This study investigates the use of sustainable hydrochar-mill-scale briquettes for promoting slag foaming in the electric arc furnace (EAF) process. Two types of biochar are tested, namely, a pristine green waste hydrochar (GWH) and its pyrolyzed char (PGWH) produced at 873 K. Each briquette weighs ≈20 g and has Cfix/OFeOx molar ratios ranging from 0.07 up to 0.90. Briquettes are charged into a molten EAF slag (600 g) at 1923 K. Successful foaming is achieved for all seven briquette recipes developed. The maximum slag foaming height is 1.6–2.5 times of the initial slag height, and the slag foaming duration is in the range of 1.5–3.4 min. GWH briquettes promote rapid slag foaming through the abrupt release of volatile matter, while PGWH briquettes promote a more gradual and long-lasting foaming process through gas production (CO and CO2) from carbothermic reduction. It is estimated that 1 kg of anthracite applied for slag foaming should be replaced by 2.4 kg of GWH or 1.9 kg of PGWH added in composite briquette form. At the given briquette addition rate tested in this study (30 g per kg of slag), no appreciable impurity (sulfur, phosphorous) transfer from hydrochar to the slag is observed.