible to add selenium to the batch by using mixtures of selenium metal and barium selenite. We have found that the percentage selenium retention improves with a reduction in the amount of selenium metal which is present in the batch.

Barium selenite, although giving improved selenium retention, still results in much of the selenium being lost in production. One further disadvantage with barium selenite is that since barium selenite has oxidizing properties in the resultant glass composition the ferrous state is low and this can result in low optical properties of the resultant anti sun bronze glass. The use of barium selenite also gives low optical properties of the selenium which is retained in the glass which also may be attributed to the oxidizing properties of the barium selenite raw material. It is believed that the selenium in the iron/selenium chromophore is in a reduced state and therefore despite the large amount of selenium present when barium selenite is used most of the selenium is not in the correct oxidation state to produce the bronze colour.

In an attempt to counteract this oxidizing nature a reducing agent, such as sugar, anthracite or ferrous oxalate, may be incorporated into the batch. The use of sugar can cause an increase in the ferrous content thereby altering the optical properties of the glass. However, we have found that the use of a reducing agent can reduce the selenium retention. It is believed that the use of reducing agents containing carbon is detrimental to selenium retention because it reduces the selenium compounds in the batch to more volatile selenium metal. Sugar was found to be better than anthracite for selenium retention.It is believed that one reason for this is that sugar tends to decompose at low temperatures whereas anthracite tends to decompose at higher temperatures with the consequence that at high temperatures sugar has a reduced tendency to react with selenium compounds to produce volatile selenium metal.

We have found that for an equivalent carbon content anthracite produces a higher ferrous amount than sugar which suggests that the decomposition of the reducing agent, for example to produce carbon monoxide, at high temperatures has a greater influence on the production of ferrous.

In order to improve selenium retention in the batches, an oxidizing agent, typically sodium nitrate, was added to the batch. It was found that sodium nitrate greatly improved selenium retention. It is believed that the addition of sodium nitrate causes the selenium to be present in the form of non-volatile selenium compounds. We have found that since sodium nitrate is an oxidizing agent then in order to obtain the correct optical properties it is necessary to balance the oxidizing nature of the batch materials, including sodium nitrate and, if present, barium selenite, with the reducing agent. If the ratio of sodium nitrate to reducing agent is too high the glass is over-oxidized resulting in a poor bronze colour and if the ratio is too low selenium retention is reduced.Thus it is necessary to balance the sodium nitrate which is added for selenium retention with the reducing agent such as sugar or anthracite which is required to obtain the correct antisun bronze colour. We have determined levels of sodium nitrate and sugar or sodium nitrate and anthracite where not only is the correct anti sun bronze colour obtained but also 70 to 100% of selenium retention is also obtained. At equivalent batch carbon levels, anthracite is approximately twice as powerful at producing ferrous ions as sugar in batches containing sodium nitrate. Thus the amount of reducing agent present in batches containing sodium nitrate depends on the type of reducing agent employed. When the same optical ferrous levels are obtained from sugar-containing and anthracite containing batches, the best selenium retention and selenium optical figures are obtained with the batch containing sugar. We believe this is due to most of the sugar burning off at lower temperatures than the anthracite so as to produce more carbon dioxide or carbon monoxide below about 500 C.

Some further tests on a cold top simultator furnace have shown that a peroxo oxidising agent on its own (i.e. without a reducting agent) can give good optical properties to coloured glass, such as bronze glass, and can give good selenium retention. Suitable oxidising agents are calcium peroxide, magnesium peroxide, barium peroxide, zinc peroxide or sodium persulphate (disodium peroxydisulphate).

In addition, we have found that good optical properties and selenium retention can be obtained by using a batch composition in which some or all of the selenium in the composition is derived from at least one of a selenate compound, such as sodium selenate, and a selenite compound, such as sodium or barium selenite.

These latter two aspects provide the real technical advantage that the required glass can be produced not only with reduced selenium emissions but also with no NOX emissions.

Furthermore, we have found that good optical properties and good selenium retention can be achieved using lower amounts of sodium nitrate oxidant than have been used in the prior art. This provides the advantage of reduced environmental pollution resulting from NOX emissions.

If the total iron amount is increased then this can increase the ferrous and selenium optical values and can also increase selenium retention.

ExPerimental Results An initial test was carried out to determine the effect on selenium retention of the selenium-containing raw material of the batch. The raw material consisted of selenium metal, barium selenite and sodium selenite and the results are shown in Figure 1. The Co amounts and optical percent values were within the target limits at the end of the tests. The composition of the batch samples (tests 6B, 7B and 8B) are shown in Table 2. From Figure 1 it may be seen that barium selenite gives improved selenium retention compared to sodium selenite and selenium metal, (with sodium selenite and selenium metal giving substantially the same retention).

However, it should be noted that these tests were conducted in the absence of an oxidising agent which would normally be present in the batch. This could affect the degree of selenium retention. The percentage ferrous state is shown to change substantially inversely with the selenium retention whereby the batch which retains most selenium also has the lowest percentage ferrous state and the lowest ferrous optical percent. Figure 1 also shows that the selenium optical result does not correspond exactly to the amount of selenium present in the glass. Although the barium selenite gives twice as much selenium retention it only appears to give slightly more bronze colour.

Thus barium selenite has an advantage over selenium metal in terms of selenium retention but the ferrous optical value was found to be too low due to the glass being over oxidized. This problem was investigated by two separate routes, the first being by reduction of the amount of the selenium raw material and the second being by reducing the glass by means of sugar, anthracite or ferrous oxalate additions, each of these additions acting as a reducing agent. Further experimental batch compositions (tests 10, 1lB, 12, 13, and 16) are shown in Table 2. The results of the 16 hour tests are shown in Figure 2. The Co amounts and optical percent values were within the target limits at the end of the tests.A comparison between test 6B and test 10 shows that as the amount of selenium metal added to the batch is decreased the percentage retention of the selenium is increased. In addition, a drop in the ferrous optical value between those two tests suggests that selenium metal has a reducing effect in the glass. A comparison of tests 7B and llB illustrates the effect of reducing the barium selenite.

The comparison of these two tests shows that as the raw material selenium input decreases there is a tendency towards a decrease in the percentage retention of selenium. The selenium optical percent has not decreased to the same extent as the amount of selenium retained in the glass indicating that selenium optical activity is not directly proportional to the retained selenium. The experiment also shows that a reduction in barium selenite causes an increase in the ferrous optical values suggesting that barium selenite has an oxidizing effect on the glass. It has been found that oxidation of the glass is detrimental to the production of the selenium bronze colour since this affects the ferrous content.

Tests 12 and 13 show the effect of introducing sugar and anthracite into a batch containing the production level of selenium in the form of barium selenite. Both reducing agents were introduced at a level to give an equivalent batch carbon content of the 0.2 weight percent. Figure 2 shows that adding carbon to the batch in either form reduces selenium retention. The sugar batch retains more selenium and so gives greater optical selenium whereas the anthracite batch gives greater ferrous optical values. This seems to show that anthracite is a stronger reducing agent. In test 16 ferrous oxalate was substituted for rouge in the batch containing barium selenite. This gave no improvement in selenium retention or selenium optical value compared with test 11 and the only effect seemed to be an increase in the ferrous optical value.

