"Brown’s Range is expected to produce the equivalent to 148.2...

"Brown’s Range is expected to produce the equivalent to 148.2 tonnes of Dysprosium in the first three years of operation in a total of 1,719 tonnes of rare earth oxide."

The keyword here is EQUIVALENT. If A is equivalent to B then under a certain point of view A can be replaced with B without changing the outcome. In our case I have no info about how many kgs of 52% mixed concentrate have the same value of as one Kg of Dysprosium oxide of a certain purity. What I know, on the basis of what they have told us, is that whatever that ratio is the total production for the first 3 years is expected to be equivalent to 148.2 tonnes of Dysprosium.

An example.

"The barrel of oil equivalent (BOE) is a unit of energy based on the approximate energy released by burning one barrel (42 U.S. gallons or 158.9873 litres) of crude oil. ... Typically 5,800 cubic feet of natural gas or 58 CCF are equivalent to one BOE."

In the above statement the point of view under which gas is equivalent to oil is the amount of energy released with the ratio being 5,800 cubic feet of gas per barrel of oil.

Cheers.