The migration and transformation of chromium during co-processing 1 of cement raw meal mixed with chrome-polluted soil

: To efficiently dispose of chrome-polluted soil, we tested the co-processing of raw meal 11 mixed with chrome-polluted soil in a tube furnace (laboratory experiments) and a cement rotary kiln 12 (field-scale experiments). The migration and transformation reactions of chromium were analyzed 13 and the environmental risk was evaluated. The average mass balance value was 91% for the 14 laboratory experiments. In field-scale experiments, the mass balance values were 110% for the 15 control experiments and 84% when 1% soil was treated. Therefore, only a small amount of Cr was 16 volatilized into the flue gas. The average total Cr concentration in the soil samples was 403.25 17 mg/kg, and the ratio of Cr(VI) to total Cr was 1.83% or less. On average, 45.15% of Cr(III) was 18 oxidized to Cr(VI) in laboratory experiments, while 87.94% of Cr(III) was oxidized in field-scale 19 experiments, and the difference could be a result of the different calcination conditions. The 20 materials in the cement rotary kiln make full contact with oxygen, and in this high temperature and 21 oxidizing atmosphere, abundant CaO and MgO promote the oxidation of Cr(III) to CaCrO 4 . SiO 2 , 22 Al 2 O 3 , and Fe 2 O 3 reduce CaCrO 4 , which inhibits Cr(III) oxidation. The Cr concentration in the 23 cement products was well below the Chinese standard limits. Therefore, the treatment of 1% 24 chrome-polluted soil with a cement rotary kiln is experimentally safe.


Introduction 28
Chromium is widely used in steel, leather, and other industries, and the resulting chromium 29 residue can cause serious pollution. In China, the annual production of chromium exceeds 160,000 30 tonnes, the cumulative amount of chromium residue is nearly six million tonnes, and the amount of 31 chrome-polluted soil exceeds 20 million tonnes (Wang et al. 2011). Cement kilns have some 32 advantages for the co-processing of solid waste, and can increase the capacity for solid waste 33 disposal and reduce the consumption of raw materials and fuel by the cement industry(Aranda Usón 34 et al. 2013; Kosajan et al. 2020). However, the migration and transformation of Cr and other heavy 35 metals during co-processing can threaten environmental and human health. Most chromium residue 36 is in the trivalent (Cr(III)) and the highly toxic hexavalent (Cr(VI)) forms. More importantly, some 37 Cr(III) will be oxidized to Cr(VI) during co-processing in a cement kiln (Fu et al. 2021; Gong et al. 38 2020; Li et al. 2018a). Therefore, the study of the transfer and transformations of Cr during co-39 processing of waste is of great significance. 40 Alkali metal and alkali earth metal oxides affect the redox state of chromium. In cement raw 41 meal, coal, and chrome-polluted soil, chromium exists mainly in the forms CaCr 2 O 4 , Cr(OH) 3 , 42 2 Cr 2 O 3 , Cr 2 (SO 4 ) 3 , CrO 3 , and FeCr 2 O 4 . (Jiang et al. 2016) The components in cement raw meal 43 include CaO, SiO 2 , Al 2 O 3 , Fe 2 O 3 , and MgO; CaO makes up more than 60% of the raw meal (Li 44 2020). 45 Together, CaO and oxygen will oxidize Cr(III) to Cr(VI), while only oxygen, at even 1500 °C, 46 will not oxidize Cr(III). Lack of oxygen can inhibit the oxidation of Cr(III) to Cr(VI) ( The raw meal was mixed with 1%, 3%, or 5% chrome-polluted soil. The mixtures were 101 thoroughly homogenized, and 20-30 g was placed in a quartz crucible. The crucibles were preheated 102 on the top of a tube furnace, and then were placed in the furnace for 30 min. Two crucibles were put 103 into the furnace for each run, and calcination experiments were carried out twice for each mixing 104 ratio. The experiments were carried out in an atmosphere of O 2 (5%) and N 2 (95%) with the flow 105 rate of 1 L/min, and the temperature was set at 1450 °C. The mass of the samples was accurately 106 weighed before and after the experiments. 107 108

Field-scale experiments 109 2.3.1 Co-processing chrome-polluted soil in a cement rotary kiln 110
The field-scale experiments were carried out in a cement rotary kiln. The mixing of raw meal 111 and chrome-polluted soil was conducted at various mass ratios indicated in Table 2

Cement kiln system 116
A schematic of the cement kiln calcining system is illustrated in Fig.1. The chrome-polluted 117 soil was added to the raw mill, which homogenized the soil and raw meal.  Table 3 shows the distribution of Cr in the seven chrome-polluted soils. All samples were tested 127 in parallel (a and b Calcination experiments of raw meal and chrome-polluted soil were performed in a tube 137 furnace. The raw materials included cement raw meal and soil, and the combustion products 138 included clinker and flue gas. The mass and the total Cr concentration of these substances are listed 139 in Tables 4 and 5, respectively. The total Cr concentration in clinker was 1.52-4.67 times of that in 140 raw meal, and increased with the mixing ratio of soil. The total Cr concentration in clinker increased 141 under the influence of weight loss. The rate of weight loss in the calcination experiments ranged 142 from 35.02% to 36.27%, and was caused by the evaporation of crystalline water, the release of CO 2 143 and SO 2 , and the loss of organic impurities. 144 145 Table 4 The mass of raw materials and products (g) Where M rm , M s , and M cl represent the mass of raw meal, soil, and clinker, respectively. C rm , C s , 152 and C cl represent the total Cr concentration in each of these three materials. The mass balance rate 153 of Cr under the four working conditions is shown in Fig.2 were collected from the original site of a special steel plant, and chromium is predicted to exist as 164 Cr 2 (SO 4 ) 3 and CrO 3 . Therefore, most of chromium in the raw materials will be difficult to volatilize. 165 Chromium is volatilized mainly as a hydroxide in a high-temperature tube furnace. 166 Thermodynamically stable Cr compounds include CrO 3 (g), CrOOH(g), and CrO 2 (OH) 2 (g) ( CrOOH and CrO 2 (OH) 2 were the main gaseous forms in the combustion process. As shown in Table  169 1