Experiments were then conducted to assess the effect of adding sodium nitrate to the batch. The experiments showed that in the production of antisun bronze glass, considerable savings in selenium may be obtained by the use of sodium nitrate since sodium nitrate greatly increases selenium retention. Although sodium nitrate improves selenium retention, the resultant glass can be found to be low in ferrous optical value and low in selenium optical value.

Therefore, in order to obtain the correct colour for antisun bronze glass employing sodium nitrate, it is necessary to reduce the glass or increase the iron. Accordingly, the experiments were carried out to determine the effect of reducing agents in batches containing sodium nitrate.

Although sodium nitrate gives greatly improved selenium retention, the addition of sodium nitrate to an anti sun bronze batch is undesirable due to the production to NOX during batch meltdown. For environmental considerations, the optimum batch would therefore contain no sodium nitrate but nevertheless would give the optimum colour and selenium retention for antisun bronze. Table 3 showed the compositions of batches which contained sodium nitrate, as well as sugar and barium selenite. Figure 3 shows the experimental results. The Co amounts and optical percent values were within the target limits at the end of the tests.

Batch 17 employs a sodium nitrate amount of 1.04 weight percent and a starting sugar level of 0.2 weight percent batch carbon. The results show that the selenium retention is 100% but the selenium optical figure is low considering the amount of selenium in the glass. The ferrous optical figure is also very low suggesting that the glass is over oxidized. In test 18, the batch was modified by increasing the batch carbon to 0.4 weight percent but this made very little difference to the results, the optical ferrous and selenium figures rising only slightly. The selenium retention was unaffected by increasing the batch carbon. In test 19 the level of barium selenite was reduced to 1/6th of the production level in the batch (i.e. 22.5 ppm) yet the selenium optical only dropped by 10%, this suggesting that a large amount of selenium present in test 17 and 18 was not optically active. The optical ferrous figure increased with reduction of the barium selenite, this tending to support the previous results indicating that barium selenite had an oxidizing effect on the glass, with consequential reduction in the ferrous optical figure. In test 20 the sodium nitrate level was reduced by 50%, the sugar level was kept constant and the production level of selenium in the form of barium selenite was added to the batch. The selenium retention remained at 100%. In test 21 the level of selenium added in the form of barium selenite was reduced to 1/3 of the production level and the sodium nitrate was reduced to 1/4. This test produced a sharp increase in the ferrous optical figure but also a great reduction in selenium retention. This trend was continued with the introduction of ferrous oxalate to batch 22 in place of rouge.In test 23 the sodium nitrate level was increased back to 1/2 of the production level and the same level of barium selenite was used as in test 19 and the sugar amount was increased to 0.6 weight percent batch carbon. This gave for test 23 100% selenium retention and the selenium and ferrous optical figures were very close to the target figures. In test 24, the batch contained a compromise level of sodium nitrate between those used in test 21 and 23, i.e. 3/8 of the production level and the selenium and sugar levels were kept as in test 23. The selenium retention was less than 100% and likewise the selenium optical figure was reduced although as expected the ferrous optical figure came up to target. It would thus appear that a level of sodium nitrate between those assessed in tests 23 and 24 would seem to indicate an optimum batch level.

Tests were then carried out employing anthracite rather than sugar as the reducing agent in combination with barium selenite and sodium nitrate. The batch compositions are shown in Table 4 and the results are shown in Figure 4. The Co amounts and the optical percent values were within the target limits at the end of the tests.

In the previous tests employing sugar it was found that the optimum level of sodium nitrate was between about 1/2 and 3/8 percent of the production level and in these tests the 1/2 figure was employed as the starting point for the sodium nitrate levels. In test 25 the selenium was present in the form of barium selenite in an amount of 135 ppm, 0.52 grammes of sodium nitrate were added per 120 grammes of batch and an arbitrary starting level of anthracite of 0.4% batch carbon was employed. Figure 4 shows that with test 25 the selenium retention is not very high but the selenium and ferrous optical figures are near to target. In test 26 the anthracite was reduced to give a batch carbon of 0.3 in order to increase selenium retention and at the same time the barium selenite amount in the batch was reduced.The overall effect of these modifications was that the percentage selenium retention in the glass was much improved. The optical selenium was pushed down towards the target value and the ferrous optical figure was reduced only slightly away from the target figure. In test 27 the sodium nitrate level was reduced in the batch to see if 100% selenium retention could still be obtained. The selenium input was also reduced in an attempt to increase the amount of retained selenium which would be optically active. This gave much reduced selenium retention, but a greater proportion of the selenium retained was optically active and the ferrous optical figure was improved. The level of 0.52 of sodium nitrate remained the best for the selenium retention but there may be some scope to reduce the anthracite amount further so as to increase the selenium retention.It was found that anthracite appears to have a much more profound effect on barium selenite than sugar, tending to reduce retention. A comparison of test 23 and test 26 shows that both gave approximately the same ferrous optical figure and retained the same amount of selenium, and that both have the same sodium nitrate level although batch 23 contained 0.6 weight percent batch carbon in the form of sugar and batch 26 contained 0.3 weight percent carbon in the form of anthracite. Taking into consideration the higher level of barium selenite in batch 26, which would tend to make this batch more oxidized, it may be said that anthracite is at least twice as powerful as sugar at producing ferrous ions in the glass. However, the best optical selenium value was in the sugar batch.

A further series of tests was carried out on batches containing sugar, selenium metal and sodium nitrate. Table 5 shows the compositions of the batches and Figure 5 illustrates the results of the tests. The Co amounts and optical percent values were within the target limits at the end of the tests. Test 28 employed a sodium nitrate level of 1.04 weight percent, a sugar level of 0.4 weight percent batch carbon and half the production level of selenium metal. As is shown in Figure 5, this batch gave 100% selenium retention and the target selenium optical figure but a low ferrous optical figure. Test 28 was modified in test 29 by reducing the sodium nitrate and selenium metal content in the batch and increasing the sugar content. This gave 100% selenium retention and an increase in the ferrous and selenium optical figures.In test 30 the selenium was reduced even further and the other components of the batch were kept constant and this produced a predictable reduction in the selenium optical figure but also an increase in the ferrous optical figure, despite the previous observations that a reduction in selenium input has an oxidizing effect on the glass. In test 31 the sugar was increased with the aim of increasing the ferrous optical figure and the result was that the ferrous and selenium optical figures were pushed nearer to the target figures and there was no effect on the 100% selenium retention. By gradually decreasing the selenium and sodium nitrate contents of the batch and increasing the sugar content, ferrous and selenium optical figures were brought to target. The ferrous values shown in Figure 5 confirm the gradual reduction of the glass.

A still further series of tests was carried out with anthracite as a reducing agent, selenium metal and sodium nitrate. The compositions are shown in Table 6 and the results shown in Figure 6. The Co amounts and optical percent values were within the target limits at the end of the tests.

As has been mentioned above, anthracite tends to be twice as effective at producing ferrous ions in the glass as sugar so it may be introduced at lower levels. In test 32 the equivalent batch to that used in test 30 was assessed, but with anthracite introduced at half the batch carbon level as sugar. Figure 6 shows that the selenium retention in test 32 is only 66% but the selenium and ferrous opticals are quite close to the target figures. Test 33A was carried out by repeating test 32 but using calcium sulphate (anhydrite) rather than salt cake in the batch. With the use of anhydrite, both the selenium and ferrous optical figures were reduced slightly suggesting that anhydrite is more oxidizing and selenium retention was improved. In test 34 the level of sodium nitrate was reduced in the batch and this greatly reduced the selenium retention.This is a clear indication that sodium nitrate improves the retention of selenium in the batch.

