Black Liquor Oxidation as a Means of Efficient Chemical Recovery in Paper Mills ^†

Abstract

The chemical recovery cycle is essential for every pulp mill producing pulp via chemical pulping. The purpose of this cycle is to recover inorganic chemicals used for pulping that can facilitate heat and electric energy cogeneration. New methods such as white liquor oxidation or black liquor gasification can increase the efficiency of the cycle and help to decrease the consumption of chemicals, thus contributing to more environmentally friendly pulp and paper production. This work focuses on assessing white-liquor-processing methods and evaluating their impact on chemical consumption in further pulp-processing stages. Model balances were established for a large paper mill with a capacity of 0.75 mil. tonnes of pulp and paper production, requiring around 100 tonnes per hour of white liquor for pulping. The results indicate that major savings on the purchase of chemicals can be realized, namely, more than 0.8 tonnes per hour and more than 1.2 tonnes per hour of pure sodium hydroxide in the cases of partial white liquor oxidation and full white liquor oxidation, respectively. Greenhouse gas emissions can be reduced by more than 10 thousand tonnes per year of CO₂ equivalent as a result. The economics of the proposed technology’s implementation are favourable, indicating a simple payback period of less than three years for a certain combination of chemical and utilities costs.

1. Introduction

Ongoing climate change stresses the need to find and implement solutions for sustainable low-carbon industrial production. This includes switching to low-carbon energy sources and restructuring industry to implement new technologies and processes consuming lower amounts of energy and materials and generating less waste and fewer emissions [1]. Heavy industry, including pulp and paper production, is a major contributor to industrial emissions; thus, it is a priority to focus on in innovation in these sectors [2]. The large-scale continuous processes used in this sector enable the achievement of large absolute savings in energy and material consumption, even with a slight increment in process improvement.

Chemical pulp mills producing kraft pulp consume large amounts of white liquor for the woodchip-cooking process. A significant amount of fresh sodium hydroxide is also required to improve the Na-S balance in white liquor and to enable oxygen delignification, and some is used in the bleaching stages as well [3]. Apart from concerns regarding economic expenditures, NaOH consumption contributes 0.63 to 1.91 t of CO₂ equivalent per t of consumed crystalline NaOH to the carbon footprint of pulp mills, depending on the employed production technology and calculation assumptions [4,5].

Novel technologies for liquor oxidation (white liquor, green liquor, and black liquor) are currently used in some pulp mills, and others are under development. White liquor is oxidized so that it may be used later as a caustic in the oxygen delignification and alkali extraction stages in bleach plants [6]. The degree of oxidation, yielding either a partially or fully oxidized white liquor, determines where the resulting liquor can be used.

This contribution aims to develop conceptual white liquor balances of a large-scale paper mill in the base state and in two cases applying either partial or full white liquor oxidation. As a result, the potential for sodium hydroxide consumption reduction is quantified, and the associated reduction in greenhouse gas emissions is estimated. An economic evaluation is performed to estimate the required investment and operational costs and evaluate the economic feasibility of the proposed technology’s implementation. Thus, a contribution toward cleaner and more sustainable industrial production is presented.

2. Materials and Methods

White liquor is employed as the supply of cooking chemicals used to cook wood chips in a digester. The active chemicals in kraft pulping are hydroxide and hydrogen sulphide ions, OH⁻ and HS⁻ [3]. Hence, white liquor is a mixture of sodium hydroxide and sodium sulphide in water. The considered composition of white liquor is shown in

Table 1. Composition of white liquor, adapted from [7].

Component	Concentration (g/L)	Mass Fraction (-)
NaOH	90	0.0783
Na2S	39	0.0339
Na2CO3	26.2	0.0228
Na2SO4	8	0.0070
Na2S2O3	4	0.0035
Na2SO3	0.9	0.0008

.

Partially oxidized white liquor can be produced through the oxidation of sodium sulphide contained in white liquor with oxygen from air to produce thiosulphate [3,8] (1):

2 Na₂S + 2 O₂ + H₂O → Na₂S₂O₃ + 2 NaOH

(1)

In this case, up to up to 97–98% of the sulphide can be converted into thiosulphate [6]. Partially oxidized white liquor is suitable for the oxygen delignification process [3,6]. In this study, the degree to which sulphide is converted into thiosulphate is assumed to be 98%, and the reaction consumes 80% of the oxygen supplied to the reactor. A supply of oxygen is ensured by incorporating an air compressor (with 90% isentropic efficiency and 95% mechanical efficiency), which compresses air from atmospheric pressure to 2 bar (a). The air used as the oxygen source has a temperature of 15 °C, a pressure level of 98 kPa, and a relative humidity of 80%.

