v3.26.1
Segment reporting - Consolidated income statements (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Profit (loss) [abstract]      
Sales $ 1,335,121 $ 1,643,939 $ 1,650,034
Raw materials (769,357) (855,822) (907,937)
Energy consumption for production (164,174) (171,308) 28,651
Other operating income 82,835 84,378 100,992
Staff costs (270,649) (279,864) (305,859)
Other operating expense (245,899) (265,182) (270,090)
Depreciation and amortization charges (84,951) (75,463) (73,532)
Impairment loss (gain) (17,488) (43,052) (25,290)
Other (loss) gain 1,105 555 (29)
Operating profit (133,457) 38,181 196,940
Sales to third parties      
Profit (loss) [abstract]      
Sales 1,335,121 1,643,939 1,650,034
Operating segments | North America Silicon Metal      
Profit (loss) [abstract]      
Sales 284,395 386,429 505,472
Raw materials (147,940) (196,522) (240,482)
Energy consumption for production (60,067) (71,025) (72,680)
Other operating income 9,148 10,987 6,605
Staff costs (58,049) (57,992) (67,160)
Other operating expense (33,706) (34,497) (44,304)
Depreciation and amortization charges (33,200) (25,632) (32,313)
Impairment loss (gain) (11,259) (17,962) (21,008)
Other (loss) gain (281) (892) (71)
Operating profit (50,959) (7,106) 34,059
Operating segments | North America Silicon Metal | Sales to third parties      
Profit (loss) [abstract]      
Sales 284,272 365,429 465,647
Operating segments | North America Silicon Metal | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 123 21,000 39,825
Operating segments | North America Silicon Alloys      
Profit (loss) [abstract]      
Sales 265,833 279,806 283,180
Raw materials (111,876) (134,153) (137,307)
Energy consumption for production (33,948) (29,051) (27,886)
Other operating income 701 134 3,896
Staff costs (43,747) (43,342) (37,744)
Other operating expense (32,438) (29,340) (26,840)
Depreciation and amortization charges (16,730) (17,209) (15,183)
Other (loss) gain (296) (43) (115)
Operating profit 27,499 26,802 42,001
Operating segments | North America Silicon Alloys | Sales to third parties      
Profit (loss) [abstract]      
Sales 265,248 240,352 251,936
Operating segments | North America Silicon Alloys | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 585 39,454 31,244
Operating segments | Europe Manganese      
Profit (loss) [abstract]      
Sales 363,929 367,498 277,508
Raw materials (242,845) (230,727) (226,463)
Energy consumption for production (37,372) (21,924) 42,624
Other operating income 29,934 24,902 36,628
Staff costs (33,649) (31,355) (28,326)
Other operating expense (91,745) (77,607) (69,897)
Depreciation and amortization charges (4,637) (6,550) (7,835)
Impairment loss (gain) (7,691) (2,629) (1,571)
Other (loss) gain     1
Operating profit (24,076) 21,608 22,669
Operating segments | Europe Manganese | Sales to third parties      
Profit (loss) [abstract]      
Sales 350,547 350,646 274,223
Operating segments | Europe Manganese | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 13,382 16,852 3,285
Operating segments | Europe Silicon Metals      
Profit (loss) [abstract]      
Sales 220,946 393,278 307,230
Raw materials (133,868) (210,391) (206,727)
Energy consumption for production 1,793 4,373 103,423
Other operating income 37,671 46,241 40,321
Staff costs (61,850) (71,647) (79,114)
Other operating expense (61,055) (84,209) (71,632)
Depreciation and amortization charges (13,297) (11,458) (6,325)
Other (loss) gain 8 155 79
Operating profit (9,652) 66,342 87,255
Operating segments | Europe Silicon Metals | Sales to third parties      
Profit (loss) [abstract]      
Sales 210,037 374,373 297,206
Operating segments | Europe Silicon Metals | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 10,909 18,905 10,024
Operating segments | Europe Silicon Alloys      
Profit (loss) [abstract]      
Sales 149,516 181,702 216,465
Raw materials (127,870) (147,642) (160,780)
Energy consumption for production (7,674) 7,699 52,371
Other operating income 8,048 5,967 21,149
Staff costs (27,931) (23,613) (42,069)
Other operating expense (24,139) (24,284) (44,132)
Depreciation and amortization charges (5,672) (3,692) (3,005)
Impairment loss (gain) 1,834 (3,646) (3,619)
Other (loss) gain 12 68 47
Operating profit (33,876) (7,441) 36,427
Operating segments | Europe Silicon Alloys | Sales to third parties      
Profit (loss) [abstract]      
Sales 144,965 172,148 209,627
Operating segments | Europe Silicon Alloys | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 4,551 9,554 6,838
Operating segments | South Africa Silicon Metals      
Profit (loss) [abstract]      
Sales 7,844 67,944 50,071
Raw materials (11,919) (37,452) (16,401)
Energy consumption for production (1,441) (22,786) (27,602)
Other operating income 13 65 594
Staff costs (1,938) (6,270) (5,007)
Other operating expense (1,821) (7,502) (11,223)
Depreciation and amortization charges (54) (2,863) (1,840)
Impairment loss (gain) (372) (12,953) 478
Other (loss) gain 162    
Operating profit (9,526) (21,817) (10,930)
Operating segments | South Africa Silicon Metals | Sales to third parties      
Profit (loss) [abstract]      
Sales 5,417 51,856 45,320
Operating segments | South Africa Silicon Metals | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 2,427 16,088 4,751
Operating segments | South Africa Silicon Alloys      
Profit (loss) [abstract]      
Sales 79,524 91,021 109,684
Raw materials (45,512) (41,599) (45,491)
Energy consumption for production (22,759) (34,820) (36,710)
Other operating income 98 83 (142)
Staff costs (11,213) (9,978) (10,412)
Other operating expense (3,470) (5,394) (10,718)
Depreciation and amortization charges (4,995) (4,622) (4,056)
Other (loss) gain 1,401 (6)  
Operating profit (6,926) (5,315) 2,155
Operating segments | South Africa Silicon Alloys | Sales to third parties      
Profit (loss) [abstract]      
Sales 62,576 71,581 68,813
Operating segments | South Africa Silicon Alloys | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 16,948 19,440 40,871
Operating segments | Other segments      
Profit (loss) [abstract]      
Sales 30,148 43,001 54,921
Raw materials (14,601) (25,133) (30,391)
Energy consumption for production (2,706) (3,774) (4,889)
Other operating income 41,711 44,978 50,655
Staff costs (32,272) (35,667) (36,027)
Other operating expense (41,954) (50,271) (48,309)
Depreciation and amortization charges (6,366) (3,437) (2,975)
Impairment loss (gain)   (5,862) 430
Other (loss) gain 99 1,273 30
Operating profit (25,941) (34,892) (16,555)
Operating segments | Other segments | Sales to third parties      
Profit (loss) [abstract]      
Sales 12,059 17,554 37,262
Operating segments | Other segments | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales 18,089 25,447 17,659
Adjustments/Eliminations      
Profit (loss) [abstract]      
Sales (67,014) (166,740) (154,497)
Raw materials 67,074 167,797 156,105
Other operating income (44,489) (48,979) (58,714)
Other Operating Expense Adjustment 44,429 47,922 56,965
Operating profit     (141)
Adjustments/Eliminations | Intercompany Sales [member]      
Profit (loss) [abstract]      
Sales $ (67,014) $ (166,740) $ (154,497)