Further tests were carried out in an attempt to assess whether any agents could be substituted for sodium nitrate to give equivalent increased selenium retention. Other low melting and oxidizing materials which were tested were sodium perchlorate, barium peroxide and sodium hydroxide. Sodium hydroxide is not oxidizing but it was considered that it may aid selenium retention by early melting and give some indication of whether oxidation or early melting is responsible for increased selenium retention. The compositions of the batches tested are shown on Table 7 and the results are shown in Figure 7. The Co amounts and optical percent values were within the target limits at the end of the tests. Tests 35 and 36 assessed barium peroxide and sodium perchlorate respectively and these batches were based on that used for test 30 and the results of test 30 are indicated in Figure 7 for purposes of comparison. Batch 37 contained the same weight of sodium hydroxide as the oxidizing agent used in the other batches.

The results from batches 30, 35 and 36 showed that sodium nitrate gave the best retention and also gave the glass with the lowest ferrous optical figure. Sodium perchlorate gave good optical results but the selenium retention was very low. In addition, the pollution from this raw material may be worse than the NOX emissions from the sodium nitrate. Batches 37 and 10 gave similar results and there appears to be no additional benefit in adding sodium hydroxide.

To summarize the previous results, tests 6B, 14, 35, 33A, 31, 26 and 23 were considered to give the potentially best optical results, with all optical ferrous and selenium values being within 80 to 120% of the target figures.

Batches 6B, 14 and 35 do not contain sodium nitrate.

Batch 6B contains selenium metal at a level of 135 ppm without oxidizing or reducing agents. Although the correct colour is obtained, the selenium retention is no better than that of a conventional production furnace and the only advantage of this batch is that it does not contain sodium nitrate. Batch 14 contains barium selenite at a selenium level of 67.5 ppm, which is half the level of that in batch 6B, and sugar at a level of 0.2 weight percent batch carbon.

The retention obtained is an improvement on the present level in production but is not 100%. Batch 35 contains 35 ppm of selenium metal, sugar to a level of 0.6 weight percent batch carbon and sodium perchlorate. This batch gives the correct optical properties, which is anomalous as the glass only contains 15 ppm of selenium. In addition, the pollution from the perchlorate would tend to rule out this batch for production.

The remaining four batches all contain the same level of sodium nitrate with some form of reducing agent and either selenium metal or barium selenite as the selenium raw material.

Of the selenium metal and barium selenite batches, better retention is obtained when sugar is used as a reducing agent rather than anthracite. The best batch of these four batches was the selenium batch with sugar which was used in test 31. However, anthracite is a more conventional production raw material.

As has been mentioned above, the use of sodium nitrate as an oxidising agent can lead to NOX emissions which are undesirable. In view of the results from employing alternative oxidising agents to sodium nitrate, such as barium peroxide, further tests were carried out employing peroxo compounds as oxidising agents. These compounds do not result in NOX emissions and in addition they are less oxidising than sodium nitrate and so it has been discovered that an addition of carbon is not always essential. A number of further tests were carried out on glass batch compositions including peroxo compounds as the oxidising agent and excluding added carbon. The peroxo oxidising agents comprised calcium peroxide, sodium persulphate, magnesium peroxide, zinc peroxide, and barium peroxide. The results of the tests are shown in Table 9.In each test, sodium selenate was the source of selenium. It should be noted that the optical values for the colour for the target glass were optical ferrous 100%, optical cobalt 100% and optical selenium 114%. For each sample, the amount of the oxidising agent has been indicated in the form of an equivalent amount of calcium peroxide, and the equivalent value is specified as the weight of calcium peroxide in a 500kg batch composition.

(The results of four tests employing calcium peroxide are illustrated in Figure 9). It will be noted that with a calcium peroxide level of from about 0.6 to 1.0 kg per 500 kg of batch the optical values are substantially around the desired value. Sodium persulphate, magnesium peroxide and barium peroxide also gave good optical values, with zinc peroxide giving less good optical values. It will be seen that these peroxo compounds can be employed to give desired colour characteristics of the glass composition without employing carbon-containing reducing agents. In addition, the amounts of selenium in the resulting glass can be quite high indicating good selenium retention when compared to the prior art. Figure 10 shows the relationship between the amount of selenium in the resultant glass with the level of added oxidant for various oxidising agents.Good selenium retention was obtained using calcium peroxide, barium peroxide and sodium persulphate at amounts of from 0.4 to 1.0 kg per 500 kg batch.

Further experiments were then conducted to investigate the use of different selenium sources employing calcium peroxide as an oxidising agent and in the absence of any carbon-containing reducing agents. The selenium sources employed were sodium selenate, selenium metal, sodium solenite and barium selenite. The results are shown in Table 10 (batches El and E4 are also shown in Table 9). This shows that good optical properties and good selenium retention can be obtained without requiring a reducing agent and employing selenium sources other than selenium metal. Both sodium solenate and sodium selenite gave improved selenium retention when compared to selenium metal.

Table 10A shows the results of a number of additional tests on samples including calcium peroxide as oxidant and sodium selenate as the source of selenium. The samples differed primarily by varying the amounts of iron, expressed as weight percent Fe203 in the batch and selenium expressed as ppm in the batch. It will be seen that both the optical Se levels and the Se retained amounts in the glass vary with the initial Fe and Se amounts. It is believed that there is a strong correlation between the two properties of the glass dependent upon the initial starting amounts. It is postulated, without being bound by theory, that for bronze glasses the bronze colour results from a complex of ferrous ions and selenium ions. The optical activities of the Se and the ferrous and the Se retention appear to be determined by the Fe content initially.The total amount of initial iron appears not directly to determine the amount of ferrous in the glass.

Following analysis of these results, some further tests were carried out to assess the effect of changes which may occur to the batch if it were melted on a production furnace. The first consideration was the source of sulphate in the batch. A production unit may use anhydrite rather than salt cake and also the level added to the batch may require alteration. In addition, wet batch is almost exclusively used in production furnaces, the water being present as 4% by weight of the sand. Accordingly, the batch employed in production would be moist. In order to obtain the correct optical ferrous level in the glass the total iron content of the batch is often adjusted. The effect of the above batch alterations on the colour and selenium retention in the bronze glass was therefore investigated. The compositions tested are shown in Table 11 and the results are shown in Figure 11. The Cho304 optical percent values for tests 33A, 33B, 38, 39 and 40 were within the target limits.

The first two batches 33A and 33B gave very good optical results. Batch 38 illustrates the effect of increasing anhydrite to a batch SO3 value of 3. The optical ferrous, optical selenium and selenium retention values are lower with this batch. It would therefore appear that increasing the anhydrite, and therefore the SO3 level in the glass, decreases selenium retention. The decrease in optical ferrous may be due to the oxidizing nature of the SO3 in the batch. The moist batch in test 39 has the effect of increasing the selenium optical value. With the increase in iron in the glass to 120% of its original value in test 40, both the selenium and ferrous optical values are improved.