Fully oxidized white liquor can be produced through the oxidation of white liquor with pure oxygen in pressurized reactors if the residence time and temperature of the white liquor are increased [3]. In this case, together with Equation (1) given previously, a further oxidation reaction (2) takes place [6,8]:

Na₂S₂O₃ + 2 O₂ + 2NaOH → 2 Na₂SO₄ + H₂O

(2)

In this oxygen-based system, the amount of sulphide converted into thiosulphate can be 98–99%, and the amount of sulphide converted into sulphate can be up to 60% [6]. Fully oxidized white liquor can be used for the oxygen delignification process, but it can also be useful in the peroxide-bleaching stages and for gas-scrubbing applications. The total oxidation of white liquor is performed in a pressurized reactor using pure oxygen. In the reactor, reactions (1) and (2) take place with conversions of sodium sulphide and sodium thiosulphate equal to 98% and of 60%, respectively. As the system is pressurized, the pressure of fed oxygen is set to 6 bar (a). It is assumed that the reactions consume 97% of the oxygen supplied to the reactor. The amount energy consumed for pure oxygen production constitutes 0.5 kWh/Nm³ of the produced oxygen [9]. To compare the compositions of basic white liquor (BWL), partially oxidized white liquor (POWL), and totally oxidized white liquor (TOWL), the following parameters (composition indicators) are calculated (3)–(5) [10], with the concentrations of each compound based on the respective NaOH equivalents. The considered density of all white liquors is 1150 kg·m⁻³ [10].

Active Alkali (AA) = c(NaOH) + 12 c(Na₂S)

(3)

Effective Alkali (EA) = c(NaOH) + c(Na₂S)

(4)

Sulfidity (S) = 100 c(Na₂S)/AA

(5)

Figure 1 presents the white liquor oxidation process and highlights possibilities for chemical replacement.

The real plant operational data presented in [11] allow for the quantification of the material streams involved: a model pulp mill with a production rate of about 13,000 tonnes of bleached pulp per day consumes about 3.5 t/h (tonnes per hour) of fresh sodium hydroxide solution (concentration of 120 g/L) together with 5.8 t/h of white liquor and 1.1 t/h of pure oxygen. In the “EO” bleaching section (alkaline stage), 4 t/h of fresh sodium hydroxide is used together with 0.04 t/h of pure oxygen for pulp bleaching. Another 4 t/h of fresh sodium hydroxide is used in the peroxide-bleaching stage. The total amount of potentially replaceable sodium hydroxide is 11.5 t/h, out of which 7.5 t/h can be replaced by partially oxidized white liquor and a further 4 t/h (peroxide bleaching) can be replaced by fully oxidized white liquor.

Key equipment investment cost estimation, following the exponential method, was used to estimate the investment cost of equipment or the plant based on existing cost data of the same equipment with different capacities, with the well-known “six tenths rule” applied to the exponent value [12]. Indexation using the Chemical Engineering Plant Cost Index (CEPCI) was used to recalculate the investment costs of past investments such that they aligned with current conditions [13]. The factor method was used for the estimation of the total investment cost (TIC) of a plant, where TIC was assumed to be the key equipment investment cost multiplied by a factor of 5 [13]; this value is recommended for liquid-handling plants installed in existing industrial areas. Assumed materials and utilities costs are summarised in

Table 2. Costs of materials and utilities.

Material/Utility	Cost (EUR)	Unit	Reference
Electricity	100	MWh	[14]
Pure NaOH (crystalline)	150 to 500	t	[15]

; 8500 working hours per year are considered.

3. Results

3.1. Comparison of White Liquors’ Compositions

The composition indicators of BWL, POWL, and TOWL are compared in

Table 3. Composition indicators of basic white liquor (BWL), partially oxidized white liquor (POWL), and totally oxidized white liquor (TOWL).

Composition Indicator	BWL	POWL	TOWL
Active alkali (g/L)	130.0	110.0	97.5
Effective alkali (g/L)	110.0	109.5	96.0
Sulfidity (%)	30.76	2.70	3.05

.

