What changed in CENTURY ALUMINUM CO's 10-K — 2023 vs 2024

Item 1. Business . Facility Ownership Hawesville 100% Owned Sebree 100% Owned Mt.

Holly 100% Owned Grundartangi Facility 100% owned; long-term ground lease Vlissingen Facility 100% owned; long-term ground lease Jamalco 55% Joint venture interest; long-term ground lease Chicago Corporate Office Long-term office lease Bauxite Mining Properties Century has access to large bauxite deposit areas with mining rights that extend in many cases more than 12 years from the date of this Form 10-K.

The Company obtains bauxite from its own resources in Jamaica. Tonnes of bauxite are reported on a zero-moisture basis in millions of dry metric tonnes (mdmt) unless otherwise stated.

As used in this Form 10-K, the terms “mineral resource,” “measured mineral resource,” “indicated mineral resource,” “inferred mineral resource” and “mineral reserve” are defined and used in accordance with subpart 1300 of Regulation S-K.

Under subpart 1300 of Regulation S-K, mineral resources may not be classified as “mineral reserves” unless the determination has been made by a qualified person (as defined under subpart 1300 of Regulation S-K) that the mineral resources can be the basis of an economically viable project.

The approach to mining at Jamalco is such that in the qualified person's opinion, the documented mineral reserves are too small to merit reporting and as such, mineral reserves are not included in the discussion below. Refer to “Property History and Condition” below for more information on why reportable reserves are not available.

Part or all of the mineral deposits (including any mineral resources) in these categories may never be converted into mineral reserves. Further, except for the portion of mineral resources reclassified as mineral reserves, mineral resources do not have demonstrated economic value.

Estimates of inferred mineral resources have too high of a degree of uncertainty as to their existence and may not be converted to a mineral reserve.

Therefore, it should not be assumed that all or any part of an inferred mineral resource exists, that it can be the basis of an economically viable project, or that it will ever be upgraded to a higher category.

Likewise, it should not be assumed that all or any part of measured or indicated mineral resources will ever be converted to mineral reserves. Management relies on estimates of our recoverable mineral reserves, which estimation is complex due to geological characteristics of the properties and the number of assumptions made and variable factors, some of which are beyond our control.

The information that follows is derived from the technical report summary relating to the property prepared in compliance with Item 601(b)(96) and subpart 1300 of Regulation S-K by Aluminium Industry Professionals Inc. (“Aluminpro”), which we engaged as a qualified person as defined under subpart 1300 of Regulation S-K.

No employee of Aluminpro is an employee of the Company, and Aluminpro is not affiliated with the Company or with any other entity that has an ownership, royalty, or other interest in Jamalco or the mining properties. The scientific and technical information concerning our mineral resources in this Form 10-K have been reviewed and approved by Aluminpro.

Portions of the following information are based on assumptions, qualifications, and procedures that are not fully described herein.

Reference should be made to the full text of the Technical Report Summary dated February 27, 2025, with an effective date of December 31, 2024, filed as Exhibit 96.1 to this Form 10-K (the “Jamalco TRS”). 26 Property Location and Description Jamalco holds two Special Mining Leases, Special Mining Lease 130 ("SML 130") and Special Mining Lease 169 ("SML 169") in the southwestern region of Jamaica within the Parishes of Clarendon and Manchester and one Special Exclusive Prospecting License 580 ("SEPL 580") is in the St Catherine Parish in the southeastern region of Jamaica.

On October 7, 1991, under SML 130, the Government of Jamaica granted Jamalco the rights to explore and mine the bauxite in the leased area for a period of 40 years, expiring in 2031. The center of SML 130 is located at 18°02’N and 77°25’W and covers 137.3mi 2 .

The lease guarantees that no other company will be allowed to mine for bauxite in the leased area. Jamalco may surrender lands to the Government of Jamaica that it has mined out and restored or lands it no longer wishes to retain free of charge.

Conversely, the Government of Jamaica may rescind lands under the mining lease that it deems necessary in the national interest of Jamaica: such lands will not be made available to other bauxite users.

On January 1, 2003, the Government of Jamaica granted Jamalco under SML 169 the rights to explore and mine the bauxite in the lease area for a period of 40 years, expiring in 2043. The center of SML 169 is at 18°10’24’’N and 77°33’15’’W and covers 46.0mi 2 .

The conditions of the lease are the same as the conditions for SML 130. On June 7, 2016, the Government of Jamaica granted Jamalco under SEPL 580 the right to explore exclusively for bauxite in the license area. The license is renewable annually and currently valid until June 2025.

The center of SEPL 580 is at approximately 18°04’N and 77°25’W and covers 59.7mi 2 . Jamalco’s refinery is located at 17°54’N and 77°14’30’’W and the dedicated port is located at Rocky Point on Jamaica’s south coast at 17°49’N and 77°10’W approximately 8.7mi to the southeast of the refinery.