Further experiments were carried out on a pilot plant, which is an electrically heated cold top furnace, the primary objective being to produce good quality tinted float glass having target optical characteristics and to maintain a high level of selenium retention in the glass. Alternative oxidising agents and selenium sources were investigated for the production of glass having equivalent spectral properties to the target glass. In addition, the furnace gas emissions were monitored to provide an assessment of the environmental effect of the production process.

A range of batch compositions to produce anti sun bronze glass was evaluated on the furnace. Table 12 shows the batch compositions tested as well as a standard batch. The compositions were based on a standard float glass formulation but was modified for the various tests. The batch SO3 content derived from anhydride was fixed at 2kg per 1,000kg of glass. The silica source was dried Chelford HS50 sand with a moisture content of 0.5%. During the tests, no additional water was added to the mixer so as to simulate wet sand which normally contains up to 4% moisture. Apart from initially adjusting the iron content of the glass, the amount of rouge was kept constant to provide a total iron content of 0.38%.In the same manner, the amount of cobalt oxide in the glass was kept constant to provide a cobalt amount of 38+/-2 ppm in the glass, this providing the correct optical density. With these components being fixed, the principal chemical variables which were investigated to provide the bronze tint were the selenium source and concentration, the oxidant type and contribution, and the reductant contribution.

The reductant employed was usually anthracite, but sucrose was also assessed and the experiments also took into account the indigenous organic carbon contribution which is associated with sand and with dolomite.

The initial batch formulation which was examined on the production unit was derived from batch 33A which was run on the cold top simulator and is discussed hereinabove. This contained 35ppm of selenium metal, 34ppm of cobalt oxide and 2,800ppm of total iron oxide. The nitre contribution from sodium nitrate was equivalent to half that of a conventional melting tank and the batch carbon content was 0.2kg per 1,000 kg of glass. It was found that modifications were required to the element concentrations in order to achieve the optimal spectral specification. The cobalt oxide was increased from 34 to 38ppm and the rouge content was changed from 0.28 to 0.38 wt% analysed as iron oxide in the glass (i.e. from 0.65 to 1.07 kg per 500 kg catch) and the final rouge content remained constant during the remainder of the tests.In addition, the batch carbon content, added as anthracite, was reduced from 0.15 to 0.1keg per 500 kg of glass. The resultant glass achieved 100% selenium retention but the bronze colour was too pink, resulting from the spectral contribution of selenium being in excess of 130%.

Accordingly, the elemental selenium content was reduced from 35ppm to 23ppm in four stages until the spectral selenium contribution was 117% with 105% ferrous and 92% cobalt.

These figures were obtained from glass produced from batch 7 and the resultant glass broadly conformed with the requirement of the spectral standard of optical selenium 116%, optical ferrous 100% and optical cobalt 100%.

A number of additional experiments were then conducted on modifications of batch 7, the results being shown in Table 13. With the elemental selenium content being fixed at 23ppm, the batch nitre and carbon content were varied to determine the minimum amounts required in the batch whilst still maintaining spectral targets. In 10 batches the nitre was varied between 0 and 1/2 of conventional glass tank amounts and the additional batch carbon was varied from 0 to 0.2kg per 1,000 kg of glass. Batch 17, the compositon of which is shown in Table 12, contained 1/8 of the nitre amount employed in the conventional glass tank batch (i.e. 0.55 kg per 500 kg batch) with the minimal amount of additional batch carbon of 0.033kg per 1,000 kg of glass. The resultant glass had a spectral result of selenium 107%, ferrous 98% and cobalt 99%.The results showed that using sodium nitrate as an oxidising agent, selenium metal as the selenium source and anthracite as a carbon addition, a selenium retention of 100% could be obtained with satisfactory optical spectral values of selenium and ferrous. The three most suitable batches were batch 7, 9 and 17 and the composition of batch 9 is shown in Table 12. Batch 9 contained 3/8 of the nitre amount relative to the conventional level, namely 1.65kg nitre per 500kg of batch. Batches 7, 9 and 17 had additonal batch carbon levels of 0.2, 0.1 and 0.033kg per 1,000 kg of glass respectively.

Further tests were then carried out on the cold top production furnace to assess alternative selenium sources and oxidising agents. The batch compositions tested are shown in Table 14.

Previous tests (discussed hereinabove) had shown that barium selenite could be employed as a selenium source but whilst this gave improved selenium retention the optical qualities of the glass were not satisfactory. It was found that when barium selenite was employed as the selenium source, an oxidising agent was required to promote the bronze tint in the glass. It was considered that unless an alternative to the conventional sodium nitrate oxidising agent could be found there was little justification in employing barium selenite as a selenium source. Accordingly, tests were carried out on the production unit employing oxidising agents other than sodium nitrate and other selenium sources. Disodium peroxy-disulphate, Na2S2O8, referred to hereinafter as sodium persulphate, was employed as an oxidising agent.The sodium persulphate can also provide a sulphate source for refining of the glass. The sodium persulphate oxidising agent was employed at a value set by the sulphate contribution of 2kg per 1,000 kg of glass and no anthracite was added as batch carbon so that only indigenous organic carbon from the sand and dolomite remained in the batch. This constituted batch 19, the composition of which is shown in Table 14. The glass ribbon was grey and the optical selenium level was 72%. Further sodium persulphate could not be added to improve the selenium retention otherwise the batch SO3 would have exceeded beyond the adjudged optimum of 2kg per 1,000 kg of glass for the electric melting operation.

Due to the limit on sulphate level which can be added to the batch, a more oxidised selenium source was employed, this being sodium selenite and the anhydrous form containing 67.3% SeO3 as analysed by X-rayed diffraction. Batch 20, the composition of which is shown in Table 14, contained 45 grams of sodium selenate solely as the selenium source which translated to a theoretical 45 ppm concentration based on glass weight. No further oxidising agent was added although the standard anhydrite content was restored to provide the batch SO3 of 2kg per 1,000 kg glass. The resultant glass had a bronze tint and provided an optical profile of selenium 104%, ferrous 106% and cobalt 95%. The chemical retention of selenium in the glass was only 21 ppm yielding only 46% efficiency of selenium retention and a reduction to 25 grams of sodium selenate in the batch merely resulted in a lower chemical selenium retention in the glass with dilution in spectral intensity.

Batch 23, the composition of which is also shown in Table 14, combined the advantages of sodium selenate with the potential activity of the sodium persulphate oxidising agent. The selenium theoretical level was calculated at 35 ppm and the persulphate contribution was set at the maximum batch SO3 lever of 2kg per 1,000 kg of glass (this being equivalent to 1.25kg of sodium persulphate in 500 kg of batch). This achieved a bronze tint which met the target optical parameters and the optical evaluation was selenium 118%, ferrous 98% and cobalt 97%. Chemical analysis of the glass revealed selenium retention of 30ppm equating to an efficiency of selenium retention of 86%. Thus, batch 23 achieved not only good selenium retention and optical characteristics but also did so without any NOX emissions.

Alternative commerically available oxidising agents were then employed as oxidising agents, these being barium peroxide (95% active ingredient) and calcium peroxide (75% active ingredient). Other oxidising agents, particularly magnesium peroxide (25% activity) and zinc peroxide (55% activity) were not tested on the production furnace but are described above in relation to the cold top simulator.

