By Wang Peilin

Chromium salt is an important inorganic chemical product, which is widely used in electroplating, tanning, paint, pigment, medicine, national defense and military sectors. However, the production of chromium salt, especially that of basic products (i.e. sodium dichromate, also known as sodium bichromate), is typically at the cost of high energy consumption, high material consumption and heavy pollution.

Technological innovation brings in new hope

   Limestone and dolomite need to be added to the mixture of chromite and soda ash during the roasting process with calcium compounds, which usually causes heavy pollution to the environment and waste of a big amount of resources. Accordingly, fewer and fewer calcium compounds were used as additives in the process. Finally, a new process – the calcium-free roasting process was developed and first put into commercial production in Germany in 1960s. Since then, the calcium-free roasting process has been vigorously growing in developed countries, making it possible for producers to centralize production and for regulators to set up environmental protection rules across the whole chromium salt industry.

   China's chromium salt production was still 100% reliant on calcium or less calcium roasting process till the early 1990s. Later, a liquid phase oxidation production technology with various processes was developed in the 21st century, turning out to be a good alternative to the roasting process that commonly results in heavy pollution at high temperature of above 1 200℃, and thus it has been put on the list of clean production processes nationwide.

   At present, there are six producers using roasting process, and the approved capacity is 238 000 tons, of which five adopt calcium-free roasting with the approved capacity at 218 000 tons combined and the output at 318 000 tons; the rest one adopts less-calcium roasting process, and the approved capacity is 20 000 tons. Table 1 gives a comparison of the main technical indicators of roasting process-based (with calcium, without calcium) and liquid phase oxidation-based sodium dichromate.

   Table 1   Key technical indicators of Chinese sodium dichromate production 

Treatment of three wastes paves way for healthy development

   The three wastes (refer to waste gas, waste water and waste residue) are the barriers to the development of chromium salt. Especially the untreated chromium slag poses heavier pollution to the environment. In the 1990s, some enterprises obtained economic benefits at the expense of the environment. The production & operation environment was harsh: chromium-containing sewage was usually emitted; the chrome-containing roasting tail gas was handled, but more often did not meet the discharge standards, resulting in heavy pollution; waste residue, because of a big amount of chromium residue in it (with 2.5 ~ 3.0 tons/ton of products based on calcium roasting process), had a high content of Cr6+, and was hard to be well treated. In the early stage, the roasting process was adopted to reduce Cr6+ in chromium residue to Cr3+, and then it was piled up instead of being comprehensively used.

   In order to solve the pollution problem generated during the chromium salt production and protect the environment, the relevant government departments have issued a number of laws, regulations, industrial policies and standards applying to the chromium salt industry, and have raised the access to the chromium salt industry continuously, in other words, producers that are not up to the standards or compliance are constantly being eliminated. In the context, the chromium salt industry is moving toward stable development.

   In 2004, under the direction of the State Environmental Protection Administration, six chromium residue disposal technologies were selected: using chromium slag as cement mineralizer; replacing dolomite with chromium slag for sintering ironmaking; detoxification of chromium slag in whirlwind furnace; using chromium slag as green glass coloring agent; dry detoxification of chromium slag; and improved chromium residue wet detoxification. Among them, the first and the fifth technologies have been widely used. The effective use of waste residue is regarded as a step for producers to reduce consumption of materials and energy, trim production cost, and improve the economic benefits.

   Cumulatively, 70 companies in China have been engaged in the production of chromium salt since 1958. By the end of 2005, their combined chromium salt output reached 2 million tons, generating more than 6 million tons of chromium slag, only about 2 million tons of which were handled and the remaining 4 million tons of chromium slag were stacked without any hazard-free disposal. Around 2010, however, all the stored chromium slag was disposed in a harmless manner and then was put in comprehensive utilization.

   In 2018, there were seven domestic sodium dichromate producers, four of which had a capacity of 50 000 t/a and above each, and the rest three had a capacity of 20 000-30 000 t/a. The combined output was 340 000 tons. The domestic sodium dichromate market has almost achieved the targets: clean, large-scale, centralized, automated, and environmentally-friendly production. However, a new round of intense competition and revolution in the industry is underway.