As shown in Table 3, the values of AA and EA are somewhat lower in the oxidized liquors compared to the BWL. The sulfidity in the oxidized liquor is much lower than that in the BWL. If the sulfidity is too low, the lignin content of the pulp may be relatively high, and the degree of carbohydrate degradation may be severe, which leads to low pulp strength. However, if the sulfidity is too high, emissions of reduced sulphur compounds may increase, and the corrosion rates in the recovery process may be high [3].

3.2. White Liquor Oxidation

The mass balance of the bleaching plant yielded a potential for the replacement of NaOH solution (concentration of 120 g/L) by POWL equal to 7.5 t/h, corresponding to 0.8 t/h of crystalline NaOH. The potential for the replacement of NaOH solution with TOWL obtained using the bleaching plant balance amounted to 11.5 t/h, which is equivalent to 1.2 t/h of crystalline NaOH. Annually, up to 7–10 thousand tonnes of sodium hydroxide can be saved, resulting in a decrease in a paper mill’s carbon footprint related to purchased chemical by 6 to 19 ktons of CO₂. Considering a paper mill producing 13 kton of pulp per day, this would translate into 0.01 to 0.03 tons of CO₂ per ton of produced pulp.

A substantial economic effect can be achieved as a result. Crystalline NaOH prices have varied between EUR 150 to 500 per ton in Europe in the last few years [15]. Thus, the achievable annual savings amount to EUR 1 to 5 mil.

3.3. Economic Parameters

Basic economic parameters were estimated for three plant variants. They include POWL plant; TOWL plant A, in which only totally oxidized white liquor is produced; and TOWL plant B, in which both types of oxidized white liquor are produced. The cost of electric energy for compressors, white liquor pumps, and oxygen production is included in the operational costs.

As shown above, the best variants for installing a white liquor oxidation plant are POWL only or the TOWL variant 2. Even with NaOH prices as low as 150 EUR/t, the achievable profit is significant, and the simple payback period, which is indicative of a project´s feasibility, is only in the order of several years.

4. Discussion

Table 3 provides an insight into the compositions of basic and oxidized white liquors. These differences might influence the consumption of chemicals in further pulp production stages and final product quality. An experimental investigation and subsequent oxidation parameter tuning are necessary in real pulp and paper mills [16] in order to implement the technology successfully and avoid any significant disruptions in the production process.

The economic parameters estimated in

Table 4. Economic indicators of considered white liquor oxidation plant layouts for NaOH price equal to 150 EUR/t.

Parameter	POWL Plant	TOWL Plant A	TOWL Plant B
Investment costs (mil. EUR)	1.5	3	3.5
Electricity costs (EUR/h)	5	11	23
Savings (EUR/h)	120	60	180
Profit (EUR/h)	115	49	157
Profit (mil. EUR/year)	0.98	0.42	1.33
Payback period (years)	1.5	7.1	2.6

are extremely sensitive to the price of NaOH. Previous years brought significant changes in its market price, which were caused by unprecedented fluctuations in the price of electricity. Despite the volatile NaOH market prices, the payback periods reported in Table 4 for various plant configurations are favourable, even in the case in which a low NaOH price (150 EUR/t) was adopted for calculations. The investment into a white liquor oxidation plant is attractive when compared to the simple payback periods of mid-scale projects usually regarded as acceptable by industrial companies (5 to 7 years). Apart from the promising project economy, the corresponding carbon footprint reduction is another important investment incentive for any environmentally aware company. In this sense, the reported NaOH carbon footprint range [4,5] is too wide and requires a separate life cycle study to confirm the claimed environmental benefits.

Besides white liquor oxidation, other alternatives such as green or black liquor oxidation could be considered, each having a different impact on both chemical consumption in the pulping process and a paper mill’s in-house steam and electricity generation. A more complex study dedicated to the techno-economic and environmental evaluation of each of those investment opportunities would contribute to the efforts to decarbonize the pulp and paper industry.

5. Conclusions

A conceptual assessment study on the implementation of white liquor technology in a large-scale paper mill was presented, including basic chemical balances and the identification and quantification of potential chemical savings. The financial savings and emissions reductions resulting from three white liquor oxidation plant layouts were estimated. The simple payback period of 1.5 to 7 years obtained for the conservative NaOH price estimate (150 EUR/t) is promising and shows the potential of such a savings measure’s adoption by paper mills. A further study dedicated to the techno-economic and environmental comparison of available oxidation technologies applied to various stages of the chemical recovery cycle in paper mills is the next goal.