middling

Japanese: middling

…Particles containing two or more minerals are called single-edged. In addition, intermediate products in ore dressing and coal dressing that are intermediate in quality between concentrate and waste rock are also called single-edged, middling, or middle ore. Single-edged particles are gradually broken down into individual mineral particles by crushing. …

From [Coal Preparation]

… The high-quality coal obtained as a product of coal washing is called clean coal. The intermediate-quality product is called middlings. The term low-grade coal is also used to refer to low-quality coal. …

*Some terminology explanations that mention "middling" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…このように2種以上の鉱物が共生した粒子のことを片刃と呼ぶ。また，選鉱･選炭における中間産物で，その品位が精鉱と廃石の中間にあるものも片刃，あるいはミドリングmiddling，中鉱などと呼ばれる。片刃粒子は粉砕によってしだいに単体の鉱物粒に分かれる。…