By-Product

When you make cheese from milk, a lot of clear liquid is left beside the curds. This is called whey. For every block of cheese you lift out, far more whey flows out with it.

For a long time, there was hardly anyone to sell this liquid to. Many places poured it into ditches or fed it to livestock. Then people learned it contains protein, and the technology to collect it took hold. Today it is dried into powder and sold as a proper product.

This is where it splits from waste. If it sells, it is a by-product. If it doesn't sell and only costs money to dispose of, it is waste. The same whey crossed from one side to the other.

Mines work the same way. When copper is refined with electricity, the sludge that settles at the bottom holds gold and silver. The mine wasn't dug to get those metals, yet they get collected too.

We don't decide how much by-product there is. Splitting an ore to get rare earths is a good example. From this ore, the rare earths inside come out together, in fixed ratios.

So if you need more neodymium, you can't make only more neodymium. The ore called bastnasite is one such case. In a 2015 paper, the figures for the days when the Mountain Pass mine in the United States ran to get europium show that getting 1 ton of europium oxide meant mining ore that held 450 tons of cerium oxide and 300 tons of lanthanum oxide. The ratios change depending on the ore body. To get the element everyone wants, you end up with mountains of elements that hardly anyone is buying.

This industry even has a name for the problem: the balance problem. The cost of storing and handling the pile-up ends up added to the price of the element that does sell.

The low price is also a different story from economies of scale. Economies of scale mean the cost per unit falls the more you make. When the other elements pouring out while you chase a high-demand rare earth are cheap, it is because more comes out than buyers want. Not every by-product is cheap. How much it is worth is set by those who want it.

Not everything that comes out together is a by-product. "By-product" is the word used when you can tell which one is the main product. "Joint product" is the word used when the things that come out together are worth about the same, so it is hard to call one main and the other secondary.

When crude oil is refined, gasoline, diesel, and kerosene come out together, and it is hard to call any one of them a sideshow. These are called joint products. By contrast, at a cheese plant it is clear that cheese is the main product and the whey is not.

So this name follows how much something is worth, not what it is made of. When the value changes, the name changes too.

That has really happened. Before electric lights spread, there was a mesh mantle that was placed over a gaslight flame to make it glow brightly. The product that sold back then was thorium, and rare earths were a sideshow attached to it. Today it is the other way around: rare earths are the product, and thorium is a burden that is hard to handle. Yesterday's by-product can become today's main product.