Evaporation and its Effects on Physico-Chemical Properties of PbO-containing Slag

PbO as well as other volatiles will evaporate at high temperature. In the measure of relevant slag property, the slag will be kept at high temperature for a long time (even several hours). The continuous evaporation will result in uncertainty of the measured results. In another point of view, that means the measured result is no longer in consistent with the pre-determined slag composition, but a new one changed obviously. For high or medium PbO content slag, the weight loss can reach to 20∼40 % at high temperature. The effect is not negligible. In this work, the evaporation and physicochemical property (melting point and viscosity) measurement of typical PbO-containing slag were jointly analyzed based on calculation, TG-DSC, melting temperature and viscosity measurement. A deviation method based on traditional measurement ways was suggested and practiced. It is proved that to take physicochemical property value with traditional method and the get the in-time composition to match the measured values is feasible. how obtain in-time composition is also discussed and tried. The work will be of extensive reference significance to other slag system with higher volatiles.


Preface
Up to now, pyro-metallurgy is still the main route for Pb and part of Zn production.In recent years, a lot of pyro-metallurgical process have been developed for Pb smelting, such as QSL, SKS, Kivcet process, Ausmelt/Isasmelt, Kaldo, and bottom blowing/side submerged blowing and so on [1,2].In all these processes, PbO and ZnO containing slag is common.How to control the slag physicochemical properties, such as melting point, viscosity, surface tension, heat and electric conductivity, heat transfer property, component activity.etc., is of great importance to smelting process designing, process control, inclusions removal, product quality, as well as other technical and economic indexes.
It is well known that PbO and ZnO, flouride and chloride are material trend to evaporate at high temperature [3][4][5][6][7].In the physicochemical property measurement, these volatiles will evaporate continuously at heating or high temperature keeping process, so to the production process.The higher, the temperature is, or the longer the keeping time is, the great the loss of volatile from the slag is.That means the slag composition will be changed continuously.As a result, the measured slag physicochemical properties will change [5][6].It is referred as uncertainty of slag property for brief.
In oxidation smelting or PbO direct reduction step, the main slag is PbO-FeO-Fe2O3-CaO-SiO2-ZnO system with higher PbO and ZnO content.Only a few of research related to these slag [13][14][15][16].but they focused on phase equilibrium and gave little practical results on slag properties.As what they pointed out that the PbO and ZnO evaporation can affect the accuracy of physicochemical properties measurement.That means that up to now there is no reliable PbO and ZnO containing slag physicochemical property data to support pyro-metallurgical production process development and optimization.
Our group measured the weight loss of PbO slag with TG-DSC.It is detected that the weight loss (i.e., evaporation) to slag with initial PbO content (determined in slag sample preparing) of 21.54 % and 43.09 % can reach to 21.62 % and 38.69 %, respectively, at temperature between 736 °C to 1450 °C [17].Thus, in the physicochemical property measurement of slag at high temperature, the evaporation is absolutely not neglectable.The traditional slag property measurement method in not fit these slags again [5,6,17].Then how much the evaporation effect is? What is the effect mechanism?How to make it to get the correct property data?How to get the real value?These questions should be replied urgently for PbO and ZnO containing slag control as well as to pollution control.

Weight loss of some PbO and ZnO containing slag at high temperature
The weight loss of some PbO and ZnO containing slag were measured at high temperature as listed in table 1 and figure 1 [17].To avoid the effect of moisture in the samples, only the weight loss at higher temperature, i.e. above 410 °C were taken.It can be seen that the weight loss is very large.That means the slag composition will change obviously.The higher the PbO content is, the great the weight loss is.To the slag with FeO/SiO2, CaO/SiO2 and PbO at 1.54, 0.4 and 43.09 % respectively, the weight loss can reach to 38.69 % when the temperature arises from 736 °C -1450 °C.From figure 1, it is clear that to a sample, the weight loss will increase with the temperature increase.Of course, the weight loss will also increase with the keeping time increase at a high temperature.
To all of the physicochemical property measurement, the slag (or flux) will experience heating, melting and high temperature keeping.If the slag evaporation happens and the weight loss is great like listed in table 1 and figure 1.The effect of evaporation to slag property measurement result is certainly not negligible.Especially at the temperature above melting point.

The consideration to the uncertainty of slag property measurement
In traditional method, to take melting point measuring as an example, the slag composition is determined first, the sample is prepared, and then heating slowly, until the sample smelt at a temperature.The temperature is considered as melting point of the slag with the initial composition.This is no question to the slag sample without strongly evaporation.But to the samples like in table 1, the slag composition will change continuously, because the evaporation.The final temperature corresponding to the melting point with traditional method is not in accordance with the initial composition again, but a composition changed from initial one [5-6, 18, 19].In another point of view, that means all the physicochemical property results are not accurate without consider the evaporation.The difference in heating route, temperature, and keeping time can deduce to the difference weight loss, as a result, the difference composition changes and different property measurement result.That the main cause of uncertainty of slag property measurement.
Then how to solve this uncertainty?Since the evaporation leads to the chemical composition change, results in the property measurement result not in accordance with the original composition, to take the in-time composition to corresponding with the property measurement result by traditional method will be reasonable theoretically.Here how to acquire the in-time composition become the key problem.