Barium peroxide was added to a batch to provide an equivalent oxidising capability as the nitre in batch 17, the resultant batch being batch 24 which is indicated in Table 14. The sodium selenate input was fixed at 35 grams per 100 kg of batch corresponding to a selenium concentration of 35 ppm in the glass. After 2 days residence in the production furnace, this batch provided glass samples which, when analysed, revealed optical properties of selenium 118%, ferrous 94% and cobalt 99%. The chemically retained selenium was estimated to be 29 ppm giving an operating efficiency of selenium retention of 83%.

A composition was then tested employing calcium peroxide in combination with sodium selenate. A bronze tinted glass was attained. The oxygen contribution was calculated to provide a concentration in the batch of 84 grams of oxygen per 500 kg of batch, this being equivalent to that generated by the sodium persulphate. The sodium selenate addition was 30 grams in a 500 kg batch, this being equivalent to a notional 30ppm selenium in the glass. The resultant glass composition exhibited a bronze colour with a lower than target optical selenium. The chemically analysed ferrous contribution was also low, suggesting over-oxidation. An empirical adjustment was made to lower the oxygen content so as to provide a higher ferrous level, this being achieved by reducing the calcium peroxide to a level of 400 grams per 500 kg of batch.The resultant batch 30, the composition of which is shown in Table 14, realised a chemically analysed ferrous of 24% in the glass and an optical selenium value of 113%. The chemically retained selenium was analysed at 28 ppm providing an efficiency of selenium retention of 93%.

Table 15 shows the results of batches 19, 20, 23, 24 and 30 in combination with batches 7 and 9. The results of additional batch 22, employing sodium persulphate, are also shown in Table 15.

During the formation of the glass there is a complex interaction of selenium with other trace elements, including iron, which can affect the ultimate properties of the glass.

It is difficult to confirm which species of selenium, for example, selenite, selenate, atomic selenium, polyselenides, alkali selenides or iron selenide, may be present in the glass compositon because the selenium concentration employed to generate the tint can be so low as to be near the analytical detection limit. This hinders the identification of the species which are frozen into the glass matrix.

However, from analysis of the oxidizing and reducing components which have been employed in the batches and which have provided target compositions, a number of factors can be deduced.

It is believed that the preferred range for the iron redox ratio is from 0.26 to 0.32, the optimum value being 0.29 which is equal to a ferrous iron concentration in the glass of 22.5%. This ratio is indicated with results shown in Table 13 and it is to be noted that batch 17 shown in Table 13 had prescribed optical characteristics resulting from the fine adjustment between the niter and carbon levels. Also, the calculation of an optimum peroxo compound contribution could be made when seeking a target optical selenium specification.

The tests showed that excessive oxidation or redution of the batch components resulted in the loss of chemically retained selenium in the glass and this also resulted in a bleached state or highly reduced blue colour in the glass.

Some of the good bronze tinted compositions rely on a combination of oxygen with carbon to achieve the desired colour. However, analysis of an ordinary batch without added anthracite, revealed a background organic carbon contribution from the raw material source. The mean level of carbon contributed is 0.01 weight % with a coefficient of variation of +/- 30%, this translating into a background free carbon content of 50 grams per 500 kg of batch or a batch carbon content of 0.12 kg per 1,000 kg of glass.

Using the basic equation C+O2=CO2 and the available reductant and oxygen from the various batch components, a factor X, can be calculated: aram atoms of carbon = X gram moles of available oxygen Preferred compositions exhibiting the target spectral specification have the property that X = 1.5 +/- 0.2, this indicating a net reducing condition. Thus, the oxygen contribution from the more expensive peroxo compounds used to balance the background carbon content can be gauged accurately.

In the production of selenium-containing glass it is important to consider the environmental and health and safety aspects of the production processes. It is desirable to reduce Se emissions as much as possible since selenium is regarded as a toxic substance with an occupational exposure standard of (OES) 0.lmg/m3. For external assessment the OES must be reduced to 1/50th of the external standard at ground level. SOx and NOX emissions are also undesirable.

The cold top electric furnace can limit particulate carry over in the furnace melt and can reduce the emission of volatile constituents. This is particularly important with elemental selenium and its compounds when manufacturing bronze tinted compositions. Tests for selenium in the vicinity of the furnace were carried out and it was found that in the vicinity of the furnace the potential exposure to selenium was less than l/lOth of the curent Health and Safety Executive long term exposure standard for selenium. In addition, the selenium emission from the electric cold top furnace was measured and this was estimated to be only about 10% of that of a conventional furnace. For the nitre-free batch 30, the NOx emission was only 30% that of a conventional furnace at equivalent load. Thus, the present invention can provide real environmental advantages.

TABLE 2 BATC@ES CONTAINING NO SODIUH NIIRATE

TEST NUHBER 6B 10 7B 8B 11@ 12 13 16 RAW MAT@RIAL CEELFORD SAND 74.56 74.56 74.56 74.56 74.56 74.56 74.56 74.56 BEN BENNEIT LIMESTONE 5.07 5.07 5.07 5.07 5.07 5.07 5.07 5.07 WARHSWORTE DOLOMITE 18.46 18.46 18.46 18.46 18.46 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 .0037 .0037 .0037 .0037 .0037 SODA ASH 21.73 21.73 21.73 21.73 21.73 21.73 21.73 21.73 SODIUM NITRATE SALTCAKE .363 .363 .363 .363 .363 .363 .363 .363 AN@YDRITE BATC@ SO3 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 ROUGE .165 .165 .165 .165 .165 .165 .165 @@RROUS OXALATE .417 ANTERCAITE 0.024 SUGAR .0475 BATCE CARBON 0.2 0.2 SELENIUM METAL .0135 .0045 Z@NC SE@@NIDE BARIUM SELENITE .0452 .0226 .0452 .0452 .0226 SODIUM SELENITE .0296 SODIUM SELENATE Se ppm in BATC@ 135 45 135 135 67.5 135 135 67.5 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM @YDROX@DE MOISTURE (@ OF SAND) TABLE 3 BATCHES USED TO INVESTIGATE THE USE OF SUGAR WITH.BARTUM SELENITE AND SODIUM NITRATE

TEST NUMBER | 17 18 19 20 21 22- 23 24 RAW MATERIAL CHELFORD SAND 74.56 74.56 74.56 74.56 74.56 74.56 74.56 74.56 BEN BENNETT LIMESTONE | 5.07 5.07 5.07 5.07 5.07 5.07 5.07 5.07 WARMSWORTH DOLOMITE | 18.46 18.46 18.46 18.46 18.46 18.46 18.46 18.46 Co3O4 t .0037 .0037 .0037 .0037 .0037 .0037 .0037 .0037 SODA ASH 1 21.08 21.08 21.08 21.41 21.58 21.58 21.41 21.50 SODIUM NITRATE | 1.04 1.04 1.04 .52 .26 .26 .52 .39 SALTCAKE .363 .363 .363 .363 .363 .363 .363 .363 ANHYDRITE BATCE S03 | 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 ROUGE .165 .165 .165 .165 .165 .165 .165 PERROUS OXALATE 0.417 ANTHRACITE SUGAR 1 .0475 .095 .095 .095 .095 .095 .1425 .1425 BATCH CARBON I .2 .4 .4 .4 .4 .4 .6 .6 SELENIUM METAL - I ZINC SELENIDE BARIUM SELENITE .0226 .0226 .0075 .0452 .015 .015 .0075 .0075 SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH | 67.5 67.5 22.5 135 45 45 22.5 22.5 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM HYDROXIDE MOISTURE (% OF SAND) TABLE 4 BATCHES USED TO INVESTIGATE THE USE OF ANTHRACITE WITH BARIUM SELENITE AND SODIUM NITRATE B

