Of the remainder of this 80 tons of coal, it will be exchanged with the producers of all the other consumption goods that the miners and mine owner require. In this context it is quite immaterial whether the producers of these consumption goods exchange their commodities for coal, to be used for their own consumption, or to be used as constant capital, in their enterprises.
If 20 tons of coal, out of the 60 tons available for circulation, is used by the miners and mine owners, for their own consumption – say the equivalent of £10 wages and £10 profit, if the rate of surplus value is 100% - then 40 tons = £40 (£20 wages and £20 profit) is still available to exchange for other consumption goods.
Suppose these consumption goods are represented by cloth. The miners exchange 20 tons of coal - £20 – for 20 metres of cloth, and the mine owners do the same. It makes no difference whether the miners and mine owners obtain this cloth by exchanging coal with a cloth manufacturer directly, who needs this coal to run their business, and fuel steam engines, or whether some of it is used as fuel for the fires of his workers, or even his domestic servants.
What constituted revenue for the miners and mine owner has been exchanged for revenue in the hands of the cloth manufacturer.
“But for the coal producer the cloth is an article of consumption, and both cloth and coal represent for him revenue; the coal, revenue in its non-realised form; the cloth, revenue in its realised form.)” (p 190)
The coal producer may only have used 10 tons of their output for their own use (5 as wages, 5 as profit) and then exchanged another 10 tons with the producers of steel, wood, and machines, leaving an additional 10 to exchange with the cloth manufacturer, but this is only possible if they then exchange the additional 10 metres of cloth they obtain, to get additional steel, wood, or machines to meet their needs for constant capital.
In reality, of course, what this means is that they consume £10 of coal themselves, they sell £10 to the steel, wood and machine makers, plus £10 to the cloth maker, and they then spend £30 with the steel, wood, and machine producers.
“The whole quantity of coal, iron, timber and machinery which are reciprocally replaced in this way by the exchange of constant capital for constant capital, of constant capital in one natural form for constant capital in another natural form, has absolutely nothing to do either with the exchange of revenue for constant capital or with the exchange of revenue for revenue. It plays exactly the same role as seed in agriculture or the capital stock of cattle in cattle-rearing. It is a part of the yearly product of labour, but it is not a part of the product of the year’s [newly- added] labour (on the contrary it is a part of the product of the year’s labour plus the pre-existing labour), which (conditions of production remaining the same) replaces itself annually as means of production, as constant capital, without entering into any circulation other than that between dealers and dealers and without affecting the value of the part of the product which enters into the circulation between dealers and consumers.” (p 190-1)