The Finland-based company's ore reserves have risen from 9.37 million tonnes to 12.3Mt at an average grade of 0.94% lithium oxide.
The reserve at its largest deposit, Rapassaari, has grown 55% from a 2019 estimate to 8.21Mt at 0.89% Li2O.
The company is aiming to start producing battery-grade lithium hydroxide in 2024, using its own ore.
CEO Hannu Hautala said the Rapasaari increase would extend Keliber's planned mining operations by more than two years.
A 2019 study, improved in 2020, had estimated a 13-year operation based on the then-9.3Mt in reserves at several spodumene deposits, producing 15,000t of battery-grade lithium hydroxide annually from a plant to be built 50km from the mining area.
Keliber is aiming to publish an updated definitive feasibility study in early 2022.
"By continuing our exploration activities and systematic drilling programme, we will ensure Keliber's position as a lithium hydroxide producer well into the future," Hautala said yesterday.
Keliber had €18.1 million cash on hand at the end of June.
Sibanye-Stillwater announced a €30 million investment in February to take a 30% stake in Keliber, marking the South Africa-based miner's first foray into the battery minerals sector.
It followed this with a deal in July to acquire a nickel processing plant in France for €65 million and today agreed to invest US$490 million for a 50% stake in ASX-listed ioneer's flagship Rhyolite Ridge lithium-boron project in Nevada.