TEST NUMBER 25 26 27 RAW MATERIAL CHELFORD SAND 74.56 74.56 74.56 BEN BENNETT LIMESTONE 5.07 5.07 5.07 WARMSWORTH DOLOMITE 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 SODA ASH 21.41 21.41 21.50 SODIUM NITRATE .52 .52 .39 SALTCAKE .363 .363 .363 ANHYDRITE BATCH SO3 2.0 2.0 2.0 ROUGE .165 .165 .165 FERROUS OXALATE ANTHRACITE .048 .036 .036 SUGAR BATCH CARBON .4 .3 .3 SELENIUM METAL ZINC SELENIDE BARIUM SELENITE .0452 .015 .0117 SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 135 45 35 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM HYDROXIDE MOISTURE (% OF SAND) TABLE 5 BATCHES USED TO INVESTIGATE THE USED OF SUGAR WITH SELENIUM METAL AND SODIUM NITRATE

TEST NUMBER 28 29 30 31 RAW MATERIAL CHELFORD SAND 74.56 74.56 74.56 74.56 BEN BENNETT LIMESTONE 5.07 5.07 5.07 5.07 WARMSWORTE DOLOMITE 18.46 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 .0037 SODA ASH 21.08 21.41 21.41 21.41 SODIUM NITRATE 1.04 .52 .52 .52 SALTCAKE .363 .363 .363 .363 ANHYDRITE BATCH SO3 2.0 2.0 2.0 2.0 ROUGE .165 .165 .165 .165 FERROUS OXALATE ANTHRACITE SUGAR .095 .1425 .1425 .19 BATCH CARBON .4 .6 .6 .8 SELENIUM METAL .00675 .0045 .0035 .0030 ZINC SELENIDE BARIUM SELENITE SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 67.5 45 35 30 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM HYDROXIDE MOISTURE (% OF SAND) TABLE 6 BATCHES USED TO INVESTIGATE THE USE OF ANTHRACITE WITH SELENIUM METAL AND SODIUM NITRATE

TEST NUMBER | 32 33 34 RAW A & B MATERIAL CHELFORD SAND 74.56 74.56 74.56 BEN BENNETT LIMESTONE 5.07 4.82 5.07 WARMSWORTH DOLOMITE 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 SODA ASH | 21.41 21.68 21.50 SODIUM NITRATE | .52 .52 .39 SALTCAKE | .363 .363 ANHYDRITE .352 BATCH S03 | 2.0 2.0 2.0 ROUGE | .165 .165 .165 FERROUS OXALATE ANTHRACITE .036 .036 .036 SUGAR BATCH CARBON .3 .3 .3 SELENIUM METAL | .0035 .0035 .0035 ZINC SELENIDE BARIUM SELENITE SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 35 35 35 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM HYDROXIDE MOISTURE (% OF SAND) I TABLE 7 BATCHES USED TO ASSESS POSSIBLE ALTERNATIVES TO SODIUM NITRATE

TEST NUMBER 30 36 35 37 @0.

RAW MATERIAL CHELFORD SAND 74.56 74.56 74.56 74.56 74.56 BEN BENNETT LIMESTONE 5.07 5.07 5.07 5.07 5.07 WARMSWORTH DOLOMITE 18.46 18.46 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 .0037 .0037 SODA ASH 21.41 21.73 21.48 21.04 21.73 SODIUM NITRATE .52 SALTCAKE .363 .363 .363 .363 .363 ANHYDRITE BATCH SO3 2.0 2.0 2.0 2.0 2.0 ROUGE 0.165 0.165 0.165 0.165 0.165 FERROUS OXALATE ANTHRACITE SUGAR .1425 .1425 .1425 BATCH CARBON .6 .6 .6 SELENIUM METAL .0035 .0035 .0035 .0045 .0045 ZINC SELENIDE BARIUM SELENITE SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 35 35 35 45 45 SODIUM PERCHLORATE .52 BARIUM PEROXIDE .52 SODIUM HYDROXIDE .52 MOISTURE (% OF SAND) TABLE 8 BEST BATCHES

