إِذِ الْأَغْلَالُ فِي أَعْنَاقِهِمْ وَالسَّلَاسِلُ ...

When iron collars will be rounded over their necks, and the chains.

means, the chains will be attached to the iron collars, and the keepers of Hell will drag them along on their faces, sometimes to the boiling water, and sometimes to the Fire.

Allah says:

... يُسْحَبُونَ ﴿٧١﴾

فِي الْحَمِيمِ ثُمَّ فِي النَّارِ يُسْجَرُونَ ﴿٧٢﴾

they shall be dragged along, in the boiling water, then they will be burned in the Fire.

This is like the Ayat:

هَـذِهِ جَهَنَّمُ الَّتِى يُكَذِّبُ بِهَا الْمُجْرِمُونَ

يَطُوفُونَ بَيْنَهَا وَبَيْنَ حَمِيمٍ ءَانٍ

This is the Hell which the criminals denied. They will go between it and the fierce boiling water! (55:43-44)

After describing how they will eat Zaqqum (a bitter tree of Hell) and drink Hamim (boiling water),

Allah says:

ثُمَّ إِنَّ مَرْجِعَهُمْ لإِلَى الْجَحِيمِ

Then thereafter, verily, their return is to the flaming fire of Hell. (37:68)

And Allah says:

وَأَصْحَـبُ الشِّمَالِ مَآ أَصْحَـبُ الشِّمَالِ

فِى سَمُومٍ وَحَمِيمٍ

وَظِلٍّ مِّن يَحْمُومٍ

لاَّ بَارِدٍ وَلاَ كَرِيمٍ

And those on the Left Hand -- how (unfortunate) will be those on the Left Hand? In fierce hot wind and boiling water, and shadow of black smoke, (that shadow) neither cool nor (even) pleasant.

until,

ثُمَّ إِنَّكُمْ أَيُّهَا الضَّآلُّونَ الْمُكَذِّبُونَ

لاّكِلُونَ مِن شَجَرٍ مِّن زَقُّومٍ

فَمَالِـُونَ مِنْهَا الْبُطُونَ

فَشَـرِبُونَ عَلَيْهِ مِنَ الْحَمِيمِ

فَشَـرِبُونَ شُرْبَ الْهِيمِ

هَـذَا نُزُلُهُمْ يَوْمَ الدِّينِ

Then moreover, verily, -- you the erring-ones, the deniers (of Resurrection)! You verily, will eat of the trees of Zaqqum. Then you will fill your bellies therewith, and drink boiling water on top of it. And you will drink (that) like thirsty camels! That will be their entertainment on the Day of Recompense! (56: 41-44, 51-56),

إِنَّ شَجَرَةَ الزَّقُّومِ

طَعَامُ الاٌّثِيمِ

كَالْمُهْلِ يَغْلِى فِى الْبُطُونِ

كَغَلْىِ الْحَمِيمِ

خُذُوهُ فَاعْتِلُوهُ إِلَى سَوَآءِ الْجَحِيمِ

ثُمَّ صُبُّواْ فَوْقَ رَأْسِهِ مِنْ عَذَابِ الْحَمِيمِ

ذُقْ إِنَّكَ أَنتَ الْعَزِيزُ الْكَرِيمُ

إِنَّ هَـذَا مَا كُنتُمْ بِهِ تَمْتَرُونَ

Verily, the tree of Zaqqum will be the food of the sinners. Like boiling oil, it will boil in the bellies, like the boiling of scalding water.

(It will be said:) ''Seize him and drag him into the midst of blazing Fire, then pour over his head the torment of boiling water. Taste you (this)! Verily, you were (pretending to be) the mighty, the generous! Verily, this is that whereof you used to doubt!'' (44:43-50)

i.e., this will be said to them to rebuke and ridicule them.