Mining by Jamalco is subject to the Mining Act of 1947 ("the Mining Act") and its subsequent amendments and the lease agreements. It provides the holder of a mining lease with full access to land granted with the exclusive right to explore and mine bauxite.

Many lands are held by private owners and, prior to prospecting or mining, Jamalco is required to give notice to the owner and provide compensation. 27 Refer to the Jamalco TRS in Sections 2 through 5 for more information on the Jamalco property location, history and topography.

Infrastructure Jamalco has three office locations, a refinery, a rope conveyor and a railway. Jamalco’s mining and refinery operations may be reached by a toll road from the center of Kingston, approximately a one hour drive west of Kingston, the administrative capital and main business center of the island.

Jamaica has multiple daily air and sea connections to the United States, Canada, and United Kingdom. The bauxite deposits are distributed throughout the region in communities of local residents, served by an extensive road system and infrastructure providing services to these communities. The deposits are connected to the central stockpile at St Jago by internal private haul roads.

Access to the deposits is also available through public roads, but these roads are generally not used for transporting bauxite. Jamalco haul roads occasionally cross public roads. The crossings are controlled by contractors directly employed by Jamalco.

Many deposits occur on the higher Manchester plateau which is connected to the stockpile at St Jago via an aerial ropeway cable conveyor over the face of a steep, rugged limestone escarpment.

The conveyor is 3.4 km in length with a capacity up to 1,000 tonnes of bauxite per hour from the load station at Mount Oliphant, at an elevation 1750 ft or 533m, to the rail head at St Jago, elevation 150 ft or 46 m.

From St Jago, the bauxite is transported to the alumina refinery by a dedicated 18.0 km rail system, controlled and operated by Jamalco 24 hours per day, 6 days per week and approximately 9 rail trips are made each day transporting bauxite.

At the refinery the bauxite is processed through the Bayer Process resulting in red mud residue which is disposed of within the residue lakes near the refinery. Alumina is transported from the refinery by the rail line to the Rocky Point Port on the southern coast, again a distance of 18.0 km.

Approximately 900 employees work in the refinery and related facilities such as the laboratory and port. Jamalco has employed three mining contractors to carry out the following tasks: road development, pit preparation, mining and stockpiling, train loading and rehabilitation of mined pits. The combined workforce for operations and maintenance is 328 contractor employees.

Refer to the Jamalco TRS in Sections 14 and 15 for more information on infrastructure. Property History and Condition Studies and exploration for bauxite first began in Jamaica in 1944, with bauxite mining beginning in 1952.

Alcoa was first granted mining concessions for the property in Clarendon, creating a mining joint venture in 1959 and beginning bauxite mining and exporting in 1963. The Company also built an alumina refinery in Clarendon that commenced operations in 1972 at an annual capacity of 0.5mdmt drawing on bauxites from the Mocho Mountain region upper Clarendon.

Jamalco was formed in 1988 when the Government of Jamaica acquired a 50% interest in Alcoa’s mining and refining operations, with Alcoa remaining the managing partner. In December 2014, Alcoa sold its stake in Jamalco to the Noble Group. In May 2023 the Company purchased the interest from the Noble Group.

General Alumina Jamaica Limited is the managing partner of the Jamalco joint venture. Jamalco's bauxite consists of many deposits that occur as infilling within depressions on an eroded limestone surface. Referred to as a karst topography, these depressions result from the solution of the limestone over time.

The shape of a typical deposit crudely resembles an inverted, flattened cone whose surface may cover many hectares. Exploration for bauxite continues to be conducted on a regular basis to maintain sufficient mineral resources and reserves to meet refinery supply.

At the start of mining, most identified deposits were on public land where the Company had ready access for drilling and mining within the constraints of restrictions imposed by the Mining Act. Currently, most of the remaining bauxite deposits are on private land parcels, which may have multiple owners.

This situation of multiple ownership of deposits, typical of Jamaica, calls for a unique approach to detailed exploration, mine planning and bauxite extraction which has resulted in a lack of reportable mineral reserves.

Specifically, Jamalco has a policy that once a consolidated land position is attained, then production in-fill drilling and mining is immediately initiated in order to minimize the outlay of capital. As such, it is Aluminpro’s opinion that the reserves are too small to merit reporting.

This practice has been the basis for mine planning, extraction, blending and processing of bauxites in this area over the last 50 years and has supported a technically and 28 economically viable operation. Despite our operations, under subpart 1300 of Regulation S-K, we are considered an “exploration stage” company because we do not have mineral reserves to disclose.

The Jamalco facilities, including equipment, are in a maintained condition. Net book value of these facilities and equipment as of December 31, 2024 of $190.7 million is included in Property, plant and equipment, net on the Consolidated Balance Sheets. Jamalco has prepared capital projects to increase the proportion SML 169 bauxite to 55 % of the refinery feed.