TEST NUMBER 6 14 35 33 31 26 23 RAW A & A & MATERIAL CHELFORD SAND 74.56 74.56 74.56 74.56 74.56 74.56 74.56 BEN BENNETT LIMESTONE 5.07 5.07 5.07 4.82 5.07 5.07 5.07 WARMSWORTH DOLOMITE 18.46 18.46 18.46 18.46 18.46 18.46 18.46 Co3O4 .0037 .0037 .0037 .0037 .0037 .0037 .0037 SODA ASH 21.73 21.73 21.73 21.68 21.41 21.41 21.41 SODIUM NITRATE .52 .52 .52 .52 SALTCAKE .363 .363 .363 .363 .363 .363 ANHYDRITE .352 BATCH SO3 2.0 2.0 2.0 2.0 2.0 2.0 2.0 ROUGE .165 .165 .165 .165 .165 .165 .165 FERROUS OXALATE ANTHRACITE .036 .036 SUGAR .0475 .1425 .19 .1425 BATCH CARBON .2 .6 .3 .8 .3 .6 SELENIUM METAL .0135 .0035 .0035 .003 ZINC SELENIDE BARIUM SELENITE .226 .015 .0075 SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 135 67.5 35 35 30 45 22.5 SODIUM PERCHLORATE .52 BARIUM PEROXIDE SODIUM HYDROXIDE MOISTURE (% OF SAND) TABLE 9 COLD TOP SELENIUM BRONZE RESULTS COMPARISON OF OXIDIZING AGENTS LEVELS OF OXIDANT SCALED TO AN EQUIVALENT WEIGHT OF CALCIUM PEROXIDE | -----GLASS LEVELS-----| |----COLOUR VALUES----| |-EATCH-| Sample CaO2 Fe2O3 Se % Fe2+ Optical Optical Optical Oxidant Kg/500 ppm Ferrous Co Se Kg batch E 1 0.8 0.382 38 30% 110% 113% 128% CaO2 E 2 0.4 0.375 26 31% 122% 110% 52% CaO2 E 3 0.6 0.375 26 31% 128% 115% 108% CaO2 E 4 1.0 0.380 32 26% 117% 114% 130% CaO2 E 9 0.0 0.392 8 29% 138% 125% 68% Na2S2O8 E10 0.2 0.396 13 2@% Na2S2O8 E11 0.4 0.391 20 27% 125% 119% 102% Na2S2O8 E12 0.6 0.386 38 26% 122% 129% 151% Na2S2O8 E13 0.8 0.368 38 26% 101% 118% 136% Na2S2O8 E20 0.6 0.380 19 29% 128% 125% 83% MgO2 E21 0.8 0.385 14 29% 138% 127% 85% MgO2 E30 1.0 0.381 24 30% 119% 116% 112% MgO2 E31 1.2 0.386 30 26% 123% 120% 121% MgO2 E26 0.6 0.380 14 30% 136% 116% 62% ZnO2 E27 0.8 0.382 12 30% 130% 114% 66% ZnO2 E28 0.6 0.373 31 28% 128% 121% 120% BaO2 E29 0.8 0.382 37 25% 123% 143% 189% BaO2 E32 1.0 0.381 36 27% 117% 114% 92%MgO2/Na2S2O8 REFEATED BATCHES E1R 0.8 0.391 25 28% 125% 121% 112% C3O2 E13R 0.8 0.391 30 26% 124% 128% 143% Na2S2O8 TABLE 10 COLD TOP SELENIUM BRONZE RESULTS COMPARISON OF SELENIUM SOURCES WITH 0.8 # 1.0 Kg/500Kg batch CALCIUM PEROXIDE AS OXIDANT SS = SODIUM SELENATE M = SELENIUM METAL SS'ITE= SODIUM SELENITE BS'ITE= BARIUM SELENITE |------GLASS LEVELS------| |-----COLOUR VALUES-----| |-BATCH-| Sample CaO2 Fe2O3 Se % Fe2+ Optical Optical Optical Selenium Ferrous Co Se Source E 1 0.8 0.382 38 30% 110% 113% 128% SS E 4 1.0 0.380 32 26% 117% 114% 130% SS E 7 0.8 0.383 23 26% 116% 111% 103% M E 8 1.0 0.396 28 26% 123% 129% 134% M E37 0.8 0.388 30 27% 128% 114% 87% SS'ITE E38 1.0 0.392 40 24% 126% 116% 100% SS'ITE E39 0.8 0.388 11 28% 133% 113% 66% BS'ITE E40 1.0 0.387 15 31% 131v 110% 80 BS'ITE TABLE 10A |--GLASS LEVEL---| |-----COLOUR LEVEL-----| |-----BATCH----| @SAMP@CaO2 Optical Optical Optical Se Fe2O3 Se % Fe2+ Ferrous Co Se SOURCE Fe/Se --- E23 0.8 0.379 15 24% 124% 119% 79% SS 25S/.38 E24 0.8 0.340 27 27% 109% 108% 99% SS 30S@.33 E24R 0.8 0.334 25 27% 103% 116% 94% 30S@.33 E25 0.0 0.340 19 30% 114% 113% 86% SS 25S@.33 E34 0.8 0.388 32 27% 124% 131% 144% SS .38/25Se E35 0.8 0.332 23 28% 106% 123% 106% SS .33/30Se E36 0.8 0.344 13 29% 124% 149% 94% SS .33/25Se E41 1.0 0.400 21 29% 133% 143% 137% SS 0.40/30Se E42 1.0 0.356 18 29% 117% 116% 108% SS 0.35/30Se E43 1.0 0.309 17 27% 100% 116% 71% SS 0.30/30S2 E44 1.0 0.399 19 27% 128% 116% 105% SS 0.40/25Se E45 1.0 0.353 16 29% 118% 116% 78% SS 0.35/25Se E46 1.0 0.309 13 29% 101% 120% 68% SS 0.30/25Se E47 1.0 0.401 16 29% 121% 110% 118% SS 0.40/20Se E48 1.0 0.359 15 31% 122% 117% 114% SS 0.35/20Se E49 1.0 0.311 11 29% 103% 111% 48% SS 0.30/20Se SS= SCDIUM SELENATE 10th COL GIVES OR LEVELS OF Fe2O3 & Se USED WHEN Fe2O3 or Se WAS CHANGED TO STUDY Fe/Se INTERACTIONS Fe in wt%; Se in ppm.

TABLE 11 VARIATIONS ON TRIAL BATCH

TEST NUMBER 33 38 39 40 RAW A & B MATERIAL CHELFORD SAND 74.56 74.56 74.56 74.56 BEN BENNETT LIMESTONE 4.82 4.69 4.82 4.82 WARMSWORTH DOLOMITE 18.46 18.47 18.46 18.46 Co3O4 .0037 .0037 .0037 .0037 SODA ASH 21.68 21.68 21.68 21.68 SODIUM NITRATE .52 .52 .52 .52 SALTCAKE ANHYDRITE 0.352 0.528 0.352 0 .352 BATCH SO3 2.0 3.0 2.0 2.0 ROUGE .165 .165 .165 .222 FERROUS OXALATE ANTHRACITE .036 .036 .036 .036 SUGAR BATCH CARBON .3 .3 .3 .3 SELENIUM METAL .0035 .0035 .0035 .0035 ZINC SELENIDE BARIUM SELENITE SODIUM SELENITE SODIUM SELENATE Se ppm in BATCH 35 35 35 35 SODIUM PERCHLORATE BARIUM PEROXIDE SODIUM HYDROXIDE HOISTURE (% OF SAND) 2.198 (4%) TABLE 12 Standard and Experimental Batch Compositions Standard 33A or Batch Batch 1 Batch 7 Batch 9 Batch 17 Chelford Sand 310.50 kg 310.50 kg 310.50 kg 310.50 kg 310.50 kg Dolomite 75.00 76.90 76.90 76.90 76.90 Limestone 21.00 20.00 20.00 20.00 20.00 Soda Ash 85.20 89.64 89.64 89.64 90.50 Salt Cake 3.73 0.00 0.00 0.00 0.00 Anhydrite 0.00 1.50 1.50 1.50 1.50 Sodium Nitrate 4.00 kg 2.20 KG 2.20 KG 1.65 kg 0.55 kg Rouge 1.04 0.65 1.07 1.07 1.07 Anthracite 0.00 0.15 0.10 0.05 0.016 Selenium 200 ppm 35 ppm 23 ppm 23 ppm 23 ppm Cobalt Oxide 40 ppm 34 ppn 38 ppm 38 ppm 38 ppm Glass Analysis SiO@ 72.6% 72.5% 72.4% 72.5% CaO 8.4 8.4 8.4 8.3 Fe2O3 0.278 0.379 0.369 0.382 Al2O3 1.1 1.1 1.1 1.1 Mg3 3.9 3.9 3.9 4.0 Na 0 12.7 12.7 12.8 12.7 K2O 0.67 0.65 0.62 0.64 SO 0.20 0.20 0.20 0.19 SnO2 0.10 0.08 0.10 0.10 Ferrous@ 25.0 Z 21.0 X 23.0 Z 23.0 Z Fe2+/Fe3+ 0.34 0.26 0.29 0.30 Se 26 ppm 30 ppm 25 ppm 25 ppm Co3O4 34 ppm 34 ppm 37 ppm 38 ppm Cl 232 ppm 232 ppm 249 ppm 234 ppm TABLE 13 ANTISUN BRONZE FLOAT - [500 kg. batch form@lation]

BNAITTCf?Ei 2.2 kfj. 1.rr Ic(J. 1.1 !J. (1rr k!J. O.2Irj r? o Q a (1) ew C 2 ~~~~~~~ ~~~~~~~ (()) 113 t 3O1 n L ~ g I - (J.2(; I).2 100 ~ |tS O =. cn " o J 0.1 2'j112 ANTIlUtACIT- O2!I O LVP (g)3 O- 23 I1E I '!'.

