Hancock bought 500,000 shares in Vulcan in June for A$200,000, but sold part of his stake in early September.
Since mid-September, he has been solidly buying, culminating in his acquisition of 2.5 million shares for $2.75 million on Friday.
It gives Hancock a 5.2% stake in Vulcan.
In October, Hancock debuted on the 2020 Financial Review Rich list at number 40 with a net worth of $2.05 billion.
His mother topped the list with a net worth of $28.9 billion.
On Friday, Vulcan released a prefeasibility study for its Zero Carbon Lithium project in Germany, outlining annual production of about 40,000 tonnes of lithium hydroxide, as well as 74 megawatts of renewable energy.
The project has a post-tax net present value of €2.25 billion and capital costs of €1.74 billion.
Vulcan shares have risen from just A28c in January last year to Friday's closing price of $6.44, valuing the company at $508.3 million.