How is sugar made white?

Sugar does not come out of a cane field white. The juice is brown, cloudy and full of impurities.

Lime is what clears it.

Milk of lime is stirred into the juice, and carbon dioxide is bubbled through. The lime and the gas form fine chalk particles. As those particles sink, they carry the impurities down with them. What is left behind is clean juice, ready to be boiled into sugar.

Every sugar mill in India works this way.

Worth knowing
Almost none of the lime stays in the sugar. It is added to collect the impurities, then filtered out again. Lime is used across industry like this, as the material that carries unwanted things away rather than as an ingredient in the finished product.

Source: Indian Institute of Sugarcane Research (ICAR).