C t ~, 2!) zit CJIV NV N ' N . flACITE ib !J. ~~~~~~ ~~~~~~~~ 23 J(1 o 2O4kj 2(;1313 2293 11374 7 i 7 , 1 ' t 4 (33 10 ?3 22 9j C\4 a L PSICS (Q) ;:Iritii (ii.ii.iii.1 i C,1 .. j1i:iI Itriiri ( O,' ) I)fl(f.iI nirriL)er > (it'roti N (rii: r~ L - ~U E, r|cw i) ; : Q N ~ 1 / LLJ O' 7CD~. ~ "t C = C a, ; C a: O Ct Z o Z > '1 TABLE 14 Batches containing Alternative Oxidising Agents Batch 19 Batch 20 Batch 23 Batch 24 Batch 30 Chelford Sand 310.50 kg 310.50 kg 310.50 kg 310.50 kg 310.50 kg Dolomite 76.90 76.90 76.90 76.90 76.90 Limestone 20.00 20.00 20.00 20.00 20.00 Soda Ash 90.50 90.50 90.50 90.50 90.50 Anhydrite 0.00 1.50 0.00 1.50 1.50 Rouge 1.07 kg 1.07 kg 1.07 kg 1.07 kg 1.07 kg Sodium Persulphate 1.25 0.00 1.25 0.00 0.00 Calcium Peroxide 0.00 0.00 0.00 0.00 0.40 Barium Peroxide 0.00 0.00 0.00 0.50 0.00 Sodium Selenate 0.00 0.045 0.035 0.035 0.030 Cobalt Oxide 38 ppm 38 ppm 38 ppm 38 ppm 38 ppm Selenium 23 ppm 0.00 0.00 0.00 0.00 Glass Analysis (HFT) 1335 1338 1342 1344 2952 SiO2 72.5% 72.5% 72.6% 72.6% 72.8% CaO- 8.4 8.4 8.3 8.3 8.4 Fe203 0.379 0.38 0.367 0.363 0.372 Al2O3 1.1 1.1 1.0 1.0 1.0 Mg 4.0 4.0 4.0 4.0 3.9 NasO 12.7 12.7 12.8 12.8 12.6 K2O 0.63 0.62 0.58 0.58 0.60 SO 0.18 0.20 0.20 0.20 0.20 SnO2 0.06 0.06 0.06 0.10 0.07 Ferrous 22.0% 25.0 % 24.0 % 22.0 Z 24.0 % Fe2+/Pe3' 0.29 0.33 0.32 0.27 0.31 Se 12ppm 21 ppm 30 ppm 29 ppm 28 ppm Co3O4 38ppm 38 ppm 39 ppm 38 ppm 37 ppm Cl 218ppm 224 ppm 227 ppm 227 ppm -238 ppm TABLE 15 BRONZE TINTED FLOAT GLASS Batch Oxidising Wt Kg Reducing Wt Kg Source Target Chem Efficiency Chem Ratio Opt Opt Opt No. Agent per Agent per Se Se Se % Fe2+ Fe2+/ Se Fe Co 500Kg 500Kg ppm ppm % Fe3+/ % % % Batch Batch * 7 NaNO3 2.2 Anthracite 0.100 Element 23 30 21 0.26 117 103 9 NaNO3 1.65 Anthracite 0.05 Element 23 25 100 23 0.29 112 96 93 17 NaNO3 0.55 Anthracite 0.016 Element 23 25 100 23 0.29 107 98 99 19 Na2S2O8 1.25 Element 23 12 52 23 0.29 72 111 98 20 - 0.045 Na2SeO4 45 21 47 25 0.33 104 106 95 22 Na2S2O8 1.25 Na2SeO4 25 27 100 26 0.31 88 115 92 23 Na2S2O8 1.25 Na2SeO4 35 30 86 24 0.32 118 98 97 24 BaO2 0.50 Na2SeO4 35 29 83 22 0.27 118 94 99 30 CaO2 0.40 Na2SeO4 30 28 93 24 0.31 113 93 91 * In addition to any added carbon all batches contained 0.050 kg C per 500 kg batch from raw materials.

Claims (23)

CLAIMS:

1. A glass batch composition for production of a coloured glass, the composition including selenium or a source of selenium, characterised in that the composition includes in combination an oxidising agent for increasing selenium retention in the resultant glass and a reducing agent for providing the glass with desired colour characteristics.

2. A composition according to claim 1 wherein the glass is a bronze glass and the composition further includes iron.

3. A composition according to claim 1 or claim 2 wherein the oxidising agent comprises sodium nitrate, sodium perchlorate, calcium peroxide, sodium persulphate, magnesium peroxide, zinc peroxide or barium peroxide.

4. A composition according to any one of claims 1 to 3 wherein the selenium in the composition is present in the form of one or more of selenium metal, sodium selenite, sodium selenate or barium selenite.

5. A composition according to any foregoing claim wherein the reducing agent comprises carbon or ferrous oxalate.

6. A composition according to claim 5 wherein the carbon is in the form of anthracite, sugar, starch, coke or charcoal.

7. A composition according to claim 6 wherein the sugar is in the form of sucrose.

8. A method of producing a coloured glass containing selenium, the method comprising melting the glass batch composition according to any one of claims 1 to 7 to form a molten glass and forming the molten glass into a desired shape.

9. A glass batch composition for production of a coloured glass, the composition including selenium or a source of selenium characterised in that the composition includes a peroxo oxidising agent.

10. A composition according to claim 9 wherein the peroxo oxidising agent comprises calcium peroxide, magnesium peroxide, barium peroxide, zinc peroxide or sodium persulphate (disodium peroxydisulphate).

11. A method of producing a coloured glass containing selenium, the method comprising melting the batch composition according to claim 9 or claim 10 to form a molten glass and forming the molten glass into a desired shape.

12. A glass batch composition for production of a coloured glass, the composition including selenium, characterised in that the composition includes at least one of a selenite compound and a selenate compound which comprises some or all of the selenium in the composition.

13. A composition according to claim 12 wherein the selenite compound comprises sodium selenite or barium selenite.

14. A composition according to claim 12 wherein the selenate compound comprises sodium selenate.

15. A method of producing a coloured glass containing selenium, the-method comprising melting the batch composition according to any one of claims 12 to 14 to form a molten glass and forming the molten glass into a desired shape.

16. A glass batch composition for production of a coloured glass, the composition including selenium, characterised in that the composition includes up to 0.4 wt% of an alkali metal nitrate.

17. A composition according to claim 16 wherein the composition includes from 0.055 wt% to 0.4 wt% of alkali metal nitrate.

18. A composition according to claim 17 wherein the composition includes from 0.1 wt% to 0.33 wt% of alkali metal nitrate.

19. A composition according to any of claims 16 to 18 wherein the alkali metal nitrate comprises sodium nitrate.

20. A composition according to any one of claims 16 to 19 wherein the alkali metal nitrate is in combination with a reducing agent containing carbon.

21. A method of producing a coloured glass containing selenium, the method comprising melting a glass batch composition in a cold top melter to form a molten glass, the glass batch composition including up to 0.4 wt% of an alkali metal nitrate, and forming the molten glass into a desired shape.

22. A glass batch composition for production of a coloured glass substantially as hereinbefore described.

23. A method of producing a coloured glass containing selenium substantially as hereinbefore described.