An overall capital investment of $70.5 million has been estimated to cover mine development and installation of the necessary equipment from 2025 to 2027. Refer to the Jamalco TRS in Section 5 for more information on the property history and condition. Mining Method and Processing Operations Jamalco’s resources are based on exploration done by Jamaican Bauxite Institute (“JBI”).

For an accessible deposit and planning production drilling, ArcGIS software is used to produce a background image showing the created grid, JBI grid and JBI drill holes, which is then uploaded to GPS receivers. This is utilized in the field by the drillers to peg production drillholes. Additionally, a 3-D outline is generated within the orebody outline.

This process is also completed in AutoCAD, where both exploration and production drillhole coordinates are used to generate a surface from which the 3-D outline is produced. Once the production drilling is completed with assays, AutoCAD plans are prepared with grades and thicknesses that are passed on to the mine operators. Production drilling is essentially quality control in-fill drilling.

Parcel maps are created for the production crew, showing color-coded grade information which enables selection of mining for stockpiling. Once mined, Jamalco utilizes the Bayer Process to extract alumina from bauxite. The processing plant is a fixed plan for ore crushing and washing.

Refer to the Jamalco TRS in Sections 12 and 14 for more information on the mining method and processing operations. Bauxite Mineral Resources The tables shown below of resources were prepared using the results of the procedures performed by Aluminpro. As stated above, Jamalco has no reportable mineral reserves.

Summary of Attributable Bauxite Mineral Resources Exclusive of Mineral Reserves at December 31, 2024: (tonnes shown in millions) Tonnes (1) AvAl 2 O 3 (2) (%) ReSiO 2 (3) (%) P 2 O 5 (4) (%) Measured mineral resources — — % — % — % Indicated mineral resources 27.0 38.22 % 3.03 % 1.06 % Measured + Indicated mineral resources 27.0 38.22 % 3.03 % 1.06 % Inferred mineral resources — — % — % — % (1) Represents the Company's 55% interest in the above-quoted mineral resources.

The reference point for the mineral resource is the in situ predicted dry tonnage. (2) Available Alumina at Low Temperature (3) Reactive Silica (4) Phosphorous Cut-offs are based on a 35% AvAl 2 O 3 grade for the overall pit.

The exception is Porus Victoria Township location where the value of this bauxite for blending allows for a cut-off of 30% AvAl 2 O 3 to be applied. ReSiO 2 grades should not exceed 6% on a pit basis; where high silica zones are encountered in mining such bauxites are flagged and are to be avoided in the extraction process.

P 2 O 5 and goethite-hematite ratios are monitored for blending purposes, however no specific cut-offs are applied to constrain the resources. No call factors are applied to the above resource tonnages and grades. Metallurgical recovery is approximately 84% but has not been applied to these tonnages.

The bauxite transfer price to the refinery is $12.46 per tonne and tonnage is reported on a bone-dry basis. All resource estimation is based on an in-situ density of 1.44 tonnes/m 3 . 29 The Company does not have mineral resource information at December 31, 2023 or December 31, 2022 for comparative purposes.

Refer to the Jamalco TRS in Section 11 for more information on the mineral resource estimates, including key assumptions used. Exploration Activity Exploration at the property is completed using auger drilling. Deposits are identified by the slight topographic depressions across the limestone relief suggesting the potential for karst hosted bauxite mineralization.

Aerial survey assists in the selection of potential deposits and a few prospecting auger holes allow for confirming the exploration target. At the start of operations, much of the area was owned by the government or by Jamalco and the local communities were less sensitive to bauxite development.

Currently, the ownership of land parcels requires Jamalco to obtain sufficient rights before exploration is completed. Recent exploration has not been material to operations.

The following table shows a summary of the deposits, drill holes, and assays completed at the property as of September 30, 2024: Deposits Holes Meters Sampled Analyses SML 130 152 4,542 24,244 15,890 SML 169 172 9,727 57,095 37,464 Internal Controls We have internal controls and procedures designed for quality assurance and quality control on the Company’s production activities and associated information for the estimation of mineral resources and reserves.

These internal controls include surveying of drillhole collar location, drill sampling, collection and security, database verification and security and quality assurance and quality control programs. Internal controls used by the Company are informed by internal reviews and by reviews, audits, and studies performed by third-party mining consultants.

We recognize the risks inherent in exploration, such as the geological complexity, interpretation and extrapolation of data, changes in operations, ongoing mine planning, or permitting requirements, macroeconomic conditions and new data, among others. See Item 1A. Risk Factors of this Form 10-K for more information on risks. Item 3.

Legal Proceedings We are a party from time to time in various legal actions arising in the normal course of business, the outcomes of which, in the opinion of management, neither individually nor in the aggregate are likely to result in a material adverse effect on our financial position, operating results and cash flows.

For information regarding material legal proceedings pending against us at December 31, 2024, refer to Note 17. Commitments and Contingencies to the consolidated financial statements included herein. Item 4. Mine Safety Disclosures Not applicable. 30 PART II