 Corresponding author: Mahdi Haroun Copyright © 2023 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Reducing chromium discharge in tanning: The salt-free chrome tanning process Mahdi Haroun * and Manal Mohammed Ahmed Department of Industries, College of Applied and Industrial Sciences, University of Bahri, P.O. Box13104-Khartoum, Sudan. GSC Advanced Research and Reviews, 2023, 15(01), 015–020 Publication history: Received on 20 February 2023; revised on 31 March 2023; accepted on 03 April 2023 Article DOI: https://doi.org/10.30574/gscarr.2023.15.1.0100 Abstract Nowadays, there is increasing concern about environmental protection in global industrial activities. The sustainable leather industry is currently focusing on cleaner processing methods to promote a greener environment. However, the discharge of waste streams containing several pollutants poses a significant social threat to the leather sector. The conventional pickling and chrome tanning process is particularly problematic due to the enormous amounts of chlorides and chromium emissions, which have been a longstanding issue that has not been satisfactorily resolved. In order to minimize the emissions of chromium and chloride integrately, a salt-free and high exhaustion chromium tanning method was designed and optimized. The study's results demonstrated that the chrome tanning process could be improved by conducting it in a salt-free environment, resulting in a smooth process and a chromium absorption rate increase to 99% compared to 75% in the conventional process. Moreover, the dosage of Chrome required was minimized from 1.5% to 0.8%, resulting in a significant reduction of residual Chromium concentration in the spent liquor from above 1150 mg/L to 40 mg/L. The novel chrome tanning process significantly decreases the amount of chromium-containing sludge generated and facilitates the disposal of spent liquor with a reduced amount of chrome and chloride. The novel process effectively addresses long-standing problems and meets the requirements of the modern sustainable leather industry. Keywords: Leather manufacture; Pickling free-salt; Chrome tanning; Aromatic sulphonic acid 1. Introduction Tanning turns animal skins into leather, making it more durable and resistant to decomposition. Despite the development of other tanning methods, chrome tanning remains the most important and dominant method in the leather industry. This is because chrome-tanned leather offers superior quality, stability, and versatility that cannot be matched by other tanning methods [1]. The standard chrome tanning process has three stages: pickling, tanning, and basification. During pickling, pelts are treated with a salt solution containing sulfuric and formic acid in an amount of 1.0-1.8% on split limed pelt weight. The pickle float should contain at least 6% sodium chloride to protect the collagen from acid swelling, which would negatively affect the leather's properties. The pH of the pickle float is controlled at 2.5- 3.0 to modify the reactivity of the carboxyl groups and allow effective penetration of chromium into the pelts' inner layers, while preventing excessive chromium combination on the surface that causes coarse grain. Chrome tanning is initiated during the pickling phase [2]. Typically, 6-8% of basic chromium sulfate (chrome powder) is used in the chrome tanning process, which contains about 25% Cr2O3 of 33% basicity. This corresponds to 1.5-2.0% of Cr2O3 on the limed pelt weight to ensure minimal chromium levels in the leather and produce high-quality products with excellent properties, including hydrothermal stability, mechanical strength, and organoleptic properties [3]. After the chrome has completely penetrated the pelts, basification occurs to neutralize acids and facilitate the combination of chrome with the collagen. Basification agents such as sodium hydrogen carbonate and magnesia are commonly used to achieve a final pH of 3.8-4.2 in the float, which promotes the combination of chromium to the carboxyl groups and adequate crosslinking of chromium between collagen fibers [4].