The new method to get the property of slag with volatile
To take the melting point measurement as an example.The Hemispheric temperature (traditional melting point measuring) method was used.The sample was heated at rate of 15 ℃/min, until the height drops down to 50 % of initial height.The temperature is taken as the melting point Tm.The weight loss at the same heating rate was carried out in the same time.Details is shown in figure 2.
To the pre-melted slag sample, the Tm is 1285 °C and the weight loss is less.The evaporation is neglectable.To the mixed slag sample, the measured Tm is 1385 °C, and the weight loss at 1385 °C is about 8.3 %.If the composition at 1385 °C can be obtained or calculated based on weight loss, the measured Tm should be in corresponding to the new composition, not the initial one.

Determination of evaporation component and in-time composition
The vapor pressures of PbO, ZnO, Pb, Zn at different temperature was calculated as table 2. From the results, it can be seen that if no reduction happens, the evaporation component in slag should be PbO first because of its higher vapor pressure and then ZnO.At reduction atmosphere, PbO, ZnO can be reduced and evaporates at the metal gas state.The test results support this conclusion.The collectives of condensed material are mainly PbO as listed in table 3, especially to higher PbO slag.So, it is easy to combine the TG results to get the in-time composition at the melting point by traditional way, and then to match the melting point with the in-time composition.
Apart from this way, another method can be used to get the in-time composition by roasting and composition analysis, especially to the complicated evaporation [20].To roast the slag, sample up to different temperature at the rate of melting point measurement and then cooling down quickly to analyze composition of the rest sample.The device is shown as figure 3.One of the typical test results of the slag sample 3 with 20% PbO is shown in figure 4. It can be seen that with the temperature increase, PbO and ZnO content in slag decreased.At the same time, content of another component (not volatile) increased.If the melting point of the sample is at T2, the matching composition is shown in T2 marked vertical line.

The practice property measurement to slag with PbO and ZnO
It has been clear that the new melting point Tm measuring method is composed of two steps, measuring Tm in traditional way first, and then getting the in-time composition to match Tm.So, to the viscosity.Generally, slag viscosity is measured with rotating cylinder viscometer at different temperature, i.e., heating the sample up to a temperature above melting point first, viscosity measuring, and then lower temperature to another point, measuring repeatedly.Finally, relation between viscosity and temperature can be obtained as shown in figure 5.According to the new method, the viscosity of sample 3 at 1150 ℃ is about 0.25 Pa‧s.The matching in-time composition is taken as vertical line T1 in figure 4, not the composition of sample 3 in table 3. The in-time composition is about 9%PbO-7%ZnO-25%SiO2-45%FeO-14%CaO, a great difference from initial composition.In the same way, the viscosity of a serious different slag composition at different temperature can be picked up from the η-T relation of one sample by traditional method.

Evaluation of measurement error with traditional method
Measurement uncertainty in another point reflects the deviation from the real value that is often difficult to get.In melting temperature measuring, the result can be affected by heat transfer, delay of particle reaction, partial melting, sample evaporation and so on.The effect of different factors is shown in figure 6 [20].
To a slag with no volatile, the general result is as line ①.Tm measured will increase with the heating rate.If heating rate is close to zero, we get the approximate real value (line ②, point D).But to the slag with volatile, such as PbO, Tm measured will decrease with the heating rate as line ③.The trend is completely different.If the heating rate is tended to zero, the volatile will evaporate completely and the measured Tm trend to point D, i.e., the same value as no volatile slag.Suppose the effect of all other factors is the same, the only difference is volatile evaporation.The temperature difference between line ① and line ③ is the effect of evaporation.With the increase of heating rate, the evaporation is restrained and lead to Tm measured decreased.Evaporation effect is more than other effect.To sum up, the result of line ③ appeared.Then what is the real value?If at the rate of 30 ℃/min, the evaporation can be restrained effectively, the error of Tm measured can be modified only refer line ① to line ②.To move line ① down parallelly, we got line ④ that shows the result of evaporation effectively controlled.The point F is the approximate real value.Gap between line ④ and line ③ as GH reflects the error caused by evaporation.

Conclusion
Focused on slag with PbO, ZnO or other volatiles, the following points can be concluded.
(1) PbO will evaporate obviously at high temperature, lead to composition change and property measuring error that not negligible.Traditional method is not fit PbO containing slag again.
(2) The higher the temperature is, the more the volatile content in slag, and longer the keeping time at high temperature, the greater the weight loss (caused by evaporation), composition change and the effect to property measurement is.
(3) Measuring the physicochemical property by traditional method and then take the in-time composition to match measured result is reasonable and workable.It suits for slag with volatiles, such as PbO-containing slag.
(4) In-time composition is the key problem for the new method.It can be obtained through different ways.If the volatile component is only one, the in-time composition can be gotten with TG.If the evaporation is complicated, the in-time composition can be obtained by slag roasting parallelly.Predicting kinetic model for in-time composition prediction based on evaporation mechanism is the final resolution.
(5) A method for real melting temperature value and measurement error evaluation based on test at different heating time is suggested.

Figure 2 .
Figure 2. Schematic program of the new melting point measurement [4].

Figure 6 .
Figure 6.The effect of different factors on melting temperature deviation [20].

Table 2 .
Calculated vapor pressures of Pb, Zn, PbO and ZnO at different temperature, Pa.

Table 3 .
The vapour condensed material composition by test, %.