Migration and conversion of Cr 186
The Cr(VI) concentration in the raw materials and combustion products is shown in Table 6. 187 The Cr(VI) concentration in the clinker was increased significantly compared with the raw materials 188 during all experimental conditions. The increased Cr(VI) concentration can be explained as follows: 189 (1) Cr(VI) concentration increased under the influence of weight loss and (2) Cr(III) was oxidized 190 to Cr(VI) during calcination. 191 192 Table 6 The total Cr(VI) concentration in raw materials and combustion products (mg/kg) 193

Mass balance of Cr 237
The material streams for the combustion experiment of chrome-polluted soil in a cement rotary 238 kiln are shown in Fig.5. The raw materials included cement raw meal, soil, coal, and kiln dust, and 239 the combustion products included clinker, flue gas, and kiln dust. The Cr mass balance calculation 240 was carried out based on these material streams. The kiln ash was returned to the kiln and therefore 241 not included in the mass balance calculation. The input and output rates of Cr were calculated 242 according to Tables 2 and 7, as shown in Table 8. Similar to Eq. (2), the mass balance rate of Cr was 243 110% for the control condition and 84% with 1% chrome-polluted soil. The mass balance results 244 ranged from 70% to 130%; therefore, the Cr is mass balanced and is not significantly volatilized 245 into the flue gas. This is consistent with the results of the laboratory experiments.   Fig.6 shows the valence distribution of Cr in the cement kiln system for the control condition 255 and with 1% chrome-polluted soil. In Fig.6, the distribution of Cr(III) and Cr(VI) in clinker under 256 the control condition was almost the same as that with 1% chrome-polluted soil. The ratio of Cr(VI) 257 to total Cr was 88.96% and 87.46%, respectively, for the control and experimental conditions. On 258 10 average, 87.94% of the Cr(III) in the raw materials was oxidized. In the laboratory experiments, the 259 ratio of Cr(VI) to total Cr in the clinker ranged from 38.66% to 45.61%. This is significantly lower 260 than for the field-scale experiments, and can be explained by the different calcination conditions. 261 The materials in the cement rotary kiln can make full contact with oxygen, while in a tube furnace, 262 the inside of the bulk material does not have full contact with oxygen. 263 With the added 1% chrome-polluted soil, the Cr concentration in the clinker was higher than 264 in the blank condition. Total Cr increased by 40.5%, and Cr(VI) increased by 38.13%. The increase 265 in Cr can be explained by the addition of 1% chrome-polluted soil. 266 267 Fig.6 The valence distribution of Cr in the cement kiln under the control condition and with 1% 268 chrome-polluted soil 269 270

Possible reactions of Cr in the cement kiln 271
The calcination process of raw meal in a cement kiln includes preheating, decomposition, 272 sintering, and cooling, which are carried out in preheater, pre-calciner, rotary kiln, and clinker cooler, 273 respectively. Based on the chemical reactions and reaction conditions of raw meal in a cement kiln, 274 the possible reactions of Cr were analyzed in this study. is still in the solid phase; however, CaO can dissolve in the high-temperature liquid(Li 2020). 308 Dissolved CaO has increased contact area with Cr, promoting the oxidation of Cr to a certain extent. 309 In the cooling process of clinker, there is still a small amount of free CaO in the solidified material 310 that has not been chemically reacted. 311 312

Environmental implications 313
The distribution of Cr in two cement products (P·C 32.5R and P·C 42.5R) with added 1% 314 chrome-polluted soil is shown in Table 9. The Cr concentration in the cement products was lower 315 than that in the clinker, as shown in Table 7. The total Cr for the two cement products decreased by 316 40.14% and 8.09%, and Cr(VI) fell by 61.59% and 62.64%, respectively. The decrease occurred 317 because the Cr concentration in clinker was diluted after adding gypsum, and other additives. The 318 standard Gb30760-2014 "Technical Specification for Collaborative Disposal of Solid Waste by 319 Cement Kiln" indicates that the limit for Cr in clinker is 150 mg/kg. Therefore, adding 1% of 320 chrome-polluted soil to the raw meal of a cement kiln conforms to Chinese safety standards. 321 322 Table 9 The distribution of Cr in two cement products (mg/kg) 323 For the four calcination experiments in a tube furnace, the Cr mass balance values ranged from 326 84% to 99%, and the average mass balance was 91%. During the co-processing of chrome-polluted 327 soil with raw meal in a cement rotary kiln, the mass balance values were 110% and 84% in the 328