To calculate for any result, we must first know the analysis of the irons to be used in making the charge. We shall consider silicon as the guide.

In keeping track of results, the proportion of silicon in the local scrap of an establishment can be accurately estimated. With miscellaneous machinery scrap, this is more difficult; the following, however, are safe estimates:

Casting

Silicon (per cent)

Small thin scrap

2.0 to2.4

Large scrap ranges

1.50 to 2.0

Method

The analysis of pig iron is made from drillings taken from a fresh fracture. Between the very fine grain about the chilled sides of the pig and the very coarse grain in the center, average-sized crystals will be noticed in the fracture. It is here that the drillings for analysis should be made, as indicated in Fig. 131. About a 3/4-inch flat drill is best to use, as it cuts a more uniform chip from the varying grades of pig than does a twist drill.

To determine the analysis of a carload lot of pig iron, the following method is employed: Select ten pigs which will represent an average of the close, medium, and coarse-grained iron in the car. These pigs should be broken, and drillings taken from the fresh fracture. The drillings from these ten fractures are thoroughly mixed together, and about 2 ounces by weight, or a large tablespoonful by measure, is sufficient for the chemical analysis. The result is taken as the average analysis of the carload.

The smaller foundries who do not employ a chemist can get a good working analysis of their iron from the furnace from which it is bought. Or, in many cases, sample drillings are sent to a practicing chemist. Usual Silicon and Sulphur. The proportions of silicon and sulphur contained in the ordinary grades of pig iron are approximately as follows:

Grade or Pig

SILICON

(Percent)

Sulphur

(percent)

7 to 12

0.03

Silvery

3 to 5

0.03

No. 1 foundry

2.50 to 2.90

0.03

No. 2 foundry

1.95 to 2.40

0.04

No. 3 foundry

1.40 to 1.90

0.05

Calculation Of Mixture

When we have the analysis of our iron, we can proceed to calculate the mixture, bearing in mind that some of the silicon will be burned out of the iron during the heat. From 0.15 to 0.25 per cent is a fair estimate for this loss in cupolas ranging in size from 36 inches to 72 inches inside lining. This loss must be deducted from the final estimate.

Illustrative Examples

It is proposed to make a mixture for miscellaneous machinery castings which require about 2 per cent of silicon, and we wish to use one-half scrap and three other irons, whose silicon contents are as follows:

Grade or Iron

Silicon (per cent)

Silvery

4

No. 1 foundry

2.65

No. 2 foundry

2.22

No. 3 foundry

1.75

Scrap

2.00

The student should bear in mind that per cent means 1/100 or .01. To multiply a whole number by per cent, set the decimal point two places to the left in the percentage; thus 35 per cent of 5,000 =.35 X 5,000 = 1,750. In multiplying per cent by per cent, set decimal points in the percentages one place to the left before multiplying, and the result is expressed as per cent; thus 25% of 35% = 2.5 X 3.5=8.75 per cent.

Then we may have the following proportions of silicon, using the above irons:

(A)

(B)

(C)

(D)

No. 1

25%

X

2.65%

=0.6625%

No. 2

20%

x:

2.22%

=0.4440%

No. 3

5%

X

1.75%

=0.0875%

Scrap

50%

X

2.00%

= 1.0000%

Total silicon content

=2.194 %

Deduct for loss in heat

=0.20 %

Estimated silicon in result

= 1.994 %

Or, with No. 2 and silvery irons, we may have:

(A)

(B)

(C)

(D)

No. 2

45%

X

2.22%

=0.999 %

Silvery

5%

X

4.00%

= 0.200 %

Scrap

50%

X

2.00%

= 1.000 %

Total silicon content

=2.199 %

Deduct for loss in heat

=0.17 %

Estimated silicon in result

=2.029 %

In these examples, column (A) is the kind of iron; (B), per cent of this iron used in charge; (C), per cent of silicon in single grade of iron; (D), per cent of silicon to whole charge as supplied by each grade. One or more per cents in column (B) are usually decided upon before beginning calculations, and then the others are varied until the desired silicon content is obtained.

With this as a guide, it is a simple matter to find the actual weight for each grade, to make up any size of charge. For example, we wish to put 5,000 pounds on the bed and 3,000 pounds on other charges. Then, using the first mixture and the ratio 5:3 between the bed and the other charges, we have:

From column (B)

Bed

Other charges

No. 1 25%X5,000 =

1,250 lb.

750 1b.

No. 2 20%X5,000 =

1,000 lb.

600 1b.

No. 3 5% X 5,000 =

250 1b.

150 1b.

Scrap 50% X 5,000 =

2,500 lb.

1,500 1b.

Total iron

5,000 lb.

3,000 lb.

Fuel

Both anthracite coal and foundry coke are used in the cupola. Coal, owing to its density, carries a heavier load than coke, but it requires greater blast pressure and does not melt as fast as coke.

Foundry Coke

Coke, for foundry use, should be what is known as "72-hour" coke, as free as possible from dust and cinders. Coke is made up of a sponge-like coke structure which is almost pure fixed carbon, and an open cellular structure, which makes it especially valuable as a furnace fuel because it is so readily penetrated by the blast.

A representative analysis of a strong 72-hour coke is as follows:

Item

Proportion (per cent)

Moisture

0.49

Volatile matter

1.31

Fixed carbon

87.46

Sulphur

0.72

Ash

10.02

Cellular structure

50.04

Coke structure

49.96

Heat units per pound

12,937 B.t.u.

Specific gravity

1.89

Proportions Of Charge

The proportions of the bed fuel, the first charge of iron, and the subsequent charges of fuel and iron vary greatly with the size and design of the cupola, the grade of fuel used, and the method of charging. To determine the right amount of fuel for the bed, the most practical thing to do is to cut and try, especially with a new equipment.

For 36- to 48-inch cupolas, averaging 22 inches above the tuyeres for the melting zone, with a 10-ounce blast to start, the best way to proceed is to chalk off this distance inside the cupola before daubing up. Then, from a 1/2-inch rod of iron, bend a shape like Fig. 132. The distance a equals the distance from the mark inside the cupola to about 4 inches above the bottom of the charging door. When the coke is well lighted, before charging the iron, level off the bed according to this gage. The safe practice is to have the bed too high. If the bed is too high, it is indicated by slow but hot metal; if the bed is too low, the metal is dull. After the first heat, the height may be adjusted until proper melting is obtained; then try always to work to the same height.

The weight and character of the coke charged on the bed should be carefully noted. The first charge of metal should be in the proportions of 2 pounds of metal to 1 of fuel; all others in the ratio of 10 of metal to 1 of fuel. Intermediate charges of coke should be just sufficient to preserve the upper level of the bed. The layer is usually about 6 inches thick; its weight should be carefully taken.

The action of the furnace must be carefully watched, with the object of making it melt the iron charged as rapidly as possible and of bringing it down white hot. Also, the ratio of iron to fuel should be reduced as low as may be, without sacrificing either of these other objects.